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Weather Station Hardware => Ambient Weather and Ecowitt and other Fine Offset clones => Topic started by: kc59harley on March 29, 2020, 04:48:59 PM
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I was checking my data from my weather station hardware with the Gladstonefamily website weather.gladstonefamily.net and they indicate my barometer reading are high for my location. I used to have Oregon Scientific hardware but got tired of replacing sensors. The WS2902 didn't transmit to CWOP but with the new group creating a API to interface this stations output, it now does using their signup site ambientcwop.com. My question is, is there an adjustment in software to correct my barometer readings?
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Please post your CWOP ID so that we can take a look and make recommendations.
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Sorry about that.
CW7091
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Yes I have confirmed that your barometer data is not right. It is definitely sending too high of barometric pressure.
First let's determine your current display console settings.
Tell me at what your Relative Pressure is and what your Absolute Pressure is so that I can determine by how much they differ.
To show both of these, alternate your display from showing Absolute and then Relative pressure you press and hold the Pressure button and it will switch between showing one and then the other.
I need:
Your Absolute Pressure ______
Your Relative Pressure ______
Your elevation________ (80 ft or 89 ft? or something else)
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Abs - 29.83
Rel - 30.02
75 feet
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Where I live it says 82 feet but using my phone its says 75
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Step 1.
Your elevation is key in solving this problem with barometric pressure. Let's tackle your elevation first.
Your CWOP registration says that you submitted that your barometric sensor is at 27 meters (89 ft).
I see that you are at 84 ft using USGS data:
https://viewer.nationalmap.gov/theme/elevation/
Alternative elevation map for those outside of the US (Edit - I'm liking this site a bit better lately than the USGS topo site):
https://www.freemaptools.com/elevation-finder.htm
Good Latitude / Longitude converter in case you need that:
https://www.directionsmag.com/site/latlong-converter/
Forget what the phone says. That is not the most accurate information to use. It is best to use map survey data like the source I linked from USGS or from FreeMapTools.
But your barometric sensor (in your station's console) is not on the ground. We need to account for its elevation above the ground so that height needs to be included to total elevation. I'm gong to assume your WS-2902A weather station is on the first floor. Let's suppose it is at 5 ft above the ground on a kitchen counter or a shelf in the family room on the 1st floor. Perhaps it something else but I need some numbers to at least do some sample calculations. If these assumptions are wrong you can tell me what you are at and we can then make adjustments in these calculations. This would even be more important if you have this console on the 2nd floor. Your anemometer height is irrelevant for barometric calibration. Some people assume that the barometer is outside and it isn't.
Turns out that by using 89 ft (84 ft + 5 ft off the ground) matches your registration with CWOP of 89 ft or 27 meters. Good, we don't need to make any changes with your CWOP registration by using these numbers. If these numbers are wrong it is still an easy fix to email NOAA to adjust your station's barometric sensor elevation to be correct. But I'm going to assume this is right so that we can do the next calculations.
The next step is to find out what the difference between your Relative and Absolute pressure should be. This different is determined by the elevation of your barometric sensor (89 ft or 27 meters). This difference will be static (it doesn't ever change). Meaning it doesn't matter what the pressure is at any given moment as the difference between Absolute and Relative will always be the same (temperature affects this a bit but we are going to use standard temperature of 15°C or 59°F in this calculation which is the way to calculate and set configure this).
The difference between your Absolute and Relative pressure should be: 3.25 hPa or 0.096 inHg for an elevation of 89 ft or 27 meters.
Unfortunately with your weather station you can't dial in a calibration difference of that level of precision. But we can get closer by switching your console to temporarily use hPa. Then after we are done entering in this difference between Absolute and Relative you can then switch back to inHg and the calibration is still there but it was entered with the higher precision of hPa. So you enter in a difference of 3.3 hPa between Absolute and Relative. You can if you want use the less precise inHg and enter in a difference of 0.10 inHg between Absolute and Relative pressure.
Basically what we have done with this last step is tell the console that you are at 89 ft (27 meters) of elevation above sea level where the barometric sensor is. Telling the console that the difference is 3.3 hPa or 0.10 inHg says that you are at that elevation.
How I got to that number? I used the following online calculator (more details on follow-up post on how to use this site):
https://keisan.casio.com/exec/system/1224579725
Then I then see the difference between Absolute and Relative which corresponds to Atmospheric pressure and Sea Level Pressure on the online calculator (Absolute and Relative mean the same thing as those things). The result I'm giving you is the difference of both numbers after you enter in your elevation (in meters for this online calculator).
Then to convert the difference in hPa to give you a difference in inHg I used this converter:
https://www.convertunits.com/from/hpa/to/inhg
What you'll notice from the numbers you gave me before....your console is not configured for the proper elevation.
The difference between your current Absolute and Relative pressure is 0.19 inHg.
That is 6.43 hPa of difference.
Which corresponds to 54 meters or 177 ft which is way wrong. Your console thinks it is at 177 ft with your current settings.
Your difference between Absolute and Relative is about 3.1 hPa too high or 0.09 inHg too high.
After you have fixed the difference between Absolute and Relative you never touch this offset again. It remains that way to indicate that you are at 89 ft.
Step 2.
The last step is to calibrate your barometer to match your Relative to the nearest Metar.
Use this map to find nearby Metars:
https://aviationweather.gov/metar
Then below the map enter in the ID of the local Metar, and be sure to select "Decoded" and then Get METAR Data
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There are 3 METARs close-by to you:
KRNT
https://aviationweather.gov/metar/data?ids=KRNT&format=decoded&date=&hours=0
KBFI
https://aviationweather.gov/metar/data?ids=KBFI&format=decoded&date=&hours=0
KSEA
https://aviationweather.gov/metar/data?ids=KSEA&format=decoded&date=&hours=0
When looking at Metar data mb is millibars which is the same units as saying hPa. So mb = hPa (HectoPascal)
This is another reason why switching your console to use hPa makes it easier and more accurate. Then when done calibrating you can switch to show inHg and everything is still calibrated.
Use those on a good clear day around noon to calibrate against. (you can do it now to get closer but then double check the next day at noon on a fair day)
To set your Relative pressure to match the Metar you'll actually be adjusting your Absolute up or down to affect a change up or down on your Relative pressure.
That last sentence I know is confusing to understand at first. So let me say it again further spelled out.
You look at the Metar from those links I provided and then you look at your current Relative Pressure before adjusting anything.
You then determine how far off you are. Meaning how much up or down is your Relative pressure compared to the Metar?
Suppose your Relative is 6.4 hPa or 0.19 inHg too high compared to Metar.
Then you need to look at the Absolute value and whatever it shows you need to lower by 6.4 hPa or 0.19 inHg.
This is because when you bump up or down the Absolute value in your console what you are actually doing is affecting the same change in the Relative value. Because the difference between Absolute and Relative is constant (since we set that before as representative of your elevation)
That is the biggest thing to realize when adjusting the barometer on these stations. Two steps are required. First you set the difference which corresponds to your elevation. Then you calibrate Relative by adjusting up or down your Absolute value, and then checking back that Relative now is calibrated. You can't change Relative directly or what you will be doing is changing your elevation. So never adjust Relative directly after you have set your difference offset value to your elevation.
Monitor your Relative pressure over a few days compared to the Metar. Make minor adjustments till you are happy. Don't go crazy though. It is impossible to always expect your barometric pressure to be exactly equal to the Metar. This can be further explained by the fact that the Metar is some distance away and the barometric pressure isobars may be not in line with your current location. There are going to be times when a given Metar is on the same isobar as you and there are going to be times when the isobars are not going to be the same as your location. That is the purpose (to use Sea Level Pressure) of comparing neighboring stations that are at different elevation. If you want to further understand isobars and how to take barometric calibration to the next level then I suggest reading this other post that I wrote. But don't jump on this other stuff just yet. First just take is simple and really grasp the content in this post before you go looking at this next advanced topic of isobars. All you need to know for this fist step is that when you are done with this that your station will be pretty well calibrated and you'll know that things are okay even on days where you seem to drift a bit from these local Metar.
https://www.wxforum.net/index.php?topic=36579.0
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I was checking my data from my weather station hardware with the Gladstonefamily website weather.gladstonefamily.net and they indicate my barometer reading are high for my location. I used to have Oregon Scientific hardware but got tired of replacing sensors. The WS2902 didn't transmit to CWOP but with the new group creating a API to interface this stations output, it now does using their signup site ambientcwop.com. My question is, is there an adjustment in software to correct my barometer readings?
Hey, thanks for using AmbientCWOP. I just wanted to clarify that AmbientCWOP is an independent entity and not related in any way to Ambient, LLC or Nielson-Kellerman. I just noticed you said "the new group" and "their" and wanted to make this absolutely clear, as it's posted in the header and footer of every page on the site.
Sorry for the legalese :)
galfert will help you get this straight, he's VERY knowledgeable about this topic. I'll be working on a backend way to make reported pressure more accurate on AmbientCWOP here soon as well.
Thanks!
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galfert will help you get this straight, he's VERY knowledgeable about this topic.
Thank you danoh for the kind words.
I'll be working on a backend way to make reported pressure more accurate on AmbientCWOP here soon as well.
FYI - what danoh means by this is that currently AbientCWOP is sending Relative pressure to CWOP, when it should be instead taking your station's Absolute pressure and putting it through some calculations to arrive at Altimeter pressure and then send that to CWOP. I have mentioned Relative and Absolute pressure before on the previous post. Now I'm mentioning Altimeter pressure and that is yet another different thing. Most software that upload to CWOP are designed to send Altimeter pressure to CWOP using your correct Absolute (station) pressure. So if you have correctly calibrated you don't need to worry about this.
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So I used the USGS map and put in my coordinates. It was spot on and gave me a elevation of 83.8 feet. My console is on the second floor on a tall table (2 feet). I believe a second story is 10 feet. So my true elevation at the console is 95.8 feet. Correct?
Also your explanation of how you got to your numbers is quite excellent. I'll try and get it figured out. I'm wondering if I just get an update to NOAA it might correct my numbers for Gladstonefamily?
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Sorry danoh. I didn't mean to say the group was related if its not. I just got onboard signing up with it. That was the one area the console software wasn't pushing data to which is important to me.
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I'm reporting altimeter now.
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Based on the USGS map and where my console lives in my two story, I had them make the change to 95.8 feet in elevation.
My pressure readings on the console today were slightly different.
30.06 Abs
29.86 Res
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I'm reporting altimeter now.
Excellent fast work! Thank you!
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So I used the USGS map and put in my coordinates. It was spot on and gave me a elevation of 83.8 feet. My console is on the second floor on a tall table (2 feet). I believe a second story is 10 feet. So my true elevation at the console is 95.8 feet. Correct?
Well every home/building is different. Probably close enough. You can measure you walls easily I would think. Then add a bit for floor space and you should know pretty close what the elevation is at the 2nd floor.
Also your explanation of how you got to your numbers is quite excellent. I'll try and get it figured out. I'm wondering if I just get an update to NOAA it might correct my numbers for Gladstonefamily?
No sending an update registration of your elevation to NOAA for your real elevation will not correct your data which I've seen by looking at your raw data packets.
http://findu.com/cgi-bin/raw.cgi?call=CW7091
The only thing updating your elevation with NOAA will do is for station information and it might affect MesoWest that does reverse station pressure calculations. You have to fix your barometer as I have delineated. There is no way around that.
Based on the USGS map and where my console lives in my two story, I had them make the change to 95.8 feet in elevation.
My pressure readings on the console today were slightly different.
30.06 Abs
29.86 Res
That is not that much different. I'd actually say it is about the same. The difference is 0.19 inHg yesterday and 0.20 inHg today. That is within the margin of error as things change during the day.
Based on those numbers your current configuration is set to:
0.20 inHg or 6.77 hPa offset (difference between Absolute and Relative pressure)
which indicates 56.6 meters or 185.7 ft of elevation.
Obviously that is too much and corresponds to a higher elevation than what you are really at.
For 95.8 ft or 29.2 meters your barometric offset difference should be 3.5 hPa or 0.10 inHg.
I highly recommend dialing it in with hPa units for greater precision. Your station will waver less even when you later switch to showing inHg. This is because the equipment is actually really using hPa internally and then converting to inHg if you ask for that. There is always a loss of accuracy due to unit conversion.
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Kc59harley,
I see you did an adjustment the other day. What is the difference between your Absolute and Relative pressure now?
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30.11 Abs
30.21 Rel
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30.11 Abs
30.21 Rel
Okay good. If you calibrated that difference using hPa then the precision is actually greater than the 0.10 difference you now have. Meaning that you gave me a difference in inHg units and that is less precise than hPa. Hopefully you dialed in that difference using hPa which for your elevation of 96 ft should be 3.5 hPa, because if not and yo used just the 0.10 inHg difference then your elevation is actually about 92 feet instead of 96 feet. You can't precisely dial in 96 feet using inHg. This is because every 0.01 inHg is about 9.3 feet. 92 feet is closer to the next jump of another 0.01 inHg which would take your elevation up to around 102 feet. Meaning if you are using inHg then dialing in 92 feet is better than 102 feet if your real elevation is 96 feet. The reason has to do with the precision of each unit given the number of significant digits that they hardware gives you. The difference you need is 3.5 hPa between Relative and Absolute and that converts to 0.10335494 inHg and that is impossible to dial in at that precision when you only get 2 decimal precision using inHg and it gets rounded to just 0.10 inHg of difference. That little bit of extra precision translates to the loss of precision of 92 feet instead of 96 feet.
The whole previous paragraph is just nit picking and an effort to try and explain why you should calibrate using hPa instead of inHg units, if you care about having the most precisely calibrated station. It will be less annoying later, and your barometer will track more true even when looking at inHg.
So far in this post I've only been talking about setting the difference between Relative and Absolute pressure. The only thing that setting that difference does is basically configure your elevation for your barometric sensor. We still need to then actually calibrate your barometer to your local Metar.
From the data that you are now sending to CWOP to me it looks like your barometer needs this second step of calibrating to the local Metar. To do this you need to adjust your Absolute pressure down by about 2.7 hPa (or about 0.08 inHg if you are just stuck using inHg which I discourage using). This change that you perform on Absolute will cause the Relative to change by the same amount...which is the result we are looking for.
Please don't misunderstand.....After going through these two steps of 1. entering the difference for elevation 2. calibrating with Metar, then you can switch the console to show you inHg and everything will be that more precise. Switching to hPa for the two calibration steps is only temporary as you calibrate. The Metar shows you SLP in mb which is hPa.
So you are almost there I think.
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So I'm trying soak this in because I've never done this or understood it before. I'm still trying to understand the math that you started with on the hPa and inHg and how you got to the numbers, sorry. I walked through it but the light didn't go on. I just put in what you said I should. I really want to understand it better so it makes some sense.
For instance the calculator https://keisan.casio.com/exec/system/1224579725. I'm not sure what you put in to get there.
What I used was the less precise inHg of 0.10 in the Relative pressure that you said to adjust.
Math was never my strong suit.
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Okay no problem. I can show you the math and do this step by step. The math thus far has only been more complicated because I'm trying to do two things. I'll be repeating some of what I've already said so that this can serve to also help others.
- I'm trying to convey how accuracy is lost due to unit conversion, because of the loss of precision in using inHg.
- I'm also trying to provide you with both inHg and hPa numbers so that you can see how each unit type is affected. Mostly so that you can use inHg if you are really that set against using hPa.
Before I go into the math I suggest we only talk about hPa so that I don't have to do as many unit conversions and so that there is less math to show. Once we are done, then you switch your console back to inHg and everything will be perfect. This is all good practice for me as I'm using this experience to hopefully write up a good how to that everyone can understand.
How to switch to between inHg and hPa units
- Press and hold SET for 2 seconds to enter setup mode.
- Repeatedly tap the SET button to get to the adjustment of Pressure units of Measure (pressure units will be flashing).
- Press WIND+ button to switch between hPa, mmHg or inHg.
- Press LIGHT/SNOOZE to exit setup mode.
Set Absolute and Relative Offset
- Look at console and press Pressure button to switch between Absolute and Relative and see what the current difference (offset) is between these now that hPa is the selected units. In your case it needs to be 3.5 hPa of difference.
The basics
We have determined that your barometric sensor (in the console) is at 96 feet of elevation. This was determined via the USGS website using your location and then adding to that the height off the ground for your console which is on the 2nd floor in your case.
The first step in calibration is to tell the console that you are at 96 feet. Unfortunately these Fine Offset consoles do not have a way to directly tell the console what your elevation is. We must therefore tell the console at what elevation it is at indirectly by configuring the Absolute and Relative pressure difference.
Absolute Pressure is the raw pressure of the barometric sensor.
Relative Pressure is the adjusted pressure for your elevation.
The higher up you are from sea level the lower the Absolute (station) pressure is. There is less atmosphere above you hence the lower pressure. To be able to compare weather fronts and weather conditions we need to take this Absolute station pressure and correct it for the elevation you are at. For most people this means that Relative pressure has had a given amount of pressure added to the Absolute value to correct for the elevation above sea level. Of course some people live at sea level and some few live below sea level as there are some few places on Earth that are below sea level, like Death Valley, CA.
The higher up in elevation you are the greater the difference between Absolute and Relative pressure.
How to use Keisan Online Calculator
https://keisan.casio.com/exec/system/1224579725
This online calculator has been designed to use metric units. We must therefore use meters and hPa.
For the first step in determining the offset difference between Absolute and Relative pressure we only need to enter in one number into the Keisan online calculator. All we need to enter in is your stations barometric sensor elevation (ground elevation plus height above ground). But we must enter this in meters. For 96 feet that equals is 29.3 meters (Google can convert that for you. Just enter into Google: 96 ft to m)
You keep all the other default numbers in the Keisan online calculator. The only number we need to change is the 1000 m to be 29.3 m. Then you click on the Execute button. The results will show 1,009.74 hPa as the Atmospheric pressure. The only purpose for this result is to see by how much it differs from the Sea-Level pressure. Therefore 1013.25 - 1009.74 = 3.51 hPa.
That is how I got 3.5 hPa as the difference between your Absolute and Relative pressure for your elevation.
You can change the starting sea-level pressure to be anything you want, but if you enter in 29.3 meters the difference will always be 3.51 hPa for that elevation.
*** Do not change the shown default temperature of 15°C (which is 59°F) as this is the standard temperature to use for this calculation. Changing the temperature will affect the results and we don't want that. We need an offset that works all year long. This temperature value is also what is used for Altimeter pressure calculations which is a different subject that we don't need to cover at this point. ***
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Thank you galfert for the explanation. It really helped me understand the process better and why the need to "dial it in". It is at 3.5 now. I was off. I went through the settings and looked at ABS reading in hPa at 1016.3 and REL at 1019.7. That made it 3.4 so I adjusted the REL to 1019.8. I think now Its right.
Part of the issue for me was just getting the SET routine down. I pulled the manual off the internet and read through the settings.
Again thank you for taking the time to teach a knucklehead barometric readings. It helps.
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Okay good all that only accomplishes Step 1.
You now need to do Step 2. Which is to calibrate to your local Metar.
*** The most important concept of this second step is to never directly adjust Relative pressure. Yes we need to calibrate Relative to be the same as the local Metar but you do this adjustment indirectly via the Absolute value.
- Look at the local Metar and look at the Sea Level Pressure.
- Then compare the Metar to your current Relative pressure.
- Whatever that difference is is the amount that you need to move Absolute up or down.
- Once you adjust Absolute by that amount when you exit the setup you then look at Relative again and it should have gone up or down by that same amount.
The previous process holds true because the offset you dialed in before of 3.5 hPa will always stay true. Relative and Absolute will always be 3.5 hPa different from each other ...because that corresponds to your elevation.
A good analogy is that if you take a drawing compass and put two pencils on each end and the set the distance apart of the pencils to a given amount and lock that down, then when you draw with one pencil and you make marks up and down they will always differ by the same distance from each other. That distance or difference between them represents your elevation. That is how much atmosphere you need to add to your elevation to get you to be equal to sea level.
If you take a look at this graph you can see that the barometric pressure changes but the difference between Absolute and Relative stay the same. That difference corresponds to your station's elevation (this graph below isn't your data..only used as an example). It is the amount of pressure that needs to be added to Absolute to make up for you elevation loss of pressure.
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Key Points to keep in mind:
Changing what you see for Absolute pressure makes Relative pressure go up or down by the same amount.
Directly changing what you see for Relative pressure changes your elevation (that was Step 1, and we don't mess with this again once set). You only need to do Step 1 once, unless you move the console or reset your console.
Do not adjust Relative pressure directly when calibrating to Metar. You need to affect changes in Relative when calibrating by actually adjusting the Absolute pressure by given amounts that correspond to how much up or down you need to affect Relative.
As an initial adjustment to calibrate with Metar I would recommend lowering Absolute by 2.7 hPa (as this will also automatically lower Relative by 2.7 hPa).
Then let that ride for a day or so and we can take a look if further fine tuning is necessary.
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If you are wondering how I've determined to lower Absolute by 2.7 hPa (which then causes to automatically lower Relative by the same 2.7 hPa)...this is how I've determined that...
Right now the Metar that matches your isobars is KPAE and it is showing a SLP of 1016.2 hPa.
Your station right now is showing 1019.0 hPa.
Your station is exactly 2.8 hPa too high in comparison to KPAE. I'm having you only do 2.7 hPa because I'd rather do a bit less and then do that extra 0.1 hPa later if necessary. I also don't know your exact Relative right now as I only know your Altimeter as that is what is being transmitted to CWOP and the two differ a bit. That is why I say only change it by 2.7 hPa. KPAE is not as close to you as other Metar. But right now you are better in line with KPAE than the other Metars that are closer.
As isobars change there will be better times to then possibly fine tune and calibrate against them. You'll have to wait till the isobars run through you and through them.
As you can see from this map the isobars are not favorable right now as they are passing through KPAE and YOU. The other Metars KBFI, KRNT, and KSEA are not ideal right now for isobar lineup. This map was very difficult to even draw isobars on. There is a lot going on as you right now are in between two low pressure fronts.
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Ideally you would want to be able to draw an isobar like this...
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But notice that it doesn't agree with the numbers on the map. We just can't draw this line. I'm only showing it to you because this is the situation we are waiting for to be more certain you are well calibrated to local Metar. This map also shows you why sometimes the local Metar may be different than your station and at other times it will be the same. Don't go crazy thinking something is wrong. This is totally normal. You will only be the same as local Metar when the isobars run through your location and through the Metars. For more on understanding how to get these maps I suggest referring to this other post that I wrote some time ago which is rather more advanced to take things to the next level of barometer calibration:
https://www.wxforum.net/index.php?topic=36579.0
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I have identified a problem with AmbientCWOP Altimeter formula. I've notified Danoh. This may throw off some of these final calculations as we fine tune the barometer. We will need to revisit. Still do the 2.7 hPa adjustment as that will get us closer.
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I have identified a problem with AmbientCWOP Altimeter formula. I've notified Danoh. This may throw off some of these final calculations as we fine tune the barometer. We will need to revisit. Still do the 2.7 hPa adjustment as that will get us closer.
Not the altimeter formula. It was the conversion from inHg -> mb rounding error to 1 decimal place. It was fixed yesterday, but I didn't restart the worker process for it to take affect. ](*,)
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Danoh,
Thank you. Looking good now.
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OK, I reduced the abs by 2.7. I looked the process up in the manual for Calibration (uses TEMP & MAX MIN buttons).
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Just reviewing the readings for the past couple of hours with all the changes made and it looks very good. Thank you galfert for your excellent tutorial. I had no idea the process to tweak the pressure sensor readings. Like I indicated I had to view the online manual to understand how to make changes to the Absolute pressure settings. Its slightly different than the Relative settings.
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You are welcome! It does look good. We will keep an eye out on it and see how it looks over the coming days. But I think so far it is looking to be spot on.
It is spot on not because of this graph below, since what I've compared your station to the other Metars and isobars that I've extended on the map to draw that conclusion. Though I do like this graph below that clearly shows your adjustment jumping right into the not to always be trusted red analysis line.
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By the way feel free to now switch back to your display showing inHg if you prefer that. Although if we need to make adjustments you'll need to switch back to hPa and then adjust and then switch back to seeing inHg.
I have two displays side by side. A WS-2902A and a WS-2000 and one shows inHg and the other is set to show hPa so that I can see both but I prefer hPa even though I'm in the US...just got used to it and when I look at METAR data I look at the mb (hPa). They do both need to be calibrated individually as they are two separate consoles with their own barometric sensors. In my case neither of them is supplying data to any online service as that is done by my GW1000 and Meteobridge....so that is a third barometer I need to calibrate. All three work on the same premise and the same two steps are required. For anyone dealing with multiple displays and/or a GW1000 know that it is impossible for them to match perfectly all the time. These sensors do their thing and are constantly taking reading and especially as pressure changes it is not uncommon for one to be typically 0.1 hPa off from the other. You need to look at periods of no change to really seem them match each other more perfectly...but even still there are always little blips in sensor readings. That is within the margin of error of the sensors. They do all track pretty consistently to only be off 0.1 hPa or 0.2 hPa.
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I just signed up to share my CWOP data and I'm noticing a related absolute pressure issue to the one discussed in this thread but I'm still a little confused as how to fix it. My station data can be found here: http://www.findu.com/cgi-bin/wxpage.cgi?call=FW7290&last=12
My Relative Pressure is spot on accurate to the local METAR and has been for the 6 months I've owned the station. My issue is that my absolute pressure is extremely different and I'm unsure of how to correct the issue on the station console itself. I followed the calculations and see an offset needed of 8.73hPa (the station is at 73.76m). Currently the station is showing an offset of 808.6hPa (that's not a typo, my currently relative pressure is 1003hPa and absolute pressure is showing as 194.4hPa).
As your previous posts say the difference between REL and ABS stays constant on the station, how can I correct the ABS reading to ensure the REL pressure stays as accurate as it currently is? I see in the Calibration section of the station manual ABS can be adjusted. Should I adjust it to incorporate the offset I calculated? Reset it to factory settings first?
Thank you for helping me figure this out!
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What does your console say your ABS barometer is??
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What does your console say your ABS barometer is??
123.86 inches is the current ABS reading on the console. When I change the units, it reads 194.4hPa.
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As your previous posts say the difference between REL and ABS stays constant on the station, how can I correct the ABS reading to ensure the REL pressure stays as accurate as it currently is?
You really can't fix ABS without messing up REL. You just have to mess up REL to set the proper offset. Then you'll re-calibrate the REL by way of adjusting the ABS.
I see in the Calibration section of the station manual ABS can be adjusted. Should I adjust it to incorporate the offset I calculated? Reset it to factory settings first?
No, you don't adjust the ABS to dial in the offset. You adjust the REL to dial in the offset....which messes up your current REL calibration....you just have to do it. It is the only way.
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Okay, thank you! I went into the station pressure calibration, made the absolute pressure 996 hPa based on the calculation you provide in your first post, then went to the offset menu and made the relative pressure 1004.7 based on the current METAR. It's showing up as a reasonable number on my CWOP page now, so I'll have to give it some time to collect QA data and determine any future adjustments but I think that solved the issue. Fingers crossed and thanks again!
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You can change the starting sea-level pressure to be anything you want, but if you enter in 29.3 meters the difference will always be 3.51 hPa for that elevation.
This does not appear to be the case, at least not on the calculation web page being linked to.
Scenario 1: If I enter my elevation (282 meters) and leave the temperature at 15 degrees Celsius, converting an absolute pressure of 980 hPa yields a sea level pressure of 1013.22.
Scenario 2: If I enter my elevation (282 meters) and leave the temperature at 15 degrees Celsius, converting an absolute pressure of 1020 hPa yields a sea level pressure of 1054.58.
In Scenario 1, the difference between 980 and 1013.22 = 33.22.
In Scenario 2, the difference between 1020 and 1054.58 = 34.58.
The gap appears to vary by 1.36 hPa. If I go more extreme, from 950 hPa to 1050 hPa, the difference in the gaps is 3.39, about 10% of the gap magnitude.
My main question, then, is how can a weather station that uses a constant gap ever be accurately depicting relative pressure? The console should be asking for elevation and calculating relative pressure, as it would then have all the information it needed to accurately fill in the equation.
At this point I am probably just going to back into what my absolute pressure should be by trusting the errors Gladstone QC is summarizing for me. I think I have dialed in my gap between absolute and relative as well as I can for a constant (that shouldn't be a constant). I just used an absolute pressure of 1000 hPa.
Thanks for all of your informative posts!
Thanks,
sutekh137
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You can change the starting sea-level pressure to be anything you want, but if you enter in 29.3 meters the difference will always be 3.51 hPa for that elevation.
This does not appear to be the case, at least not on the calculation web page being linked to.
Scenario 1: If I enter my elevation (282 meters) and leave the temperature at 15 degrees Celsius, converting an absolute pressure of 980 hPa yields a sea level pressure of 1013.22.
Scenario 2: If I enter my elevation (282 meters) and leave the temperature at 15 degrees Celsius, converting an absolute pressure of 1020 hPa yields a sea level pressure of 1054.58.
Yes there is some variation at the extremes. This is why I recommend leaving the temperature at 15°C and the Sea Level at 1013.25 hPa to get your Relative Offset based on your elevation. This allows the calculator to give us a good enough value that will be mostly correct as that temperature and that sea level equivalence is a good average. By using these middle numbers then even when you experience extreme weather based on temperature or pressure, you won't be too far off.
My main question, then, is how can a weather station that uses a constant gap ever be accurately depicting relative pressure? The console should be asking for elevation and calculating relative pressure, as it would then have all the information it needed to accurately fill in the equation.
I totally agree. I think Fine Offset should change their WS View app to ask for Elevation and then do all this math for us. I've spoken to the engineers about it and they were a bit receptive to the idea...but made no promises.
At this point I am probably just going to back into what my absolute pressure should be by trusting the errors Gladstone QC is summarizing for me. I think I have dialed in my gap between absolute and relative as well as I can for a constant (that shouldn't be a constant). I just used an absolute pressure of 1000 hPa.
Thanks for all of your informative posts!
Thanks,
sutekh137
Well I recommend you use 1013.25 as your sea level constant to find out your relative offset. I also would never trust Gladstone QC as that analysis depends too heavily on neighbor stations. I prefer to only use the METAR stations as the only trusted sources.
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Yes there is some variation at the extremes. This is why I recommend leaving the temperature at 15°C and the Sea Level at 1013.25 hPa to get your Relative Offset based on your elevation. This allows the calculator to give us a good enough value that will be mostly correct as that temperature and that sea level equivalence is a good average. By using these middle numbers then even when you experience extreme weather based on temperature or pressure, you won't be too far off.
...
Well I recommend you use 1013.25 as your sea level constant to find out your relative offset. I also would never trust Gladstone QC as that analysis depends too heavily on neighbor stations. I prefer to only use the METAR stations as the only trusted sources.
Galfert,
Thanks again for all the time and knowledge you share on this board! Amazing stuff! =D> \:D/ =D>
I have bolded a point of confusion for me, though -- you say to use 1013.25 as my sea level constant... Do you mean put 1013.25 in as the absolute pressure, 15C for temp, my elevation, and then see what the sea level pressure is, thereby designating my offset?
Or are you saying I should tweak the absolute pressure at my elevation until I see 1013.25 as the sea level pressure? Slightly different results either way. I want to get this as close to perfect as possible and then will start calibrating to METAR via absolute pressure adjustment.
Thanks!
sutekh137
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I have bolded a point of confusion for me, though -- you say to use 1013.25 as my sea level constant... Do you mean put 1013.25 in as the absolute pressure, 15C for temp, my elevation, and then see what the sea level pressure is, thereby designating my offset?
^^^ This one.
Basically you use the online barometric calculator only to find out what your offset should be. It doesn't matter what your current pressure is nor what is going on at your local METAR for this first step. You don't have to put in 1013.25 as that is already there as a default. Same with the temperature, it is already set to 15°C.
There are 2 steps to calibrating a Fine Offset station...
- Find out your Relative Offset using the online barometric calculator. Leave 1013.25 and 15°C and only enter in your elevation (in meters). Subtract result from 1013.25 and that is your offset.
- Adjust your Absolute pressure so that it causes the Relative pressure to go up or down by the same amount. You are calibrating Relative to match the METAR, but you aren't touching the Relative directly...this step is done by only adjusting the Absolute pressure.
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Basically you use the online barometric calculator only to find out what your offset should be. It doesn't matter what your current pressure is nor what is going on at your local METAR for this first step. You don't have to put in 1013.25 as that is already there as a default. Same with the temperature, it is already set to 15°C.
There are 2 steps to calibrating a Fine Offset station...
- Find out your Relative Offset using the online barometric calculator. Leave 1013.25 and 15°C and only enter in your elevation. Subtract result from 1013.25 and that is your offset.
- Adjust your Absolute pressure so that it causes the Relative pressure to go up or down by the same amount. You are calibrating Relative to match the METAR, but you aren't touching the Relative directly...this step is done by only adjusting the Absolute pressure.
Galfert,
I understand the steps involved, just wasn't sure about the 1013.25 because my page no longer uses 1013.25 as the default (that's why I didn't know where you were coming up with it). Apparently the page remembers previously-entered values, as even when I refreshed the page in a new browser window, the absolute pressure was the same as I had last entered (cache or cookies, I guess). I will use the offset from putting 1013.25 in there. I see that converts to 29.92 inHg, which from my reading is considered the average pressure at sea level.
So I've got my offset now, and know what to compare to in order to set absolute until relative matches. I actually think once I reduce absolute some more (because that is the direction I need to go) I will end up rather close to the Gladstone graph. Thanks!
sutekh137
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Hello, I was hoping one of you could shed some light on altitude.
My house is at the elevation of 856.16 feet. My WS-2902B is located on top of a 12ft pole outside, which would put it at the height of 868.16 feet.
I'm a bit confused reading the first posts on how to calibrate, as the elevation that I put in for CWOP is the height of the actual weather station, but from what I understand in the first posts, the barometer is located in the console, which is located inside of my house. The console is sitting on a desk maybe 3 feet off the ground.
When I put in the elevation for the calculations, which elevation matters? The consoles or the station? Should my CWOP elevation be updated to be the height of the console? Is the height different small enough that this is a non issue regardless?
I eagerly await any response. Thanks!
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When I put in the elevation for the calculations, which elevation matters?
Barometric elevation (console). 856 + 3 = 859
Should my CWOP elevation be updated to be the height of the console?
Yes
Is the height different small enough that this is a non issue regardless?
In your case it is enough to make a 0.33 hPa difference or about 0.01 inHg difference. If someone has their outdoor station at 20 ft above ground it could be an error as much as a double that. I'd say in either case it is significant enough if you want to have things calibrated as precise as possible. In the grand scheme of things it wouldn't be out of the normal to be 0.3 hPa (0.01 inHg) off at any given moment from a local METAR if you did things as precise as possible. You don't want to be doubling that already acceptable margin of error, especially when it is such a simple thing to avoid by using the correct elevation. This is also why it is important to take into consideration if you move your station console between floors as it makes a significant difference. For people that may have more than one display console (on different floors) then this should be a factor to also understand.
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Should my CWOP elevation be updated to be the height of the console?
Yes
More generally speaking, it's wherever the pressure sensor is in the house, correct? So if it's 5 foot high on a wall, it's land elevation plus 5 right?
I did my barometric calculation correct and gladstone says it's good. But my general CWOP elevation may be off about 5 feet or so because I didn't account for that.
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Should my CWOP elevation be updated to be the height of the console?
Yes
More generally speaking, it's wherever the pressure sensor is in the house, correct? So if it's 5 foot high on a wall, it's land elevation plus 5 right?
Correct
I did my barometric calculation correct and gladstone says it's good. But my general CWOP elevation may be off about 5 feet or so because I didn't account for that.
Gladstone says it is correct because it is within a plausible range given ground elevation. So, send an email to CWOP support and update your elevation, then make elevation calibration adjustment on your console.
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Thank you for the responses, I have some more questions.
I've determined the following altitudes.
House: 856.16 Feet (260.95 Meters)
Console: 859.16 Feet (261.86 Meters)
WS-2902B: 868.16 Feet (264.73 Meters)
My current CWOP elevation is at 862 Feet (262.73 Meters). If it's any further help, my station's callsign is FW7441.
Since starting this endeavor, and reading your replies, it seems my CWOP elevation is off by 2.84 Feet (0.86 Meters), so once I have everything straightened and I fully understand how to proceed I will notify NOAA.
In trying to follow along with the guide, you mention using the Keisan calculator without changing any parameters other than altitude (859.16 Feet), which I've done, and got an output of (982.36 hpa). Following the equation 1013.25 - 982.36, from what I understand that gives me an offset of 30.89 hpa.
I'm a bit confused on where to go with this now. My current absolute pressure as indicated on the console as I type this is 985.6. Should I be adjusting my Absolute Pressure using the offsets to match the 982.36 hpa from the calculator at my elevation, or should I be adjusting it by the offset of 30.89 hpa following the (1013.25 - 982.36) equation. I may have completely missed something here.
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I'm a bit confused on where to go with this now. My current absolute pressure as indicated on the console as I type this is 985.6. Should I be adjusting my Absolute Pressure using the offsets to match the 982.36 hpa from the calculator at my elevation, or should I be adjusting it by the offset of 30.89 hpa following the (1013.25 - 982.36) equation. I may have completely missed something here.
Neither of those. What you do with the 30.89 is make the Relative be that much more than whatever Absolute is at the moment. But you need to use 30.9 hPa as that is the precision you have in the console. This completes Step 1. That is going to initially mess up your Relative pressure calibration that you send out. Step 2 is to then adjust Absolute so that it imparts a change in the Relative that you send out. By however much you adjust Absolute will cause your Relative to change by exactly the same amount. What you'll notice is that no matter by how much you change Absolute, the difference between Absolute and Relative will always be 30.9 hPa. Because 30.9 hPa corresponds to your elevation.
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Alright, thank you so much.
I determined that my console's current offset was 30.8, so I increased my relative offset by .1 to match the 30.9. This should match up with your procedures for Step 1.
I then looked at my local METAR which is 11 miles away, and found that I had to offset my system by 1.6 to match the METAR, so I decreased my absolute by 1.6. This should match up with Step 2.
My numbers already look closer to my METAR station. Hopefully I followed along correctly, but please let me know if I did something wrong.
Thanks again so much for taking your time to help me!
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Perfect [tup]
Now just keep an eye on it. Remember it will not always be exactly like the METAR because it matters if isobars line up or not with you and the METAR. That would be expected. You want to fine tune calibrate when the isobars line up. You can see my Barometer Calibration with iosbars post for more details on that.
https://www.wxforum.net/index.php?topic=36579.0
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I wanted to say thank you as well, as a new owner of a WS-2902, I read, and re-read, then read once again.... I think it finally penetrated this old skull of mine. In my case, I needed an adjustment of 2.2 hpa to get down to the local METAR, 5 miles distant. My main hurdle was that I saw no adjustment for ABS Pressure on the 2902 until I read this thread, which caused me to go back and re-read the manual. Hopefully this will cause my MADIS rating for barometer to go thumbs up. Anyway, just wanted to say thanks for sharing the info!
Tom
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Excellent [tup]
You bring up a good point that on the WS-2902 the it isn't obvious because it requires a different key press to get to that part of the settings.
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Galfert, as I am new to this and reading everything I can about it, do you suppose this will do the trick to get my station back active? Not really sure how the QC checking works...
CWOP Information for KN6KS (AV650) in Quartz Hill, CA, US
Thomas Hart
11:09 AM (10 minutes ago)
to cwop-support
As indicated on"Gladstonefamily.net":
Data is being seen at findu.com, but this site is not currently marked as active. To fix this, send email to CWOP-Support and ask her to activate your site.
After extensive research, I have settled upon 2470 ft AGL / 753m. This is what I have set as my barometer altitude on the configuration page on AmbientCWOP.com. My location is on an alluvial fan that has been terraced for construction of homes, so determining the actual (relatively) precise altitude has been a challenge. My Absolute barometer reading is consistent with this altitude and my offset for Relative Pressure has been set for this altitude and matches the local METAR (KWJF) 5 miles distant.
I'm not sure how the QC checking works, but if possible can we reset the errors on the Barometer so that my station becomes active again?
Additionally, I have read that there may a problem with QC on the Wind Vector part. Is this the case? I have checked, rechecked and checked once more, my sensors are aligned exactly with Magnetic north. Again, not knowing how the QC checking works, I suspect that last Sunday 6/14, when I corrected the misalignment of the sensors (Adjusted counter-clockwise a few degrees) this may have impacted the QC? Anyway, it is correctly aligned now.
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Galfert, as I am new to this and reading everything I can about it, do you suppose this will do the trick to get my station back active? Not really sure how the QC checking works...
Send an email to cwop-support@noaa.gov and confirm your elevation and let them know you'd like to complete the registration process. Make sure you send elevation in meters not feet. If you have already done this and you are seeing messages about your station being inactive on Gladestonefamily.net then it isn't your fault. And I don't think that there is anything you can do on your side. This is occurring to just about everyone at this time. It seems that Gladstonefamily.net is the one with the issue, not your station.
I'm not sure how the QC checking works, but if possible can we reset the errors on the Barometer so that my station becomes active again?
No, you can't reset QC, it is an ongoing thing. After some time the default previous month will have good QC if the data is good for that last month. You can pull up reports for differing time lengths. Eventually if all is good it clears up just because the old data stops becoming relevant. Being active or not has nothing to do with QC.
Additionally, I have read that there may a problem with QC on the Wind Vector part. Is this the case?
Yes, on Gladstonefamily.net QC the wind is messed up. It doesn't affect other sensors. Just ignore the wind stuff.
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Thank you sir. I guess I should have been more clear, what I was posting was the email I was sending to the "cwop-support@" email address. The reason I posted it here was to get your take on it. Yes, I've been active for over a month now and recently (this week) became inactive, presumably due to QC errors on my barometer and on my Wind.
As per my previous comment, I calibrated my barometer per your excellent instructions here. This email to them was to ask to be re-activated.
Thanks for clearing that up, guess I'm not the only one...
Tom
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Thank you sir. I guess I should have been more clear, what I was posting was the email I was sending to the "cwop-support@" email address. The reason I posted it here was to get your take on it. Yes, I've been active for over a month now and recently (this week) became inactive, presumably due to QC errors on my barometer and on my Wind.
As per my previous comment, I calibrated my barometer per your excellent instructions here. This email to them was to ask to be re-activated.
Thanks for clearing that up, guess I'm not the only one...
Tom
A lot of us are getting the same error message you received about requesting to be activated. There is another post about it. It appears to be an "erroneous" error message.
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Oh, thanks! Being new, I thought it was something I'd done, particularly since I remounted my sensors last weekend to get them higher above the roofline and had adjusted the north alignment. I do see that I show a normal color on the MADIS map, although when I clicked it from the gladstone site, it showed as black.
Anyway, thanks again to you both for the help!
Tom
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So after a power outage with the recent Derecho event, I had to redo this process of calibrating the WS-2902.
After scanning through this thread again re-learning how to perform the calibration steps all over again, I wrote a helper tool for future me, and anyone else who wants to use it.
The tool is hosted on my GitHub page at https://caboose700.github.io/ws-barometer-offset-calculator/ (https://caboose700.github.io/ws-barometer-offset-calculator/). Feel free to let me know if I've messed anything up, or explained anything incorrectly, and I'll update it accordingly.
Thank you again to galfert for explaining the methodology and steps. His insight was a huge help in making this tool, and wouldn't have been made without it.
Edit: Updated link to point to refined tool, which generalizes the instructions as the prior version was specifically for the WS-2902 and the calculations are useful for other stations. Thanks again to galfert for pointers and insight on how other weather stations need to be adjusted.
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Thanks!
That tool looks like an interesting idea. But it isn't ready and some things need to be fixed. We should take this offline and work on it. I'll be in touch with you via PM.
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Chase,
Nice work on getting the finishing touches to your online calculator. I've performed some tests and I'm getting good results now.
https://caboose700.github.io/ws-barometer-offset-calculator/
For anyone that wants to understand how this works....
Step 1: The purpose of this step is to provide your elevation to your console. Since it doesn't understand Feet nor Meters, the elevation has to be given as the Relative Offset that corresponds to your elevation.
Step 2: The purpose of this step is to adjust your station's factory Absolute value so that given your elevation causes Relative pressure to match your local METAR.
Thanks,
George