Author Topic: CWOP Gladstonfamily indicates barometer is off on WS2902  (Read 4782 times)

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Offline kc59harley

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CWOP Gladstonfamily indicates barometer is off on WS2902
« on: March 29, 2020, 04:48:59 PM »
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?

Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #1 on: March 29, 2020, 05:38:00 PM »
Please post your CWOP ID so that we can take a look and make recommendations.
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Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #2 on: March 29, 2020, 05:44:10 PM »
Sorry about that.

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Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #3 on: March 29, 2020, 09:25:02 PM »
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)
« Last Edit: March 31, 2020, 09:04:34 AM by galfert »
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Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #4 on: March 29, 2020, 09:58:06 PM »
Abs - 29.83
Rel - 30.02

75 feet

Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #5 on: March 29, 2020, 09:59:56 PM »
Where I live it says 82 feet but using my phone its says 75

Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #6 on: March 30, 2020, 12:03:50 AM »
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 15C or 59F 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
 [ You are not allowed to view attachments ]

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
« Last Edit: July 31, 2021, 08:34:47 AM by galfert »
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Offline danoh

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #7 on: March 30, 2020, 06:28:37 AM »
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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Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #8 on: March 30, 2020, 01:07:24 PM »
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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Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #9 on: March 30, 2020, 02:05:53 PM »
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?
« Last Edit: March 30, 2020, 02:47:26 PM by kc59harley »

Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #10 on: March 30, 2020, 02:11:28 PM »
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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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #11 on: March 30, 2020, 03:47:00 PM »
I'm reporting altimeter now.
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Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #12 on: March 30, 2020, 06:22:58 PM »
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

Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #13 on: March 30, 2020, 08:14:06 PM »
I'm reporting altimeter now.

Excellent fast work! Thank you!
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Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #14 on: March 30, 2020, 08:21:21 PM »
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.

Quote
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.
« Last Edit: March 30, 2020, 08:40:17 PM by galfert »
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Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #15 on: April 03, 2020, 10:24:50 AM »
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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Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #16 on: April 03, 2020, 11:45:21 AM »
30.11 Abs
30.21 Rel

Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #17 on: April 03, 2020, 01:07:25 PM »
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.


« Last Edit: April 03, 2020, 02:12:19 PM by galfert »
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Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #18 on: April 03, 2020, 02:03:28 PM »
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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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #19 on: April 03, 2020, 03:38:17 PM »
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 15C (which is 59F) 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. ***
« Last Edit: April 03, 2020, 04:24:39 PM by galfert »
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Offline kc59harley

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #20 on: April 03, 2020, 10:18:05 PM »
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.

Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #21 on: April 03, 2020, 11:06:55 PM »
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.
 [ You are not allowed to view attachments ]

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.


« Last Edit: April 04, 2020, 07:05:03 AM by galfert »
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Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #22 on: April 03, 2020, 11:58:57 PM »
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.

 [ You are not allowed to view attachments ]



Ideally you would want to be able to draw an isobar like this...

 [ You are not allowed to view attachments ]

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
« Last Edit: April 04, 2020, 12:09:08 AM by galfert »
Ecowitt GW1000 | Meteobridge on Raspberry Pi
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Offline galfert

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #23 on: April 04, 2020, 06:41:51 AM »
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.
Ecowitt GW1000 | Meteobridge on Raspberry Pi
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Offline danoh

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Re: CWOP Gladstonfamily indicates barometer is off on WS2902
« Reply #24 on: April 04, 2020, 06:45:30 AM »
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.  ](*,)
Ambient Weather station CWOP connector: AmbientCWOP.com

 

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