### Author Topic: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure  (Read 1336 times)

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#### galfert

• Forecaster
• Posts: 1773
##### WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure
« on: October 13, 2018, 11:23:50 AM »
Here is how I calibrate my Ambient console pressure.

It is best to do this on a clear day when the pressure is close to 1013 hpa or 29.92 inHg and you want to do this between 11am and 1pm when the pressure is most level (not rising nor falling) for the day. But this is something you don't have to wait for. You can do initial calibration with whatever weather you have now or whatever time it is now and then fine tune another day.

First thing to do is switch your console to display hpa instead of inHg. It will be more precise during the calibration steps and after calibration if you still prefer inHg then you can just switch units back to inhg, and everything will stay calibrated. So stick to hpa for calibration, you'll see why later.

The first thing we need to calculate is the offset based on your elevation. This is referred to as the relative pressure offset. Basically you are telling the console your elevation indirectly by entering in this offset. Some other brand manufactures ask you for the elevation in feet or meters which is simpler than what we have to do. So with Ambient you have more work to do, and you have to do the offset calculation manually in order to provide it to the console.

I use this website to figure out the offset:
https://keisan.casio.com/exec/system/1224575267

Lets use the example that your elevation is 57 feet. Enter in your elevation into that website calculator in meters. 57 feet = 17.4 meters. That website requires all units to be entered in metric units. Enter in your stations absolute pressure to that website, even though it isn't calibrated yet, it doesn't really matter. We just need a number. Actually any pressure you enter will work because at this point we are only interested in finding out the offset. If our pressure is 29.78 then convert 29.78 inHg = 1008.5 hpa, because again that web calculator needs metric.
I use this website to convert between inHg and hpa.
https://www.convertunits.com/from/hpa/to/inhg

Lastly in the Keisan SLP website just leave the temperature at 15 C because we aren't really interested in finding out true Sea Level Pressure right now ...we just want the offset that will be carried across to all temperatures in your console and 15 C (59F) is standard ISA temperature. Now click the Execute button for the results. The results will show you 1010.58 hpa for SLP. That number doesn't mean anything right now. What matters is the difference between your absolute station pressure 1008.5 hpa and the calculated SLP result 1010.6 hpa (I rounded to tenths), and the answer is +2.1 hpa difference. So +2.1 hpa is the offset (difference) between your consoles absolute and relative pressure. In other words +2.1 hpa represents the elevation difference between 57 feet and see level (0 ft).

In your console enter in the +2.1 offset for relative pressure by moving up or down the relative pressure to be this much more than the current absolute pressure. We aren't calibrated yet and so it doesn't matter what absolute nor what relative ends up showing at this point. For now we are only interested in having it so that the difference between absolute and relative is +2.1 hpa. This difference amount between absolute and relative will never change. Unless you move your console to the second floor of your home and then you will need to correct for 57 + whatever more the elevation is at the second floor. Make sure that your 57 feet that you are using for elevation is not ground level (as in the floor) but rather where your console is now on a table or shelf. So that 57 feet should correspond to the elevation of the console. So if it is 3 feet off the ground on a counter and your ground level is 57 feet, then your real elevation for the console is 60 feet (remember to convert to metric for entering in to the web calculator). So now go back and redo the calculation with the proper elevation of your pressure sensor. Some people mistake the elevation to that of the outdoor sensor. That isn't where the pressure sensor is located. The pressure sensor is in the console of the WS-2902A, or in the separate indoor module in the case of the WS-2000 and in the case of the ObseverIP.

That is it! Well almost. Now you need to watch it and fine tune on a better day if the weather or time of day was not perfect. Remember the fine tuning is done by moving up or down the absolute station pressure as that moves the relative up or down too by the same amount. That is why it is an offset and it matches or corresponds to your elevation. Never change the offset because your elevation doesn't change unless you move the console like I mentioned upstairs or to a taller or lower shelf or table.

Lastly you can switch back to inHg after all these steps. But you'll want to keep it at hpa for a while to monitor and fine tune. The reason for doing it all with hpa rather than inHg is because 0.01 inHg = 0.34 hpa. So for every .01 inHg that you move up or down then you are moving up or down by 0.34 hpa. So you can be more precise if you can move up or down by 0.1 hpa which equals 0.00295 inHg and that is a level of precision that you can't adjust with our equipment using inHg directly. There is nothing inherently wrong with inHg if Ambient had given us an extra decimal place and then inHg reported at .001 inHg decimal places would be more precise than 0.1 hpa (as it represents 0.034 hpa). But calibrating at 0.001 inHg would be pointless because there is nothing reporting inHg to that level of precision (common sources) to compare to and calibrate with. Realize that internally the conversion is happening to a much greater degree of precision and then things get rounded before you see them on the display. If you tried to calibrate using inHg you would find that you would be constantly needing to fine tune up or down and chasing an adjustment that would just seem impossible. Do it with using hpa and you'll remove that frustration. But knowing this you'll realize that if you kept it showing hpa instead of inHg that you are seeing that much more precise of a pressure. Also on the WS-2902A display the barometer trend +- numbers next to the current pressure would be more meaningful as those + and - numbers are referring to hpa. I'm referring to this little scale:

[ You are not allowed to view attachments ]

If all that is too complicated there is another way of doing it. You make the offset zero or basically absolute is made to match and be the same as relative. It is like telling the console that you are at sea level. Then you calibrate the absolute to match the airport's Altimeter or Sea Level Pressure (whichever you are targeting) and then you don't have to do any of the work of figuring out the offset. You've removed the offset by keeping relative equal to absolute. But then you'll never know what your real absolute station pressure is as you will only really be able to see your adjusted pressure, but some would argue that it is not that meaningful to need real station pressure. I still prefer to do it right. I don't know if there are any repercussions from taking this approach in the different reporting services if any of them need station pressure. But even if that isn't the case now it may change. But this is still an interesting option to consider even if you don't do it because I think it helps the understanding of the concepts of what we are doing.

UPDATE: I just realized that if you want to calibrate to Altimeter instead of Sea Level Pressure then the offset calculated above may not be right and be a bit off. As we used a formula to calculate the offset for Sea Level not an offset for Altimeter. There is an easy fix though. Just look at the airport's METAR and see how much the Altimeter differs from Sea Level pressure and factor in that correction to your final offset. Basically you still use your given elevation but then change the resulting offset by however much more or less the METAR Altimeter is than SLP.
« Last Edit: November 08, 2018, 07:14:50 PM by galfert »
WS-2000 & WS-2902A | ObserverIP | WeatherBridge (Meteobridge) | Ecowitt GW1000
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Windy: pws-f075acbe
Tele-Pole flag pole is here (not installed yet)

#### cjtamu

• Member
• Posts: 28
##### Re: WS-2902A and WS-2000: How to calibrate for Sea Level Pressure or Altimeter
« Reply #1 on: October 13, 2018, 02:43:19 PM »
Thank you, sir.

#### galfert

• Forecaster
• Posts: 1773
##### WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #2 on: October 13, 2018, 03:32:43 PM »
For those of you with an ObserverIP you are basically doing the same thing in the calibration screen.

If you intend to report to CWOP with a Meteobridge , the Meteobridge will calculate Altimeter from your absolute station pressure on your ObserverIP. I haven't done it yet but it probably is the same thing for any other software (WeeWx or Weather Display ..etc). Which is why I say calibrate to Sea Level Pressure and then those other software programs that send to CWOP will do the Altimeter calculation for the purpose of sending to CWOP. Unless of course you prefer to always just see Altimeter and never care about Sea Level Pressure.
« Last Edit: October 13, 2018, 04:22:57 PM by galfert »
WS-2000 & WS-2902A | ObserverIP | WeatherBridge (Meteobridge) | Ecowitt GW1000
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Windy: pws-f075acbe
Tele-Pole flag pole is here (not installed yet)

#### MrM1

• North Central Florida
• Contributor
• Posts: 140
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #3 on: November 06, 2018, 11:32:22 AM »
I do not have a local airport.  How close does the nearest airport need to be to be a valid comparison?    My nearest METAR station is either 17.3 miles (K42J) or 24.1 (KGNV).
« Last Edit: November 06, 2018, 12:02:20 PM by MrM1 »

#### galfert

• Forecaster
• Posts: 1773
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #4 on: November 06, 2018, 12:06:17 PM »
I do not have a local airport.  How close does the nearest airport need to be to be a valid comparison?    My nearest METAR station is either 17.3 miles (K42J) or 24.1 (KGNV).

Well the closer the better obviously. But there is another way to deal with further away METARs. You need to wait till those METARs around you all match up. If you are in between them then guess what?...Then your pressure is the same as theirs since you are right smack in the middle of them.*  When they don't match up it means that there is change happening and it isn't uniform yet. So wait for the uniformity.

Besides K42J and KGNV you can also consider KOCF and KFIN and KSGJ. Right now four of those are matching all showing 1017.6 hpa and you are in the middle. Seems you are around 2.4 hpa too low (or about 0.07 inHg too low).

You'll just have to be a bit more patient and do some extra fine tuning as days go by if you don't have a close enough METAR.

*There is a possibility that if surrounding METAR are all equal and you are in the middle that your pressure is actually higher or lower. This would happen if the METAR stations happen to line up on the same pressure ring and you are in the center of a higher or lower center of a pressure zone. For this reason I also recommend looking a website like Windy.com and set the map to show pressure lines. Then make sure you are not in the center of a pressure zone. You could use the pressure line to help guide you as to which METAR should be in the same zone as you.

« Last Edit: November 06, 2018, 03:23:39 PM by galfert »
WS-2000 & WS-2902A | ObserverIP | WeatherBridge (Meteobridge) | Ecowitt GW1000
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Windy: pws-f075acbe
Tele-Pole flag pole is here (not installed yet)

#### MrM1

• North Central Florida
• Contributor
• Posts: 140
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #5 on: November 06, 2018, 12:28:00 PM »

#### galfert

• Forecaster
• Posts: 1773
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #6 on: November 06, 2018, 12:30:52 PM »
My favorite way to see multiple METAR on the same map is to use the Mesowest map. This method of getting METAR data is beneficial also because you can see more live METAR data (like every 5 or 10 minutes) as opposed to the limited hourly published one on aviationweather.gov.

https://mesowest.utah.edu/

• Click on the map on your state/region
• Make sure that the Region Zone says your state name or specific reporting region
• Zoom in to your area
• Map Weather Network should say NWS or NWS RAWS if you want to see more stations run by the US Forest Service and the Bureau of Land Management
• Switch Units to Metric (explained in post above why to calibrate with Metric first and then switch back to Imperial/English units if desired)
• Change the Overlay 1 and Overlay 2 to show SLP and ALT respectively (don't use the one that just says "pressure")
• Click the Red Refresh Map button (Do not refresh the browser or you will reset everything)...just keep clicking the Red Refresh button as often as you want to get newest data
« Last Edit: November 06, 2018, 12:35:04 PM by galfert »
WS-2000 & WS-2902A | ObserverIP | WeatherBridge (Meteobridge) | Ecowitt GW1000
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Windy: pws-f075acbe
Tele-Pole flag pole is here (not installed yet)

#### MrM1

• North Central Florida
• Contributor
• Posts: 140
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #7 on: November 06, 2018, 12:33:42 PM »
Here are my options locally .   K42J does not report SLP

#### galfert

• Forecaster
• Posts: 1773
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #8 on: November 06, 2018, 12:41:07 PM »
K42J may not report SLP but using Mesowest they back calculate it based on their altitude and their Altimeter. Another benefit of using Mesowest.

FYI - The way to read this map pressure is to add a 10 in front of the number (or add a 9 if it begins with a 9) and add a decimal before the last digit. So in this case 165 becomes 1016.5 hpa for SLP and 169 becomes 1016.9 hpa for Altimeter. The reason the map is showing these two numbers is because I selected Overlay 1 = Sea Level Pressure and Overlay 2 = Altimeter (and Units were set to Metric).

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« Last Edit: November 06, 2018, 12:53:52 PM by galfert »
WS-2000 & WS-2902A | ObserverIP | WeatherBridge (Meteobridge) | Ecowitt GW1000
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Windy: pws-f075acbe
Tele-Pole flag pole is here (not installed yet)

#### CW2274

• Forecaster
• Posts: 4282
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #9 on: November 06, 2018, 02:30:56 PM »
Or, simply drive to the airport with your console and set it there.

#### MrM1

• North Central Florida
• Contributor
• Posts: 140
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #10 on: November 08, 2018, 08:52:15 AM »
So Im a little puzzled.   Using the Mesowest site,  if I average the Altimeter (I assume that would be the relative pressure for that station and not the SLP) for 9 stations that surround me from Jax to Ocala to Daytona Beach,  I get

30.04 as an average

Then if I go and average 6 PWS stations around me within 5 miles (including 3 Vp2s, 1 Acurite Pro, 1 WS-1200 and 1 WS-1001 - none are my stations) I get

29.96 as an average

seems that is a pretty decent difference?   Is that the difference in distance?  Or are that many PWS's just not accurate?

And if I use the calibration method for the off set for my altitude,  i get an even higher number once I average the MATARs stations SLPs and apply the offset to my Relative pressure.

#### MrM1

• North Central Florida
• Contributor
• Posts: 140
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #11 on: November 08, 2018, 11:17:25 AM »
I'm Also confused (forgive my ignorance) by the SLP and Altimeter readings at local airports using the MesoWest site.  It looks like I am seeing SLP as lower than the staiton altimeter reading?    Shouldn't it be the other way around.? I thought pressure got less the higher in altitude you rose up.

Yet I am seeing for instance at Gainesville Regional Airport a SLP of 1018.0 and the Altimeter as 1018.3.  I would have thought it should be the other way around.  What am I missing?

Or perhaps I am seeing this all wrong.   Is the SLP at any given Airport the elevation compensated reading?  What exactly is the airport Altimeter and Pressure layer selection on the MesoWest Map?

In the ObserverIP,  which is suppose to be Sea Level Presure,   Absolute or Relative?  As I read the manual,  relative is suppose to be SLP ... and that is number that WU pulls from my station.  BUT I am 34 meters above Sea Level.  Where is the correction from SLP coming in?   Or are the readings on the WU site for every one there actually SLP levels NOT compensated for elevation ?
« Last Edit: November 08, 2018, 01:47:40 PM by MrM1 »

#### galfert

• Forecaster
• Posts: 1773
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #12 on: November 08, 2018, 02:39:25 PM »
Many or most PWS are not calibrated properly unfortunately. Don't go by your neighbor's PWS. That is why I recommend only looking at METAR data.

Quote
And if I use the calibration method for the off set for my altitude,  i get an even higher number once I average the MATARs stations SLPs and apply the offset to my Relative pressure.

The offset is for your SLP as you aren't, "applying the offset to my Relative pressure." Once you calculate what your offset should be yes the SLP on your station could go up or down depending on whether you are increasing or decreasing whatever was the previous offset. Don't expect perfect SLP after entering in the SLP offset. Because you must now do the final calibration by bumping up or down the Absolute (station pressure) to cause the SLP to move up or down to then be calibrated. You don't calibrate Absolute (station pressure) to match anything, rather you are changing it to cause the SLP to go up or down. The offset you entered (difference between Absolute and SLP) doesn't change, which is is why it is an offset, meaning SLP is always that much different than Absolute constantly. The offset signifies your real elevation.

I think the confusion is what is Altimeter versus what is SLP. Basically Altimeter is an adjusted pressure based on ground elevation. It uses an industry standard (ISO) for temperature constant of 59 deg F always in its computation no matter what the real temperature is. Altimeter is used by airports and pilots for flight reasons. It is also used by a lot of weather TV and radio stations reporting weather well mostly because that is the data that is typically available from airports where the weather station are located. Conversely we have SLP (Sea Level Pressure). SLP is another way to report pressure and it also is mostly determined by ground elevation. But SLP differs from Altimeter in that in the SLP calculation actual real temperature is a real factor. Since the formulas are different they yield slightly different results. SLP has slightly different purpose. By comparing SLP between stations at different locations it is possible to compare the weather from these different stations regardless if the elevation is different. Meteorologist prefer to use SLP to compare locations. But Altimeter is popular only because of the TV and radio media. You need to choose if you are going to be working with SLP or Altimeter on your station. Weather Underground uses SLP. But if you were reporting to CWOP then you'd be using Altimeter. With the right hardware and software it is possible to use both SLP for some services like Weather Underground and Altimeter for CWOP.

Quote
It looks like I am seeing SLP as lower than the staiton altimeter reading?    Shouldn't it be the other way around.? I thought pressure got less the higher in altitude you rose up.
Yes SLP will be lower than Altimeter if the temperature average of the previous 12 hours is higher than 59 deg F. The difference in a given station's Altimeter versus SLP has nothing* to do with ground elevation (at lower elevations). If the temperature average is exactly 59 deg F then Altimeter will be equal to SLP. If the temperature average is lower than 59 deg F then Altimeter will be lower than SLP. Those are rules of thumb. It is actually a bit more complicated than the basics I've laid out. So for completeness at very high elevations these rules of thumb don't apply. It is though a 100% factual statement that at sea level these differences I pointed out based on temperature will be true of the relationship between SLP and Altimeter (SLP = ALT @59F). Above 500 ft there already is a small discrepancy (just a few hundreths of a milibar, so not noticeable) even if the temperature average is 59 deg F. So at 3000 feet, SLP and Altimeter are not equal (close but not equal) even if the average temperature is 59 deg F.

*Sorry to make all this sound so complicated. I wish I could just give you the basics, then give you some simple recommendations, and rules of thumbs and leave it at that. But if I do that then someone else is going to call me out on speaking half truths and say that I'm wrong. I therefore really suggest you research and learn about these topics. Learn and understand the formulas and the why we need altimeter in the aviation industry versus SLP. I understand this pretty well, but the more I learn the more I realize that I don't know as much as I think I know and the subject gets more and more complicated. You have to draw the line somewhere and tell yourself that the knowledge and information you have is good enough for your purposes.

Quote
Is the SLP at any given Airport the elevation compensated reading?  What exactly is the airport Altimeter and Pressure layer selection on the MesoWest Map?
Yes SLP is a compensated pressure based on elevation, to zero out the real elevation. To pretend that what if that weather system over you was at that very same location but if you could drill down and get rid of earth and be at sea level. Altimeter is doing the same thing but not taking temperature into account. Forget just "Pressure" on Mesowest as that is the stations Absolute Pressure (station pressure) the real pressure at that location at its real ground elevation and has little purpose (unless you live right next to the airport and you want know uncompensated station pressure).
« Last Edit: November 08, 2018, 02:57:29 PM by galfert »
WS-2000 & WS-2902A | ObserverIP | WeatherBridge (Meteobridge) | Ecowitt GW1000
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Windy: pws-f075acbe
Tele-Pole flag pole is here (not installed yet)

#### CW2274

• Forecaster
• Posts: 4282
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #13 on: November 08, 2018, 02:52:39 PM »
Weather Underground uses SLP.
Not in my case, I send and they display the altimeter, but it took me badgering them to quit using MSLP.

#### MrM1

• North Central Florida
• Contributor
• Posts: 140
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #14 on: November 08, 2018, 03:05:02 PM »
So SLP at a particular Air Port Station is what that station "believes" (based on calculations) that SPL is IF that location where at the sea shore at a given Temp?

And in my ObserverIP,   my Absolute (with my calibration set to 0.00) = My Station's pressure at the station with no correction for sensor error

And the Relative reading in the ObserverIP is the same as my Absolute as long as calibration is set to 0.00 also.   (but is supposed to be the pressure at SLP?  And would be once calibrated correctly to Absolute? )

So Relative in the ObserverIP should be the same as SLP ... so in general,  if I am above sea level (33.83 meters)  Then my Relative should be compensated UP (+) from my Absolute in the IP observer based on the offset calculation in post #1?

And weather underground is pulling from me my Relative Pressure,  So WU is not really reporting the pressure at my station, but rather the pressure at my station IF my station was at sea level ??

#### MrM1

• North Central Florida
• Contributor
• Posts: 140
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #15 on: November 08, 2018, 03:07:06 PM »
Weather Underground uses SLP.
Not in my case, I send and they display the altimeter, but it took me badgering them to quit using MSLP.
I know for certain WU is pulling the data it uses showing the pressure at my station from my Relative Reading ... whatever that is.  I have tested it with really off numbers between Absolute and Relative,  and WU pulls from my Relative reading.

#### MrM1

• North Central Florida
• Contributor
• Posts: 140
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #16 on: November 08, 2018, 03:26:00 PM »
Quote
By comparing SLP between stations at different locations it is possible to compare the weather from these different stations regardless if the elevation is different.
I am guessing this is a key statement and why WU prefers to use the ObserverIPs Relative Pressure reading.  Because if the station is calibrated correctly,  it allows a comparison from station to station regardless of elevation.

#### CW2274

• Forecaster
• Posts: 4282
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #17 on: November 08, 2018, 03:38:55 PM »
Quote
By comparing SLP between stations at different locations it is possible to compare the weather from these different stations regardless if the elevation is different.
I am guessing this is a key statement and why WU prefers to use the ObserverIPs Relative Pressure reading.  Because if the station is calibrated correctly,  it allows a comparison from station to station regardless of elevation.
Same way with the altimeter, just a different measurement that all the media outlets use, which is one reason why many choose this option.

#### galfert

• Forecaster
• Posts: 1773
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #18 on: November 08, 2018, 05:03:26 PM »
So SLP at a particular Air Port Station is what that station "believes" (based on calculations) that SPL is IF that location where at the sea shore at a given Temp?

No. Altimeter is altimeter. SLP is different. They have different purposes. It has to do with setting altimeter equipment on planes. It is because an airport needs to report its altimeter and an airplane need to calibrate to that airports altimeter and since the two of them (airplane and airport) are at different places the temperature could be different so ISO temperature is part of the altimeter equation because it allows you to calibrate to the airport when you are away from the airport and when you land it will match because temperature was never a factor. Don't conflict Altimeter and SLP. One has nothing to do with the other. They both are similar but different enough also that one has nothing to do with the other.

Quote

And in my ObserverIP,   my Absolute (with my calibration set to 0.00) = My Station's pressure at the station with no correction for sensor error

Yes, correct.

Quote

And the Relative reading in the ObserverIP is the same as my Absolute as long as calibration is set to 0.00 also.   (but is supposed to be the pressure at SLP?  And would be once calibrated correctly to Absolute? )

No. Relative would be equal to Absolute if the offset is set 0.00, not the calibration of Absolute. Relative is typically SLP. Unless you want to be one of those people that choose to deal with Altimeter and then you've calibrated for Altimeter rather than for SLP.

Quote

So Relative in the ObserverIP should be the same as SLP ... so in general,  if I am above sea level (33.83 meters)  Then my Relative should be compensated UP (+) from my Absolute in the IP observer based on the offset calculation in post #1?

Yes

Quote

And weather underground is pulling from me my Relative Pressure,  So WU is not really reporting the pressure at my station, but rather the pressure at my station IF my station was at sea level ??
Weather Underground is not pulling anything. You are sending WU your Relative pressure. That Relative pressure can be SLP or Altimeter depending on which you choose to use and calibrate your station to. Only your console shows Absolute (station pressure)..... That is unless the weather service decided to reverse calculate your station pressure based your Relative pressure that you send and your elevation which you have set in the station ID configuration. They have all they need to reverse calculate your station pressure. But typically this is not done.
« Last Edit: November 08, 2018, 05:15:39 PM by galfert »
WS-2000 & WS-2902A | ObserverIP | WeatherBridge (Meteobridge) | Ecowitt GW1000
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Windy: pws-f075acbe
Tele-Pole flag pole is here (not installed yet)

#### galfert

• Forecaster
• Posts: 1773
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #19 on: November 08, 2018, 05:38:25 PM »
CW2274,
You use a Davis Vue console. Your console has been designed to work with Altimeter. I think you are doing others a disservice by telling everyone you use Altimeter and not disclosing that you are using a Davis Vue console.

Starting with station pressure with the right formula calculation you can arrive at Altimeter and separately at SLP. That said the Davis Vantage Pro console is not designed to handle Altimeter. Likewise the Ambient stations are not designed to deal with Altimeter. Of course anyone can choose to calibrate to Altimeter instead of SLP but with temperature changes problems would occur keeping calibration in these stations that are really internally designed to be using SLP. It probably is not that noticeable. But it wasn't what the manufacturer had in mind.

I've succumbed to your jamming Altimeter down people's throat. Even me in how I word my pressure discussions after having debated you one it before. I regret that. I've change my mind yet again. If you want to tell others how you prefer to use Altimeter I think that is fine. But you aren't exactly telling the whole story as you have hardware that others don't normally have. At least not in this sub discussing Ambient. You yourself have admitted that you use the Vue console for this very reason.

So for the record people...I'm talking to the whole world....use SLP not Altimeter. Unless you know what you are doing. Unless you have the right hardware and software. Yes media and the fake news use Altimeter because they are only using the data that is available from most METAR. If you want to compare your station with Altimeter that is reported by TV and radio then do it. But know that your hardware was probably designed to use SLP.

Most 3rd party software like Meteobridge calculate Altimeter from station pressure and report that to services that require Altimeter like CWOP, but the same software reports SLP to other services Weather Underground included. If you want your Weather Underground to show Altimeter instead of SLP by all means you can try to calibrate to Altimeter but unless you have complete control of your software/hardware then it really is doing an SLP calculation. And that SLP calculation will be okay and calibrated as Altimeter till the temperature shifts in a big way because SLP takes temperature average into account. Altimeter does not. So expect issues trying to use Altimeter where the manufacturer/software developer is thinking SLP.
« Last Edit: November 08, 2018, 05:47:39 PM by galfert »
WS-2000 & WS-2902A | ObserverIP | WeatherBridge (Meteobridge) | Ecowitt GW1000
WU: KFLWINTE111  |  PWSweather: KFLWINTE111
CWOP: FW3708  |  AWEKAS: 14814
Windy: pws-f075acbe
Tele-Pole flag pole is here (not installed yet)

#### CW2274

• Forecaster
• Posts: 4282
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #20 on: November 08, 2018, 06:09:43 PM »
CW2274,
You use a Davis Vue console. Your console has been designed to work with Altimeter. I think you are doing others a disservice by telling everyone you use Altimeter and not disclosing that you are using a Davis Vue console.
I've succumbed to your jamming Altimeter down people's throat.
Excuse me? I'm not jamming a damn thing down anyone's throat. I didn't realize that me having to disclose that my Vue console has the ability to calculate the altimeter was part of the discussion. If you'll notice, the title of the thread that YOU STARTED has the word ALTIMETER in it. I have no idea what your equipment can or can't do so my posts are to try to educate, if not you, perhaps others.
BTW, I used the altimeter setting with my VP2 console also, it just not as accurate at higher and lower pressures as the Vue, but certainly usable. You don't have to have a console that specifically has the altimeter setting to use the altimeter. Good enough?

#### nincehelser

• Forecaster
• Posts: 2907
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #21 on: November 08, 2018, 06:18:19 PM »
So for the record people...I'm talking to the whole world....use SLP not Altimeter.

Say what?

In my world, altimeter is predominate.

As far as I'm aware, wunderground prefers you send altimeter.  When did they switch to SLP?
« Last Edit: November 08, 2018, 06:21:37 PM by nincehelser »

#### CW2274

• Forecaster
• Posts: 4282
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #22 on: November 08, 2018, 06:25:08 PM »
So for the record people...I'm talking to the whole world....use SLP not Altimeter.
When did they switch to SLP?
It was about the time the wheels started falling off the WU wagon, say three years ago? It took me quite a while to get them to leave my altimeter alone.

#### nincehelser

• Forecaster
• Posts: 2907
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #23 on: November 08, 2018, 06:33:55 PM »
So for the record people...I'm talking to the whole world....use SLP not Altimeter.
When did they switch to SLP?
It was about the time the wheels started falling off the WU wagon, say three years ago? It took me quite a while to get them to leave my altimeter alone.

I don't know why they tried to adjust certain stations using altimeter.   In my experience, they generally left altimeter stations alone, and were focused on adjusting those sending station pressure.

I'm really confused by this thread.  As far as I'm aware, altimeter has always been far more popular for reporting than SLP.

#### CW2274

• Forecaster
• Posts: 4282
##### Re: WS-2902A WS-2000 ObserverIP How to Calibrate for Sea Level Pressure or Altimeter
« Reply #24 on: November 08, 2018, 06:39:48 PM »
I recall now, it was WU attempt to "normalize" pressure readings by switching to MSLP. Since the altimeter is technically in inches of Hg here in the States, this was their idea for "global" continuity. I get that, just leave my altimeter alone....