Topic: CWOP Gladstonfamily indicates barometer is off on WS2902

[galfert]

Danoh,
Thank you. Looking good now.

[kc59harley]

OK, I reduced the abs by 2.7. I looked the process up in the manual for Calibration (uses TEMP & MAX MIN buttons).

[kc59harley]

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.

[galfert]

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.
 [ You are not allowed to view attachments ]

[galfert]

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.

[adamgirard]

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!

[danoh]

What does your console say your ABS barometer is??

[adamgirard]

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.

[galfert]

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.

[adamgirard]

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!

[sutekh137]

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

[galfert]

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.

[sutekh137]

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!   

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

[galfert]

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.

[sutekh137]

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

[e91071f86d]

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!

[galfert]

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.

[funsutton]

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.

[galfert]

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.

[e91071f86d]

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.

[galfert]

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.

[e91071f86d]

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!

[galfert]

Perfect 
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

[tdhart]

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

[galfert]

Excellent 
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.