cwop altimeter bogus reading

[adam5]

Hi Steve,

Have you ever seen this happen. I had stopped Cumulus for about 10 minutes. Restarted and set Cumulus to send data to CWOP. Cumulus sent an altimeter reading of 00000 (b00000 in the APRS string). I've seen this occur once before. Exiting then restarting the program "fixed it." Actual pressure readings on the Cumulus main screen and in the 10 minute logged data was fine. I've tagged in the actual APRS string from CWOP to show I'm not crazy!

DW3024>APRS,TCPXX*,qAX,CWOP-2:@142331z4459.66N/08439.48W_031/002g005t026r000P011h93b00000eCumulusDsVP

-Brian

[steve]

How soon after starting Cumulus was that packet sent? Calculating altimeter pressure is quitre a complicated process, and it has to do a special read of the VP2 barometer data to get the info that it needs; it only does this once a minute. Either the packet was sent before it had got around to doing the barometer read, or it had tried to do it but got an error back.

But you say you had to restart; did it continue to send the zero for a while, then, until you restarted?

[adam5]

How soon after starting Cumulus was that packet sent?

Real soon...like a minute or two.

Calculating altimeter pressure is quitre a complicated process, and it has to do a special read of the VP2 barometer data to get the info that it needs; it only does this once a minute. Either the packet was sent before it had got around to doing the barometer read, or it had tried to do it but got an error back.

That make sense. I'm using VirtualVP and occasionally have comm issues as a few different weather programs I have running try to communicate with the console at the same time. It seems to be a drawback of using VirtualVP, at least on my setup.

But you say you had to restart; did it continue to send the zero for a while, then, until you restarted?

I think it was just one ob, since I caught it right away and decided to simply restart the program. But if I recall, when it has occurred before, the program reported b00000 for multiple obs. But again, I think the occasional comm issues I have explains the reason, based on your explanation.

-Brian

[steve]

I don't bother logging any problems reading the barometer data, as I assume that it will happen from time to time, and it will get read again in the next minute. It's only really a problem when it happens for the first reading (or subsequent ones), so I think maybe I'll put some checks in to make sure it doesn't send zero for the pressure.

[callum91]

Regarding the CWOP barometer readings - on the website for my station, it shows lower Pressure readings than what I actually had.

Is Cumulus sending the pressure from my station, which is corrected to Mean Sea Level already, and then CWOP making a further correction?

Regards

Callum

http://weather.gladstonefamily.net/site/D3951

http://www.wunderground.com/weatherstat ... IWESTSUS32

[steve]

CWOP doesn't use MSLP, it uses altimeter pressure. I should really write an FAQ for this, as it comes up from time to time. I got the feeling that the last time I gave this explanation, the user thought I was making it up!

Cumulus calculates altimeter pressure using the station pressure (absolute pressure on the Fine Offset) and the altitude that you've set in Cumulus. So it may well come out to a different value to the corrected MSLP displayed by Cumulus.

[callum91]

Ok thanks - it seems a bit stupid then because they compare my QFE (pressure at station elevation) against QNH (mean sea level pressure) at Shoreham Airport which isn't really fair. whereas if they compared my Sea Level Pressure against the one at Shoreham Airport it wouldn't be as bad.

[Repairman77]

Cumulus calculates altimeter pressure using the station pressure (absolute pressure on the Fine Offset) and the altitude that you've set in Cumulus.

That's strange; I did some test on the Pressure settings and calibrations and found mine displays the same as the console when I adjust that for Relative pressure.

Mike.

[steve]

Cumulus calculates altimeter pressure using the station pressure (absolute pressure on the Fine Offset) and the altitude that you've set in Cumulus.

That's strange; I did some test on the Pressure settings and calibrations and found mine displays the same as the console when I adjust that for Relative pressure.

It could be that because of your low altitude that the MSLP and Altimeter Pressure come out to the same value. Mine is currently 1017.0 on CWOP (i.e. altimeter) and 1017.3 on the console and in Cumulus (i.e. MSLP). What's your CWOP ID so I can look at your data?

[Repairman77]

Hi Steve; Altimeter and Relative (set on console) is 1006.1 and Absolute is 1005.1.
Sensor is 10M up on the tower and my altitude is set to 4M ASL.

I don't have it on a website but here's the latest data attached.

[steve]

Altimeter and Relative (set on console) is 1006.1

Altimeter isn't the same as Relative (MSLP). CWOP (somewhat perversely, in my opinion) wants Altimeter rather than MSLP, which is why Cumulus has to do a separate calculation to determine Altimeter. The only place in Cumulus where you can see Altimeter is in CWOP uploads, which is why I asked for your CWOP ID. I thought you were saying it was strange that your Altimeter and Relative pressures were the same value.

[Repairman77]

Steve; I was a bit confused with how C1 used the data from the Console, so I spent some time playing with all the combinations (including calibration) and the only way I could get Relative on the console to read the same as the altimeter was to set Relative to the Altimeter reading on the console. This read correctly on C1 then. So I assumed that C1 took the Relative data not Absolute.

I am even more confused (not a rare occurrence) as if it takes Abs data then I assume it would not be able to show the correct value on C1 display. If I alter the value of Rel on the console then this same value shows on the C1 display; or is that just coincidence?

Not to worry to much as it does show correctly and do what I want it to do; it just goes out by around 1mb from 1007 (correct) to 1020 (reads 1mb high) with respect to the Altimeter reading. Perhaps that's because I am using Relative to adjust it by, but you can't adjust ABs on the console, just add or subtract on C1 calibration.

Mike.

[steve]

For 'normal' use, it does use the Relative pressure from the console. It's only for CWOP that it uses the Absolute pressure and then corrects it based on the altitude you've entered into Cumulus before sending it to CWOP. To be honest, for Fine Offset stations, it makes little difference, but the VP2 calculates MSLP correctly, taking temperature and humidity into account, and some VP2 users had spotted the discrepancy, so I had to add code to correctly calculate Altimeter Pressure to satisfy CWOP.

[Repairman77]

Steve; makes sense now; so what I am doing for just displaying it on the screen with C1 is correct.
I have no intention of setting it up for CWOP at the present time.
Thanks for the explanation.
Mike.

[meadowlark]

Hello Steve -

Is this same algorithm for determining altimiter adjustment to CWOP the same for the Oregon Scientific? Reason I am asking is that I have been watching my CWOP readings for two days and something just doesn't appear correct. I am just about three miles from the NOAA weather station here and the difference between our alltitudes is about 23 feet, yet the NOAA station is showing almost 8mb's higher than my station. The other stations listed in close proximity are all higher than my station and are receiving good QC reports. I've checked my configuration for the Oregon Scientific and believe everything is in order.

Don