APRS

[ea1aha]

There is a problem with the file that it sends for APRS. In particular the pressure measurement, that sends the absolute pressure instead of the relative one with the Offset. This causes that the pressure data that it sends are erroneous. It is possible to be solved?
My name is Juan Carlos (EA1AHA)
Thanks

[steve]

APRS requires 'altimeter pressure' to be sent, so this is what Cumulus sends. It calculates it from the absolute pressure and your altitude. There may be a bug in the way Cumulus calculates altimeter pressure for Fine Offset stations, but no-one else has reported it so far. What value have you set your altitude to in Cumulus?

[ea1aha]

altitude 65 m.

Cumulus sends several information, one to PWS, another one to weather underground and the one that it sends to the APRS server. To all it sends the data well, except the one of APRS that only sends the absolute pressure to him. To others it sends the relative pressure to them except ofset that I put to him that it is of -6.8
A greeting.

[steve]

I've just tried it with my Fine Offset station. I put an altitude of 65m into Cumulus and the current station (absolute) pressure is 1001.4 hPa. The altimeter pressure which Cumulus calculated was 1008.9; this is the value it would send to APRS. It looks about right to me (the difference is approximately 1 hPa for 10m of altitude).

I don't know why yours isn't sending the correct value. Can you give me an example of the absolute and relative pressures read from the console, with the value that Cumulus is sending to APRS?

I don't understand what this offset of -6.8 is? Have you set up your station to show the correct relative pressure? You shouldn't need to use an offset in Cumulus, if that's what you're doing.

[ea1aha]

Hello.
absolute pressure 1009.6
Relative presure 1024,5
Send to APRS 1010,3

attached configuration of cumulus (file word setyting.doc)

[steve]

It's because of the -6.8 offset that you've set. Cumulus is applying that to the absolute pressure and then working out the altimeter pressure from that. It looks correct to me.

I don't understand why you are using the offset in the first place. Do you think the absolute pressure that your station is showing is too high, or is it the relative pressure you are trying to correct? Your station is showing 1024.5, but with the offset Cumulus is showing 1017.9. If you think your station is showing the correct absolute pressure, then you should be setting the correct relative pressure on your station, not by using an offset in Cumulus.

I'm probably not explaining this very well; sorry

[ea1aha]

But data

absolute pressure 1010.1
Relative presure 1025
Send to APRS 1010,8
Send to PWS 1018,2
Send to Weather underground 1018,2

relative (1025)- offset (6,8) = 1018.2
Thanks

[ea1aha]

Pardon,
I do not speak English, use the translator for the conversion of Spanish to English.
My question is: Why it sends well the data to PWS and to Weather underground and APRS does not send the same?
Thanks

[steve]

I do not speak English, use the translator for the conversion of Spanish to English.

That's OK, I don't speak Spanish. I think we're doing OK, though

My question is: Why it sends well the data to PWS and to Weather underground and APRS does not send the same?

Because PWS and Weather Underground want sea-level (relative) pressure, and APRS wants 'altimeter pressure'. They are not quite the same thing. And because Cumulus calculates them differently, the offset you are using makes the values come out completely differently. You are using the offset in Cumulus to make the 1025 relative pressure from the station read 1018 in Cumulus (and PWS and Wunderground). But Cumulus also applies the offset to the absolute pressure, and then calculates the altimeter pressure from that.

I think the answer to your problem is that you need to set the relative pressure on your station to the correct value.

[ea1aha]

Everything understood perfectly
Thank you very much by the information.
I have understood now it. By the lost one of time. Thank you very much.