Baramotetric Pressure not reporting correctly

[KF0KFL-13]

I use Cumulus MX to generate a wxnow file for my APRS weather station. I've noticed the pressure was a lot lower compared to nearby stations. I took my dial barometer to a nearby airport to calibrate it to sea level, then adjusted my Ambient WS2902 accordingly. I noticed it was still reporting low. I'm thinking the pressure going to the wxnow file is absolute or something else!
I've attached some data that will hopefully help.

Thanks in advance!

[mcrossley]

Cumulus uses Altimeter pressure for APRS and CWOP.

Is your station altitude set correctly in both Cumulus and the console?

Cumulus calculates the altimeter pressure for your station from the absolute (or station) pressure and altitude. Is the absolute pressure reading correct?

[KF0KFL-13]

I'm pretty confident the pressure is set correctly. I never had any issues before. Tonight I'm going to roll in back to previous version to see if it fixes my issue. As I look at it now, what my station reads in InHg doesn't convert convert to what's being pushed out to the wxnow in millbars..... At least going by Google's conversion

[mcrossley]

Cumulus has used altimeter pressure since v1.9.4

What has changed is that for some stations it set altimeter = what the station supplied as sea level pressure. The Davis station have always had a true altimiter pressure value as well as sea level. The Ecowitt had a "dummy" sea level value in altimeter until MX v4.0.0 when MX started calculating the altimeter pressure.

How does the wxnow pressure value compare with a local QNH value?

[KF0KFL-13]

Can you explain what a QNH station is?
Thanks

[freddie]

Can you explain what a QNH station is?
Thanks

Any airfield supplying a METAR will report its QNH (altimeter) pressure. I think you're in the USA so it will be in inches of mercury. If you look at the raw METAR report, it is the value preceded by an A.

Example for LA International:

Code: Select all

KLAX 080553Z 31004KT 10SM SCT014 SCT030 BKN200 12/12 A3011 RMK AO2 SLPNO T01220122 10156 20122

The A3011 is the QNH value - decodes to 30.11 inches.

Source: https://aviationweather.gov/data/metar/?ids=KLAX

[KF0KFL-13]

Ok! I wasn't able to successfully roll back to an older version. I'm continuing to run the latest CMX version. In calibration settings I increased the offset of the station pressure by +0.08inhg. So far it seems to compare with nearby stations and reflect what the RELATIVE pressure on my station and dial gauge is showing. I'm guessing that CMX uses my stations ABSOLUTE pressure and calculates it for altitude for the wxnow file... instead of the stations RELATIVE pressure going to the wxnow file. Please correct me if I'm wrong. Thank you

Update:
Reread a previous reply. It confirmed my guess.
Thank you mcrossly

Cumulus uses Altimeter pressure for APRS and CWOP.

Is your station altitude set correctly in both Cumulus and the console?

Cumulus calculates the altimeter pressure for your station from the absolute (or station) pressure and altitude. Is the absolute pressure reading correct?

[KF0KFL-13]

Can you explain what a QNH station is?
Thanks

Any airfield supplying a METAR will report its QNH (altimeter) pressure. I think you're in the USA so it will be in inches of mercury. If you look at the raw METAR report, it is the value preceded by an A.

Example for LA International:

Code: Select all

KLAX 080553Z 31004KT 10SM SCT014 SCT030 BKN200 12/12 A3011 RMK AO2 SLPNO T01220122 10156 20122

The A3011 is the QNH value - decodes to 30.11 inches.

Source: https://aviationweather.gov/data/metar/?ids=KLAX

Thanks Freddie

[mcrossley]

Ok! I wasn't able to successfully roll back to an older version. I'm continuing to run the latest CMX version. In calibration settings I increased the offset of the station pressure by +0.08inhg. So far it seems to compare with nearby stations and reflect what the RELATIVE pressure on my station and dial gauge is showing. I'm guessing that CMX uses my stations ABSOLUTE pressure and calculates it for altitude for the wxnow file... instead of the stations RELATIVE pressure going to the wxnow file. Please correct me if I'm wrong. Thank you

You are correct. Altimeter is calculated for stations that do not supply it AND provide an absolute (station) pressure.

Note: Ecowitt recently changed their pressure calculations, for many stations in recent firmware they now do a "proper" sea level pressure calculation rather than a simple offset relative pressure.

[KF0KFL-13]

Thanks mcrossly

Ok! I wasn't able to successfully roll back to an older version. I'm continuing to run the latest CMX version. In calibration settings I increased the offset of the station pressure by +0.08inhg. So far it seems to compare with nearby stations and reflect what the RELATIVE pressure on my station and dial gauge is showing. I'm guessing that CMX uses my stations ABSOLUTE pressure and calculates it for altitude for the wxnow file... instead of the stations RELATIVE pressure going to the wxnow file. Please correct me if I'm wrong. Thank you

You are correct. Altimeter is calculated for stations that do not supply it AND provide an absolute (station) pressure.

Note: Ecowitt recently changed their pressure calculations, for many stations in recent firmware they now do a "proper" sea level pressure calculation rather than a simple offset relative pressure.