Davis #altimeterpressure tag

[TheBridge]

On Davis VP2…

My Davis weatherstation is 850-ft above sea level so the ‘altimeterpressure’ value should be adjusted (up) by .9 over the ‘press’ value. I see them both returning the same value. And yet my Davis console display does show the pressure correct adjusted for altitude.

Why this matters…I am working to install the Sager forecasting plugin. I notice that it is forecasting based on the ‘press’ value so forecast is constantly saying rain is coming. I changed it to use ‘altimeterpressure’ value and notice it is identical to ‘press’ hence the forecasts on both cases on identical (and incorrect)

Has anyone noticed this?

Bridge

[mcrossley]

Have you disabled LOOP2 data, or have a console with old firmware?

For Davis stations altimeter pressure is only obtained from LOOP2 data.

[TheBridge]

Thanks Mark,
Loop2 is on.
Davis Console, VP2 & Envoy all new.
I’ll read up on how to check firmwares.

Bridge

[TheBridge]

Mark,
Davis console firmware = 3.88
Davis Envoy & ISS = no firmware mentioned in manual.
Got a call into Davis tech support. See what’s what here.
Bridge

[TheBridge]

Mark,
Pls see previous 2 threads.
No surprise, Davis support didn’t help when they realized the issue did not include their software.
Wether I use ‘press’ or ‘altimeterpressure’ I get the same value. According to what I read ‘Altimeterpressure’ should provide a barometer value (which is the ‘press’ value of course) offset by elevation.
I.e. A Davis station at elevation 850-ft has a ‘press’ value of 29.5-in. Then its ‘altimeterpressure’ value should be 29.5-in +.9 (elevation compensation value of 850-ft)=30.4-in.

I tested this using:
http://192.168.1.121:8998//api/tags/pro ... erpressure

And got this result:
{"press":"29.39","altimeterpressure":"29.4"}

Bridge

Bridge

[mcrossley]

I'm out today, but I'll take a deeper look tomorrow. I just saw there was a bit of code that set altimeter = SLP if no LOOP2.

Pressure handling in general needs a bit of love from what I recall - it doesn't help that all the different stations provide different values (sometimes firmware dependent) but often call them the same thing.

[TheBridge]

Even though Cumulus ‘press’ returns (from Davis) actual pressure on the ‘press’ tag, the Davis console itself display screen does show pressure adjusted for altitude. So we should hope there is a Davis API call that could return the same adjusted-for-altitude pressure value.

Or….add an embedded equation within Cumulus to calculate ‘press’ x altitude factor** = ‘altimeterpressure’…which may make it, as you commented, a more device independent call (assuming all weather devices give you the ‘press’ value); which is an approach you may actually prefer as quicker and more universal solution.

** altitude factor is also its own equation of course
https://en.m.wikipedia.org/wiki/Barometric_formula

[TheBridge]

Mark - if interested in the equation approach, forget that wiki link in previous msg. Way too complex. Simplified version is:
“At low altitudes above sea level, the pressure decreases by about 1.2 kPa (12 hPa) for every 100 meters”

[mcrossley]

OK, I am using a WLL rather than a VP2, but both use effectively the same code: They calculate the altimeter pressure from the absolute pressure returned by the station and the altitude entered in CMX. It uses lapse rates originally from code in the NOAA MADIS system.

My altitude is 70m and I get...

Code: Select all

{"press":"1032.9","altimeterpressure":"1031.3"}

So 1.6 hPa difference.

The VP2 code is the same, it takes the absolute pressure from the LOOP2 data and uses the same Station to Altimeter pressure function.
There is an override so that if the station is a Davis and LOOP2 is disabled in the CMX config it just makes altimeter = slp.

Can you switch on data logging for a few minutes, then post the MXdiags file and I can take a look at what the station is sending.

[TheBridge]

Good morning Mark,
Here you go (attached). I just switched back to 850-ft from 5-ft as I was experimenting a bit myself and then did an MX restarted to get a current, clean log file for you.

Bridge

[TheBridge]

Mark,
I just ran this URL: http://192.168.1.121:8998/api/tags/proc ... erpressure
and DID get the right value. {"altimeterpressure":"30.3"}
Current 'press' is 29.35"
However 'altimeterpressure' value not showing up in on 'Now' page of Climate.tommueller.com. I have both station and sea level values on this webpage: https://climate.tommueller.com/index.htm
so I am overlooking something so simple in the setup I can't see it.
I'll dive in more myself before I take any more of your time.
THANKS!
Bridge

[freddie]

I have a VP2 and am at 148m (486 ft) AMSL and get the following readings currently:

Code: Select all

{"press":"1032.61","altimeterpressure":"1031.2"}

As I understand it, "press" returns the air pressure adjusted to sea level using a complex formula that includes compensation for temperature, and "altimeterpressure" returns the air pressure adjusted to sea level using the standard atmosphere (ICAO/ISA). So there should be a difference between the two unless current conditions are close to the standard atmosphere (i.e. sea level pressure 1013.25 hPa (29.92 inHg) and temperature 15 degrees Celsius). AFAIK it is temperature that has the biggest effect, so with temperatures lower than 15, press > altimeterpressure, and temperature higher than 15, press < altimeterpressure. This effect will be greater with increased altitude.

Current temperature at KOXR is 9, so using that as a proxy for your temperature I would imagine press would be slightly more than altimeterpressure.

So to summarise: if all is working okay, I would anticipate there very often being a small difference between press and altimeterpressure, the difference increasing with increased departure from standard atmosphere conditions.

An important takeaway is that press is not station level pressure.

[TheBridge]

Thanks Freddie,
Very good recap. I'm learning!
As mentioned in previous the 'altimeterpressure' value does show-up in quick test thru URL however is not showing up in my 'NOW' page?
indexT.htm file attached for reference.
Hoping to learn what I have overlooked.
Bridge

[TheBridge]

Sorry Freddie,
clicked on wrong file...
indexT.htm attached

[freddie]

You have attached the page template. The data file used to populate the page may be a better starting point: websitedataT.json.

It is the "press" value that I believe is used in the standard template. There's nothing to stop you changing that, however, and including the "altimeterpressure" value instead, and then referencing it from the page template. Just search for "press" in both templates and substitute with "altimeterpressure". Or, alternatively, add it rather than replace it.

Regarding station (absolute) pressure - I don't know whether or not there is a tag for that value.