Davis #altimeterpressure tag

[TheBridge]

Entering the following of host PC:
http://192.168.1.121:8998//api/tags/pro ... erpressure

Results in:

{"press":"29.34","altimeterpressure":"30.2"}

The ‘altimeterpressure’ correctly ties to the ‘press’ value correctly adjusted for (my) 850-ft elevation (and closey approximates KOXR reading. (KXOR is only at 4-miles from me but it is 750-ft lower.).

Using the altimeterpressure measurement (not ‘press’) will result in more accurate forecasting when using the Sager forecasting plugin for my area.

When I added the altimeterpressure webtag to websitedataT.json and reran the ‘NOW’ page, ALL the data fields on the page were blank? Are added webtag position-sensitive in the JSON file?

[TheBridge]

Freddie,
Sorry, I had been adjusting the wrong file. All good.
This thread is closed!

[freddie]

There's still something odd about your setup. press and altimeterpressure should be within about 0.05 to 0.1 inches of each other. You're showing a difference of 0.86 inches which is way too high. It actually equates to about 875 feet in altitude.

In the admin interface - under Settings -> Station settings -> Location: is your altitude set correctly, along with the units?

[freddie]

Just reading some of your earlier comments - I definitely think you've got your station altitude set to something near sea level - or perhaps the altitude setting in your console itself. This is incorrect - it should be set to the altitude of your console. This is why you are seeing such a big difference. Remember that both "press" and "altimeterpressure" are calculated values of sea level pressure at your station - one using the complex formula that involves temperature, and the other using standard atmosphere calculations.

As I said previously, the thing to remember is that neither is your station level pressure, so they should be in reasonably close agreement, and you should be able to use either with the Sager forecasting plugin and expect similar results.

[TheBridge]

I am about 850-ft above sea level and using that value in Cumulus.The current 30.2 reading also matches the Davis (in-house) console display. The Davis console is also set to 850-ft above sea level so the console is also adjusting for altitude.

My nearest airport (KOXR Oxnard) is 4-miles from me, as the crow flies, yet is near sea level. They are reporting 30.2 right now so all good.

Bridge

[freddie]

I am about 850-ft above sea level and using that value in Cumulus.The current 30.2 reading also matches the Davis (in-house) console display. The Davis console is also set to 850-ft above sea level so the console is also adjusting for altitude.

My nearest airport (KOXR Oxnard) is 4-miles from me, as the crow flies, yet is near sea level. They are reporting 30.2 right now so all good.

Bridge

That doesn't explain why your "press" tag is returning such low values. The output from the two tags should be pretty close together.

[TheBridge]

The ‘press’ value is reporting actual pressure at weather station location.
A weather station at 850-ft elev will report a value that is about 0.9 below that of pressure at sea level (which is altimeterpressure value)

[freddie]

"press" reports (or should report) mean sea level pressure. See the wiki: https://www.cumuluswiki.org/a/Full_list_of_Webtags

[TheBridge]

My testing says it does not.

[freddie]

My testing says it does not.

Which brings me back to the theory that there is something not quite right with your MX configuration. If it was a fault in the program then it would have become apparent before now, considering the large number of MX installations. I have used MX (and its predecessor) for over a decade and the "press" value has always been mean sea level pressure. This is backed up both by the documentation and the calculations used in the source code for MX.

[mcrossley]

So it must be either the user entered calibration value in the VP2 console itself. Though you said that the console is showing the correct "barometer" = SL pressure?

The only other place it could be out is if you have added an offset to the CMX presuure calibration values at some point.


As @freddie has pointed out, CMX definitely uses the SLP from the console in the #press tag - and for all user presented values.

[TheBridge]

Just to be concise…and at this moment:
I am about 850-ft >SL
METAR for nearby Oxnard airport says pressure = 30.01 at SL.(see attached)
CumulusMX was elevation set to 350-ft
My Davis display console, which has an elevation setting is set to +850-ft, reads 30.04
At the same time my Cumulus ‘press’ value is 29.14.
‘Altimeterpressure’ reading is 30.0 (see attached)

I read that the calculated adjustment for barometric at 850-ft above SL is abt 0.9

Anything I can check for you please let me know. I’d like to clear this up.

Bridge

[freddie]

CumulusMX was elevation set to 350-ft

Is this a typo, or is it really set to 350?

[mcrossley]

I went back to log file you posted above and looked at the raw data...

The last data message from the console in the log file was this...

27/01 @ 05:14 where the Barometer = 29.360

Then at 14:51 you posted another message saying CMX was reporting #press as 29.35

So, I conclude that CMX is working as we expect and reporting what the console is sending accurately.

The question then becomes why is the console displaying one value and sending another?

Looking at the LOOP2 which contains more pressure data...

The last LOOP2 message @ 05:14 has...
Barometer = 29.363
Barometric reduction method = NOAA (the only option on VP2)
User pressure offset = 0
Raw sensor pressure = 29.361
Absolute press = 29.361 (raw + user offset = station pressure)
Altimeter press = 29.361

It looks like the console is not applying any elevation correction to your readings sent via the LOOP packets. But you say the console is displaying a corrected value. I'd try restarting the console as a first step - full power down with batteries out. Then if it is still the same revert to Davis support.

[TheBridge]

Thanks Mark,
Will do!
Bridge