I know people have way more important things to work on than this minor issue, so I spent the past few days figuring out most of this for myself.
Answering my own question, the sensor calibration routine on the Settings page isn’t the appropriate place to make these kinds of calculations to raw data…its for adjusting sensors that need minor calibration tweaking. I knew this already, so why I asked if I could make data calculations there, who knows
. The better way to do data manipulation is with a virtual sensor.
To create a virtual sensor that calculates Altimeter instead of SLP, I did the following.
1. Log into the MeteoHub system.
2. Click on Weather Stations and change Station Altitude to 0…select Save.
3. Click on Sensors and enter the following information in the Virtual Sensors section.
New ID. The virtual barometric sensor should have its NewID set to a ID that is available between thb0 to thb9. On my station, my real barometric sensor is thb0, so I set my virtual barometric sensor to be thb1.
Name. The virtual sensor can have any name you choose to give it. I gave mine Vthb. You could also give it Altimeter or even QNH if you choose.
Trigger. The trigger value must be set to trigger from the real barometric sensor…in my case my real sensor is thb0, so my trigger is set to thb0.
Conversion. Copy and paste the “awk code” listed below.
awk '{ printf ''%d %d %d %d %d %d'', $3, $4, $5, ((($6/338.64)^0.1903--0.000013126*(790*3.28084))^(1/0.1903))*338.64, $7, $8 }'
The above awk code and its imbedded formula must be changed to reference the exact elevation of your station. Note the part of the awk code that contains “790*3.28084”. 790 is the elevation of my station in meters. You must change 790 to be the elevation of your station, in meters, whatever it is.
4. Once you have entered the above values into your virtual sensor, click on “Save & Restart Data Logging”
5. Wait a few moments (15-30 seconds) and hit refresh to see if your awk code is being interpreted. You can tell if the code is being interpreted by MeteoHub if the “Sensor Data” field populates with information. If you see “Never” regardless of how long you wait and hit refresh, your awk code has an error.
6. Compare the results of your virtual barometers Altimeter result to your local airport(s) reported value for Altimeter. They should be exact or very close.
7. Click on Weather Networks and then choose the appropriate sensors to be used for data upload. For Pressure, use your new virtual sensor (thb1 in my case) and not the actual pressure sensor (thb0 in my case). Enter the appropriate credentials in the CWOP field (or the APRS field if you are a ham) and click on save.
You are now configured to upload Altimeter to CWOP.
The formula I used is as follows
QNH = (Pa^N′ + K′ ha)^(1/N′)
where
QNH := altimeter setting
Pa := field pressure
ha := true field elevation
N′ := 0.1903
K′ := 1.3126×10−5
And comes from
http://www.av8n.com/physics/altimetry.htm
You’ll notice a couple of things with awk translation of the above formula.
((($6/33.864)^0.1903--0.000013126*(790*3.28084))^(1/0.1903))*33.864
1. The awk code contains extra steps that take the raw pressure data ($6) and converts the pressure data to imperial pressure units (inHg). Once the raw data is in imperial units, the awk code performs the calculations necessary to standardize local pressure to altimeter (QNH). After the standardization, the awk code converts the altimeter results
back into metric units. These extra conversions steps are necessary because the formula used to perform the calculations uses imperial derived constants. Without these conversions ahead of time, the formula does not generate correct altimeter values.
2. The double dash “--“ in the formula is the equivalent of “+” and is necessary because of programming limitations with Meteohub.
Hope this helps someone in the future.
Dave
