Hello,
I have been trying to calibrate my barometer so I can get rid of the L3 mark on my station on CWOP (ID: GW5159) So far I have been way confused on how to do this and I have probably messed things up pretty good.
I tried to go by this guide on the CWOP site :https://harriscountyskywatcher.com/2021/03/16/ambient-weather-ws-2000-barometer-calibration/
Thought I had it but I doubt it as it was worse on CWOP after I did it then before.
So, I sent email to support and they sent me to here. I've looked around and tried to re-calibrate it and still is way off.
So I have went to this site to get my elevation https://www.freemaptools.com/elevation-finder.htm
I get 367m or 1204.1 ft. on that site
Right now my barometer on my control unit is reading: REL - 1021.6 hpa and ABS - 1020.7 The nearest METAR site from me is my local airport, KN03, which is showing 1020.4 mb on the altimeter. My indoor remote sensor is showing 28.83 inHg.
So using these figures, how to I calculate what to put in for REL and ABS on my console? I know these values will change by the time someone sees this and responds, but I just need a down and dirty simple way to calculate this and what/how to change. I know from reading in here that if I change the REL, the ABS will change and vice versa.
Any help is greatly appreciated.
Jeff
If you do not see an Altitude entry field in your set-up screen you are using the old firmware which used a fixed offset. The fixed offset is the difference in pressure between your barometer's elevation and the pressure at sea level. This fixed offset/difference in pressure is called the REL offset.
Here's a guide - may be not as simple as you might like but I've included an example which should help:
Calibration – Ambient Weather WS-2000
Some display consoles do not have separate offsets [like the WS-2000] in their firmware. Although not as intuitive, you still can use offsets to properly set up and calibrate your weather station's barometer.
Here's a guide or "how-to" that shows an example of how to properly set up a "non-offset" display console: If you follow this guide, your "non-offset" console will be just as accurate as a console that has separate offset adjustments.
Our objective is to set the elevation of our barometer and to adjust our barometer for best accuracy.
Let's show an example of how these settings would need to be entered. We'll assume you have a brand new weather station, and you are setting it up for the first time. If you have an existing weather station with a "non-offset" display console and have already made some adjustments, skip to Step 3.
Step 1 - review factory default settings:
The default settings:
a) default units are inHg units (inches of mercury)
b) ABS = REL
Note. I will be using a theoretical current pressure reading of 28.53 inHg as an example.
Out-of-the-box, the barometer settings might look something like this:
ABS = 28.53 inHg
REL = 28.53 inHg
We should change to hPa units for better precision for the calibration process. After calibrating, you can switch back to your preferred units:
Step 2: Changing default units from inHg to hPa:
Change the units in the console from inHg to hPa. The console will automatically convert the units to the equivalent hPa values - no manual calculations necessary. For the illustrations (below), all values will now be in hPa (hectopascal) units. Therefore, the default setting of 28.52 inHg will be converted to 1000.0 hPa:
Default settings in hPa:
ABS = 1000.0
REL = 1000.0
Step 3: Setting the elevation
EXAMPLE: adjust settings for a 300-meter elevation:
Calculate your REL offset by using an online calculator. The calculator will calculate a "pressure difference" number, which is the REL offset number that we need.
https://www.sensorsone.com/icao-standard-atmosphere-altitude-pressure-calculator/?ab=4&v2=300&f2=m&v3=0&f3=m&f4=hPa[just modify the example inputs to your altitude and preferred units]
Based on a 300-meter elevation, the calculator calculates a pressure difference number of 35.5 (rounded) so grab a pen and paper and add it to whatever is showing as your current ABS value below:
ABS = 1000.0 (current live reading)
REL = 1035.5 (adjust REL manually and change the value until it is exactly 35.5 higher than the ABS value.
Step 4: Compare REL (relative pressure) to an airport: (adjusting for accuracy)
We want our REL to be equal to the Altimeter (setting) at the airport, so we compare the values and make note of any discrepancies between the two values. In the example below, the two values don't match:
REL = 1035.5 (our barometer's reading)
Altimeter (setting) = 1036.5 (the airport's reading)
Note the discrepancy in values. Our REL (relative pressure) is too low by 1.0 hPa compared to the airport.
CAUTION: Do not adjust/change the REL value a second time!. The offset must remain at 35.5 (for a 300-meter elevation). If you change the REL value a second time, you will change the REL offset (the difference between REL and ABS) and your elevation setting of 300 meters will be ruined.
From here on, to make further adjustments, we have to switch our attention to the ABS value.
Step 5: Matching the airport (adjusting ABS):
In Step 4 (above) we found out that our REL was too low by 1.0.
To fix that, we increase the ABS value (absolute pressure) by 1.0 which will, at the same time, automatically move the REL up by 1.0.
Your display will look like this for a 300-meter elevation.:
ABS = 1001.0 (you manually changed the ABS value from 1000 to 1001)
REL = 1036.5 (REL automatically increased by 1.0 when you increased ABS by 1.0)
Note: Our REL = 1036.5 now matches the airport's Altimeter setting of 1036.5 which is what we wanted to achieve.
Step 6: Final tuning:
Because we calibrated our barometer when the airport was reading 1036.5, our barometer will start to “drift” in accuracy when the pressure increases or decreases from 1036.5. The farther the pressure is from 1036.5 – the higher the error.
The REL pressure will, of course, change over time and 1036.5 is probably, well above the average pressure for your location. Therefore, for best accuracy, the next time the airport Altimeter setting reads closer to normal (around 1013.25), you should re-adjust your ABS value once again so that your REL value on the display console matches 1013.25 at the airport.
By fine-tuning, you are shifting the calibration curve so that you are obtaining the maximum accuracy around the average pressure for your location, rather than setting the maximum accuracy at a very high (or low) pressure.
Congratulations! You are now calibrated.
