In Part I we talked about calibrating a personal weather station’s barometer from the perspective of a beginner. The procedure that was discussed in Part I was simple by design – no discussion of correcting pressure due to temperature, no discussions about air density or pressure reduction equations, etc.
I also recommended
matching the airport’s Altimeter setting (or matching QNH if your region uses METAR “Q” codes). rather than the airport’s SLP/MSLP reading. The Altimeter(setting) ignores observed temperatures, air density (warm air vs cold air), humidity and other factors.
The usual cautions apply:
It is very important to match the airport Altimeter reading only when you and your airport are in the same weather system (the same high or low pressure zone) and have the same atmospheric pressures. For the intermediate user that wants to expand their meteorological knowledge a bit more and extract more precision and accuracy out of their barometric sensor(s) – read on.
We will use a different approach and focus our attention to our station pressure (ABS); then correct for sensor error and then match our airport’s METAR Altimeter or QNH. We have already calculated our fixed offset/elevation offset for our location so there is no need to recalculate it. As you might recall from Part I, the elevation offset was 29.7 mb for a 250 m elevation.
Calculated
1 pressure readings like Altimeter(setting), QNH and METAR SLP are based on a calibrated barometric sensor that produces an accurate and precise station pressure measurement. Keeping that goal in mind, let’s start by calibrating your barometric sensor. For the intermediate user we take a different approach by correcting/adjusting your current ABS (station pressure) reading.
1 Calculating Altimeter/QNH and MSLP/SLP will be discussed in a future post for the advanced user.Ambient & Ecowitt display consoles:
Check your console’s pressure readings. If ABS = REL, that’s probably the original factory default settings, If you believe you’ve messed them up badly — you can switch your pressure units back to inHg temporarily and put in the pressure reading from the WH32B (units will be inHg) back into both the ABS and REL fields. You are now back to the factory default pressure readings. Then switch back to hPa units to start the calibration process once again.
Now adjust the REL in your display console’s calibration page so that REL is 29.7 higher than your current live ABS reading. Refer to your weather station manual for directions how to do this. Your REL is now lock-in-step with your ABS and REL will always be 29.7 higher than the ABS reading.
Compare with your airport’s Altimeter (setting). If the airport reading is higher than your REL reading you will have to increase your ABS to match your REL reading with the airports Altimeter(setting).. When you increase/change your ABS, you will notice that your REL reading will still be exactly 29.7 higher than your ABS. Do not change the REL reading by pushing any buttons – you will ruin your 29.7 offset. In other words, if you want to change your REL reading you change your ABS reading. The two readings always move lock-in-step.
Ecowitt GW Series WiFi Gateway:
If you don’t have a display console and are using the Ecowitt GW series WiFi Gateway as your console, things are even simpler - you can’t mess up your default pressure settings. You calibrate your barometer using the WSView smartphone app or depending on the GW model, you use a web browser to configure the settings. In the GW system, you do not change the ABS or REL values directly – you apply an offset/correction to change their values. For example, if ABS is too low and you wanted to add 2.0 hPa to compensate, you would add 2.0 as an ABS offset in the calibration settings page. Similarly, if ABS is too high by 2.0 hPa you subtract an offset/correction of 2.0 hPa (you enter a -2.0 as the ABS offset).
Remember the elevation offset for a 250 m elevation that we calculated previously? It was 29.7. Enter that number into the REL offset field in the calibration screen in the WSView app or in the web browser. Now check your current REL pressure. It should always read 29.7 hPa higher than your current ABS pressure.
Now compare your REL reading to the airport’s Altimeter(setting). Chances are is that the two readings will not be exactly the same (unless your barometer happens to be perfectly calibrated). If the airport’s reading is too high compared to yours, you will have to add a ABS offset to match. For example, if the airport’s Altimeter setting is 2.0 hPa higher than your REL reading, you need to add 2.0 hPa as an ABS offset. It is important not to change the REL offset as that number is tied to the specific elevation of 250 m. Always calibrate your barometric sensor by adjusting the ABS offset and set your barometer to your specific elevation offset using the REL offset.
Let’s continue with our discussion of the Earth’s atmosphere.
Meteorological considerations – atmospheric pressure:
Pressure, temperature and weather for our discussion, refers to the bottom layer of the atmosphere < 11,000 meters (troposphere).
Standard Atmosphere (model of the atmosphere):
15°C @ 0 meters (sea level)
1013.25 hPa/mb @ 0 meters (sea level)
Temperature declines linearly at the rate of 0.0065°C per meter with altitude.
Pressure declines in a non-linear fashion (it’s on a curve) with altitude (determined by the use of an equation, e.g. hypsometric equation)
Important: Note that for standard ISA conditions (aka “standard conditions”), 15°C is the temperature at sea level – not at your location..Use the digital dutch Standard Atmosphere calculator to calculate the elevation offset for your location.Refer to
Part I – how to calculate elevation offset.
Coming up! The excitement starts. Controversies rage. We take a deep plunge into the shark-infested waters of Altimeter(setting), QNH and MSLP/SLP (Sea Level Pressure).
.… continued in Part III -
a guide for the advanced user.