### Author Topic: WS-2000 - WH32B - Pressure differences  (Read 712 times)

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#### gszlag

• Forecaster
• Posts: 461
• "barometers always need correcting"
##### WS-2000 - WH32B - Pressure differences
« on: January 08, 2021, 10:24:58 AM »
This thread is a split from this other thread at after the following post because it touched on a new topic:
https://www.wxforum.net/index.php?topic=36379.msg422065#msg422065
the point I was trying to make - apart from a difference between 2000 & 2902 is that when you have calibrated your console the barometric reading on the separate barometric pressure/temp/humidity sensor will be different from the corrected readings on your console.

Very good observation to notice that the measured pressure on the remote unit(WH32B) is different...but the important question is why it is different?

The measured pressure comes from the atmospheric pressure readings (Station Pressure) from the WH32B display that contains the barometric sensor. Half the battle to calibrate the barometer is realizing that atmospheric pressure at your elevation = Absolute Pressure in the WS-2000 display console.

I do not recall what the Absolute Pressure was set at in the factory. Does anyone know?

If you use the short cut method of calibrating(using a online calculator) you assume that a close-to-you METAR pressure reading is the corrected Relative Pressure which is equal to the equivalent sea level pressure. Enter your elevation, enter the METAR SLP pressure and then, calculate what the atmospheric pressure should be at your elevation. The difference you noted is the difference between theoretical and actual and actual plus somewhere buried in there you will likely be incorporating allowances for the accuracy of the instrument. For the WS-2000, note that the stated accuracy for the WH32B barometric sensor is +/- 2.7hPa.

NOTE: other manufacturers use separate Absolute and Relative offsets in their consoles to "correct" these readings.

I think everyone agrees that Ambient should do a rewrite of the barometer calibration section of the manual  as it can lead many a new user astray.
« Last Edit: January 08, 2021, 11:33:59 AM by galfert »
Ambient Weather WS-2000
Ecowitt WS3900 console
Ecowitt GW1000/GW1100
Ecowitt WS68: Anemometer, UV/solar
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Ecowitt WH57 Lightning sensor
Ecowitt WH32E: Outside T & H sensor
Stratus Rain Gauge (manual)
Raspberry Pi 3B+ (WeeWX/CumulusMX)
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#### galfert

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• Forecaster
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##### Re: WS-2000 - WH32B - Pressure differences
« Reply #1 on: January 08, 2021, 12:42:36 PM »
I do not recall what the Absolute Pressure was set at in the factory. Does anyone know?
All Fine Offset clone stations have their Absolute pressure calibrated at the factory or perhaps it is a function of the built in calibration of the barometric pressure sensor that is used, meaning that the sensor itself is calibrated at the time of manufacture by that company and not by Fine Offset.

Regardless the outcome is the same. All Fine Offset weather stations have a preset factory Absolute pressure calibration. Technically you cannot change this internal default. You can certainly make calibration adjustments and then be ±some-amount but were to you do a factory reset then you'd be back to that internal default.

The value for this default Absolute pressure is that no matter where you are in terms of elevation it should display whatever the real atmospheric pressure is....or basically it should show you station pressure.

The reality is that the specifications of these stations state that the accuracy is ±3 hPa (newest datasheets as old data showed ±5 hPa). What this means I feel is that if you don't conern yourself with how well that initial default Absolute pressure is and you only concern yourself with matching the Relative pressure to the local METAR then your Absolute pressure should not be off more than ±3 hPa. But for many of us that is not good enough and we want to further calibrate and fine tune not just the Relative pressure but also the Absolute pressure. In my experience helping out numerous users trying to more precisely calibrate their barometers, the correction required for the majority of stations on the Absolute pressure has been less than ±1 hPa. This means that if they just left the Absolute factory calibration in place and they only did the simple matching of the Relative to the METAR that they wouldn't be far off.

Typically doing just setting that simple Relative = METAR is more than good enough for 99% of the users with this type of station. But if you want to upload to CWOP then that changes things because you can then no longer ignore the Absolute pressure, because it is the Absolute pressure that is used to the calculate the Altimeter pressure which is something that the software uploading to CWOP does for you...but the Absolute pressure needs to be better calibrated or you'll get bad quality control analysis.

Quote
If you use the short cut method of calibrating(using a online calculator) you assume that a close-to-you METAR pressure reading is the corrected Relative Pressure which is equal to the equivalent sea level pressure. Enter your elevation, enter the METAR SLP pressure and then, calculate what the atmospheric pressure should be at your elevation. The difference you noted is the difference between theoretical and actual and actual plus somewhere buried in there you will likely be incorporating allowances for the accuracy of the instrument. For the WS-2000, note that the stated accuracy for the WH32B barometric sensor is +/- 2.7hPa.
No these are not the correct steps to take with the Fine Offset clone stations. You need to separate these concepts into two steps. The method you have described is combining concepts from two objectives into one...and that isn't the right way. Step 1 is to figure out your offset (difference) between your Absolute and your Relative...and this step does not take into account whatever is going on at your local METAR.  Then Step 2 is to calibrate against the METAR once your elevation has been effectively given to your station via the Relative offset done in step 1.

Quote
NOTE: other manufacturers use separate Absolute and Relative offsets in their consoles to "correct" these readings.
What other manufactures do is that they simply ask you what your elevation is so that the math done in my Step 1 is done automatically. That is the improvement required if Fine Offset were to make things better.

Quote
I think everyone agrees that Ambient should do a rewrite of the barometer calibration section of the manual  as it can lead many a new user astray.
I think given the current complexity of getting the Absolute pressure right the current manual procedure is okay. They only want you to match the Relative to the METAR and then you are done. That is easy and simple. They can't rewrite the manual and introduce the complexities of getting the Absolute pressure right as most people will not care as most do not upload to CWOP.
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