[aprssig] suggested list for APRS WX stations?

[Steve Dimse]

I don't know of a good technical forum to answer what you are asking, but I can take a stab at it. It is easy to make a solid state sensor to measure ambient pressure with high precision (the ability to correctly follow tiny changes), there are many that sell for less than dollar. It is impossible to make one with high absolute accuracy (the ability to return true values). That is why all consumer weather stations require entering of a calibration value. It gets more complex because solid state pressure sensors are temperature sensitive, so they all include built in temperature calibration. Unless there is a hardware failure though most sensors do well correcting for differences, at least within room temperature ranges.

I pulled up the two ham sites you mentioned. Almost all of the difference between the two is an offset error, at least going back ten days, almost certainly where the user simply entered the wrong calibration value. It is hard to calibrate a pressure sensor because you need a good station as close as possible, and a day where the region is under a large area of stable high pressure, such that there is no wind and while pressures are steady for hours. 

In CWOP our QA system looks for offset errors and notifies the owner by email of their problem. Most ignore the notices, and the uncalibrated nature of the data is something every user of CWOP data knows and corrects for.

There is another source of error, which is altitude compensation. The pressure it reads is the absolute pressure, but reported pressures are always compensated for altitude. There are different ways to do this, and can result in erroneous data if the compensation is not done correctly.

So with all of that in mind, if I wanted to build a really great pressure sensor, I would buy three different makes of higher end (about $10 each) sensors and verify the absolute pressures (after careful calibration) with each other. I would use a high precision and accuracy temperature sensor (less than a buck) to compare to the relative errors to see if there a delta between my sensors related to ambient temperature (maybe make a little environmental chamber to get stable and reproducible data points) and correct for that in each sensor if needed. Then I would dig into the details of compensation to make sure I understood every nuance of it. Then I'd write the software to take the three pressures and temperature data, do the calibration and compensation, and spit out the values.

In other words, it isn't money or hardware engineering, but rather time that goes into making a great barometer.

Steve K4HG

> On Feb 12, 2022, at 9:21 AM, Ron VE8RT <ve8rt at yknwt.ca> wrote:
>   To my situation and question.  A couple of times a day I look at the
> wx data from our local APRS linked WX stations to verify that
> everything is running as it should.  This morning I noticed an
> intolerable deviation in barometric pressure (possibly temperature too)
> from that reported by Environment Canada at the airport (CYZF)
> 
> CYZF    Ski Club VE8SKI   Prelude Lake VE8WD-2
> 
> Pressure
> 
> 1028.4mB   1051.6 mB        1033.1 mB
> 
> Temperature
> 
> -35.6 C    -32.2C           -28.9 C
> 
>   As these APRS WX stations are not my own I'm not looking to repair
> or calibrate them.  I would like to build my own, using better quality
> components.  My own searches for a forum that covers weather stations
> (beyond the off the shelf brand X or Y) has been unsuccessful.  Would
> anyone be able to direct me to a forum that may help?  
> 
>   Thanks  Ron VE8RT
> -- 
> Ron VE8RT <ve8rt at yknwt.ca>
> 
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