Barometer Calibration on WMR100

[bemanuel]

I know this subject has been covered before, but I looked through the threads and still don't have a clear answer.  I have a WMR100, and the altitude is set correctly for my location (abt 105M), but the sea level pressure reported is consistently about .50 inches below the NWS site about 2 miles away.  I thought I could use the "offset" feature in the calibration dialog for VWS to change the value sent to Weather Underground, but it changes nothing (after reading the other threads, I see that VWS ignores this).

Please don't lecture me on why this is a bad idea.  I understand this, but I don't need complete accuracy.  I just want to be within a few hundredths of an inch of mercury with the other barometers in my area.  The differential is fairly consistent so for me, simply adjusting an offset in the software would work.

Is adjusting the altitude on the base unit hardware (this would have to be a major adjustment for me, and would be quite different from actual altitude) the only thing I can do to bring me closer to reporting results closer to the NWS stations?

Thanks,

[Weather Display]

using a offset in the software for a station like the OS WMR100 is a good idea in my opinion

[tomcj2]

The manual for the  WMR100 claims the accuracy to be +- 10mb, or .3 inches.  As it is more than that you should ask to return it if it is still in a warranty period.  The only way to calibrate it is by changing the altitude.  The "corrected altitude" would not be displayed at WU.

[Judwin]

I have a WMR200, and decided to make the device display the correct SL Barometric Pressure (QNH) by altering the altitude offset to do just that, knowing that the altitude will not necessarily be the altitude I'm at.

Since  it is the QNH or Sea Level pressure that is important this may be your best bet, even though it will mean  the RAW pressure logged/displayed by VWS will not be the actual local value.

I too would like to be able to correct the RAW barometric value to display the local actual pressure or QFE in aviation circles.  Being able to apply an offset/delta value to both the pressures in VWS (QNH and QFE) would be great and surely not a major task to add, it would save me having to 'fudge' the RAW value in my MySQL database when I wish to display it.



cheers

Jonathan

[Meteorologica]

The raw pressure reading from the station is not affected by the altitude setting on the console of your weather station. The altitude setting only affects the sea level pressure correction.

Since you are not using the station as an altimeter you should adjust the altitude on the console until the sea level corrected pressure matches an official nearby station and totally ignore what altitude value you had to set the console to in order to achieve this.

[aweatherguy]

There is a difference between sea level pressure (SLP) and altimiter setting (QNH). Specifically, SLP includes consideration of the actual temperature, while QNH does not. The QNH value is what you would dial into the altimeter in an airplane to make it read correctly on the ground at your location. Be aware that pressure readings you get from airports are QNH -- not SLP. This is because they need to tell pilots where to set the altimeter.

I have not been able to determine whether the WMR100 is displaying SLP or QNH (but I suspect it is intended to be SLP).

There is also station pressure (QFE) which is the actual barometric pressure at your location.

Anyway, I've got a freeware package called Weather Station Data Logger (WSDL) on SourceForge that offers connectivity to Weather Underground and CWOP.

Weather Underground data uses the WMR100's reported SLP value (or whatever it is). You can tweak what is reported by changing the altitude on the WMR100 console; then a different value goes to Weather Underground.

The data sent to CWOP by WSDL is computed from the WMR100's reported station pressure using an altitude value entered by the user in an options dialog. This will allow you to tweak the barometer to give the proper reading by adjusting the altitude value slightly. Each 10 feet of altitude corresponds to roughly 0.01" Hg.

Right now, WSDL displays the WMR100's (SLP or QNH, whatever it is) but I've been thinking about a modification that would allow the user to display a computed QNH value instead. This would allow user tweaks via an altitude setting to make the barometer reading be spot on. Please take a look at the software package and post something on the SourceForge forum or this group if you'd like to see this change in the WSDL program.

Here's the URL for WSDL:

http://wmrx00.sourceforge.net

The SourceForge forum can be found here:

http://sourceforge.net/projects/wmrx00

If you want more information about computing SLP and QNH from station pressure, look at these excellent documents:

http://www.srh.noaa.gov/epz/wxcalc/wxcalc.shtml
http://wahiduddin.net/calc/refs/ASOS_Pressure.htm

Cheers!

    -- Weber

[Weather Display]

The data sent to CWOP by WSDL is computed from the WMR100's reported station pressure using an altitude value entered by the user in an options dialog. This will allow you to tweak the barometer to give the proper reading by adjusting the altitude value slightly. Each 10 feet of altitude corresponds to roughly 0.01" Hg.

isnt that the same as  barometer offset value?

[aweatherguy]

Technically speaking, an offset is not the same since the formulas to convert between station pressure and either SLP or QNH are mutiplicative, not additive.

Practically speaking, for small changes in altitude, it will behave like a fixed offset. So if you just need to tweak the reading because it is off a little bit this will work fine.

You would only notice a difference if there were a wild swing in barometric pressure and you had made a large adjustment.

For example, assume you live at sea level and the barometer is way, way off. Your station pressure is 28.00" and you have dialed enough altitude to convert it to 29.92" (the multiplier would be about 1.0686). The offset here is 29.92-28.00=1.82". Now lets say the station pressure drops to 27.50". The new converted pressure will be 27.50*1.0686=29.39" and the offset is now 29.39-27.50=1.89". The offset from an additive point of view has changed from 1.92 to 1.89 inches.

In this example, if your barometer actually had a fixed offset error of 1.92 inches then the altitude tweak that was perfect when station pressure was 28.00 is off by 0.03 inches when station pressure drops to 27.50.

If you want to get picky about this then, you really need to know if your barometer error is a "fixed" offset or something else. The majority of folks (including myself) don't have the equipment necessary to characterize our barometer errors to this level of detail.

As a result, I think that slight tweaks in the altitude setting are just as good as an offset for our purposes...

[Meteorologica]

Only the Davis Vantage Pros to my knowledge uitilise the temperature in their sea level pressure calculation The WMRs simply use an offset so therefore you will be sending QNH to CWOP

[aweatherguy]

First of all, forgive me if I'm reading too much into your reply. I've been trying to figure out for some time exactly what the WMR100 is doing with the so-call "sea level pressure" readout.

1) It takes a very long time for a new altitude setting to update the SLP readout (several minutes). I'm guessing the WMR100 is either busy with some calculation and/or collection of data to be used in the process.

2) I have tried known formulas for reducing station pressure to both sea level pressure (SLP) and altimiter setting (QNH) and neither result matches the WMR100's reading.

If you know exactly what the WMR100 is doing here, please enlighten me with a formula, URL or some kind of explanation. It would be much appreciated!

[Weather Display]

the wmr100 does not update the barometer all that frequently on the display though (not sure of the interval, maybe every few minutes ?)
(the wmr968 was only every 15 minutes)

[aweatherguy]

Thanks for solving one mystery! I looked back through my weather logs and sure enough, pressure never changes more frequently than every 15 minutes. So this is why changing altitude on the WMR100 console takes a while to have any effect.

The only mystery left is what are they doing to reduce station pressure to whatever they call "sea level pressure"? As noted above, none of the formulas I've been able to find can duplicate the WMR100's "SLP" display...

[aweatherguy]

1) Clarification -- I was partially wrong in stating that airports report only QNH -- if you look at a METAR report, the QNH setting is identified with the letter "A". Often, sea level pressure is also reported and identified with "SLP" 

2) I think I know what is going on with the WMR100's "SLP" reading. I have been able to duplicate the WMR100 SLP reading if I use the reported station pressure and the INSIDE TEMPERATURE reading. So -- if I am correct in this conclusion -- then I think this is wrong on the part of Oregon Scientific. The SLP calculation needs to estimate the virtual temperature of a column of air between the station and sea level -- and that should be done using the OUTSIDE temperature, not the INSIDE temperature.

Does this make sense to any other experts out there?

[stevebrtx]

I've dredged this thread up from the dead because it deals in detail about sea level and raw barometric pressures. I've read and read and something is slipping by me on this whole subject, I'd appreciate some help.

Why is the barometric pressure higher as you gain altitude? Before I do an alt correction my 200 reads something like 25.XX and then with correction we're up in the 30.xx range which seems likely. So, to get VWS to match I need a 5.02 Calibration factor. This morning I reset VWS to 0.00 and about an hour later the 200 is showing LLL and as soon as I re-entered the 5.02 calibration it reverted to 30.21 at the moment.

Now, how is what I'm putting in VWS effect the 200? And again, what am I missing in the raw/SL pressure differences?

Thanks