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Author Topic: Davis Barometer Drift With Temperature  (Read 913 times)

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Offline JEB_Tucson

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Davis Barometer Drift With Temperature
« on: January 12, 2019, 07:36:31 PM »
For those interested, I'm posting a study looking at Davis barometer drift as a function of temperature. See Power Point attachment. Let me know if you have questions or file format issues.

Thanks,
James
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Offline niko

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Re: Davis Barometer Drift With Temperature
« Reply #1 on: January 12, 2019, 08:32:13 PM »
So what's your conclusion?

Offline JEB_Tucson

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Re: Davis Barometer Drift With Temperature
« Reply #2 on: January 12, 2019, 09:27:57 PM »
Hello Nico

The purpose of the study was just to determine the pressure drift WRT temperature, specifically pressure differences. That's what the study did for a range of temperatures and local pressures. Many readers will draw different conclusions based on how they want to use this data. If the study results are meaningless to a reader then that reader should ignore the study.

For my purposes, the Mean Pressure Differences shown on each graph indicate that my Vue and ASI DBX2 are calibrated the best that one can do given a 0.1 mbar calibration step. If the magnitude of the Mean Pressure Difference was greater than 0.05 mbar then a 0.1 mbar calibration step should be made.
Since I send the Vue pressure to CWOP, I desire to keep the error in the reported pressure to 0.1 mbar or less. Others may have different error criteria.

I use my ASI DBX2 as a reference barometer for calibrating the Davis Vue console. By knowing the temperature of the Vue console, I can estimate the range of pressure differences that I could see between the calibrated DBX2 and the calibrated Vue.

I hope that answers your question,
Thanks,
James
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Offline niko

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Re: Davis Barometer Drift With Temperature
« Reply #3 on: January 12, 2019, 09:49:40 PM »
I see, can I extrapolate that to conclude that the Vue baro meets its specification?

Offline JEB_Tucson

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Re: Davis Barometer Drift With Temperature
« Reply #4 on: January 12, 2019, 10:40:07 PM »
Hello Nico

The Vue, GE Druck graph indicates that "my" Davis Vue console pressure transducer in operating well within the Davis specification of +-1.0 mbar. You can also conclude this from the Vue, ASI DBX2 graph  Of course, the Vue and DBX2 barometers get periodically single-point calibrated (due to long-term drift) to insure they meet specifications. The Vue console is setup to send altimeter/ QNH pressure, so the data going to CWOP is correct.

I should say that the GE Druck Barometer introduced no significant error into the study.

Thanks,
James
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Offline niko

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Re: Davis Barometer Drift With Temperature
« Reply #5 on: January 12, 2019, 10:54:11 PM »
Interesting, thanks  :)

Offline niko

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Re: Davis Barometer Drift With Temperature
« Reply #6 on: January 12, 2019, 11:57:39 PM »
These measurements are all indoors - correct? How do you ensure that the air pressure in the test location isn't affected by e.g. forced air heating/cooling, or wind and open windows/doors etc.? Does there need to be a correction because the air inside is locally at a different (higher or lower) temp than the outside air?

Offline JEB_Tucson

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Re: Davis Barometer Drift With Temperature
« Reply #7 on: January 13, 2019, 03:07:42 AM »
Hi Nico

Yes, all measurements are indoors.

The test was not affected by other factors because pressure differences were used.
Also, the pressure from home forced air handlers are measured in IWC, inches of water. atmospheric pressure is measured in inches of mercury. Big difference. Assuming your air handler has a return duct, the net interior pressure is a fraction of a inch of water. This is insignificant when compared to the pressure from 20 to 30 inches of mercury. 

No correction needs to be made in the study to compensate for indoor/outdoor temperature differences.

Nico, I'm curious if you own and operate a personal weather station that reports to NWS through CWOP?

Thanks,
James
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Offline niko

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Re: Davis Barometer Drift With Temperature
« Reply #8 on: January 13, 2019, 04:03:14 AM »
The test was not affected by other factors because pressure differences were used.
Got it, poorly expressed. I guess I was looking past the differential sensor test to the final objective of demonstrating reporting atmospheric pressure to an accuracy of 0.1mb.

Quote
Also, the pressure from home forced air handlers are measured in IWC, inches of water. atmospheric pressure is measured in inches of mercury. Big difference. Assuming your air handler has a return duct, the net interior pressure is a fraction of a inch of water. This is insignificant when compared to the pressure from 20 to 30 inches of mercury.
Although the net interior pressure is very small is there not a potential issue with local pressure increases? Most systems that I have owned or seen have a single central return register. Surely there is a local variation in pressure in a closed, or semi-closed, room with only supply registers. After all the study is designed to support reporting to an accuracy of 0.1mb which is only 0.04 inches of water.

Quote
No correction needs to be made in the study to compensate for indoor/outdoor temperature differences.
In the differential study I agree. My question relates to effect on the accuracy of the reported pressure of air which is heated or cooled vs the outdoor air (since we are considering a very precise measurement).

Quote
Nico, I'm curious if you own and operate a personal weather station that reports to NWS through CWOP?
Own, yes, for many years, report to CWOP, no. I have an official airport station a couple of miles away. My data has always tracked has very closely with that station and I can't hope to compete with the accuracy of several $K of equipment with Davis equipment, so for geographical and technical reasons I don't see that sending it to CWOP would have any real value.

Offline JEB_Tucson

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Re: Davis Barometer Drift With Temperature
« Reply #9 on: January 13, 2019, 06:12:00 AM »
Hello Nico

*"Although the net interior pressure is very small is there not a potential issue with local pressure increases? Most systems that I have owned or seen have a single central return register. Surely there is a local variation in pressure in a closed, or semi-closed, room with only supply registers. After all the study is designed to support reporting to an accuracy of 0.1mb which is only 0.04 inches of water."*

To the extent that air handler pressure effects barometer pressure, the study used pressure differences so those effects are cancelled. The degree that I attempt to calibrate my barometer was not the subject of the study and is not the subject of this post, but, to answer your question, I do not apply corrections for air handler, wind surface pressure on the structure, open or closed windows, return duct distance from the barometer, etc. Neither does the NWS or the FAA.

*"In the differential study I agree. My question relates to effect on the accuracy of the reported pressure of air which is heated or cooled vs the outdoor air (since we are considering a very precise measurement)."*
Again this is beyond the scope of this post, but, I'm sure there is a small effect. I believe the NWS and FAA systems consider air field temperature in their QNH reductions. As I understand it, if the pressure sensor is at the same altitude as the reference runway point, then air field temperature is not used and the sensor's pressure is "standard atmosphere" reduced to calculate QNH. In other words QFE equals local measured pressure. If the pressure sensor is at a different altitude than the reference runway point then the altitude difference and the air field temperature are used to reduce the local measured pressure to a QFE pressure. Then this QFE pressure is "standard atmosphere" reduced to sea level for the final QNH value.  I hope this helps.

Thanks,
James
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« Last Edit: February 03, 2019, 05:29:06 PM by JEB_Tucson »

Offline niko

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Re: Davis Barometer Drift With Temperature
« Reply #10 on: January 13, 2019, 10:50:11 PM »
Thank you for your time  :) I understand very well the scope of your project now.

Offline JEB_Tucson

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Re: Davis Barometer Drift With Temperature
« Reply #11 on: January 14, 2019, 02:20:20 AM »
Thank you for your comments, Nico.
James
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Offline JEB_Tucson

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Re: Davis Barometer Drift With Temperature
« Reply #12 on: January 17, 2019, 06:00:09 PM »
Based on an interest to see Davis Vantage Vue pressure druft data down to 68 deg. F, I've updated the 1st order Vue chart. See attachment.

Thanks,
James
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Offline JEB_Tucson

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Re: Davis Barometer Drift With Temperature
« Reply #13 on: January 20, 2019, 05:57:33 PM »
Hello all

I have been asked to clarify the drift. Based on the latest data, the barometer drift with respect to temperature are:

Davis Vantage Vue: -0.0216 mbar/deg. F
ASI DBX2: +0.0141 mbar/deg. F

Thanks,
James
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Offline niko

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Re: Davis Barometer Drift With Temperature
« Reply #14 on: January 23, 2019, 04:03:14 AM »
Also, the pressure from home forced air handlers are measured in IWC, inches of water. atmospheric pressure is measured in inches of mercury. Big difference. Assuming your air handler has a return duct, the net interior pressure is a fraction of a inch of water. This is insignificant when compared to the pressure from 20 to 30 inches of mercury.

I ran an experiment and the worst case pressure increase (closed room at the start of the ductwork from an HVAC unit with two inlet registers and no return) was 0.2 to 0.3 hPa. So I finally wonder no more about this  :)