Author Topic: VP2 6163 outside temp too high  (Read 2589 times)

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Offline Fuzzy

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VP2 6163 outside temp too high
« on: October 17, 2011, 03:02:01 AM »
HI

J have problem with my VP2 beacause it is shows too high outside temperature. J observe station from few days. In POland last few mornings was little frozen about -1C maybe -2C. when J go at morning to my VP2 in my yard, FARS and rain collector is completly frozen but on console it is 0,5- 0,8C. What can be a problem? What can J do. J thought that davis VP2 is good station but my technoline ws3650 shows better temperature even cheap station for few dollars have better data. J`m very disappointed.
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Offline kered

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Re: VP2 6163 outside temp too high
« Reply #1 on: October 17, 2011, 03:10:15 AM »
The temperature doesn't have to go to 0.0şC to freeze or show frost, anywhere from 3şC +/- downover depending on the wind. Maybe its correct but just feels colder, ie windchill
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Online johnd

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Re: VP2 6163 outside temp too high
« Reply #2 on: October 17, 2011, 03:23:35 AM »
Why do you think that there's necessarily anything wrong? As you make more careful measurements, so it reveals more about the physics of what we call temperature as it relates to different materials.

I can't make out from your post whether your FARS fan was spinning or not, but assuming that it was then the VP2 would certainly have been measuring air temperature, whereas the rain gauge material would probably have cooled below air temperature on what I'm guessing was probably a clear (ie non-cloudy) night, because of loss of heat radiated (ultimately) into space. Put another way, the rain gauge and the surrounding air were not in temperature equilibrium and your VP2 was measuring what it was designed to measure, ie _air_ temperature.

None of this rules out absolutely that there might be a temperature error on the VP2, but radiation cooling of surfaces is a more likely explanation.

BTW, unwetted surfaces in temperature equilibrium cannot experience wind chill.
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Offline Fuzzy

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Re: VP2 6163 outside temp too high
« Reply #3 on: October 17, 2011, 04:40:55 AM »
Everything in my yard was frozen. For sure air temp was below 0. Accuracy +/- 1C for such good station is embarrassing. Do you think that J should calibrating temp on console?
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Offline Flag

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Re: VP2 6163 outside temp too high
« Reply #4 on: October 17, 2011, 05:42:35 AM »
Everything frozen in the yard does not imply you are assuming things correctly. Has to be some more facts rather than impressions.


Offline Fuzzy

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Re: VP2 6163 outside temp too high
« Reply #5 on: October 17, 2011, 06:02:12 AM »
OK.More facts. J work in chemical factory, in my labo we have thermohigrometr with certicate. when J compare two dates in certificate was 1,2C lower then Davis. It means that Davis show me about 1,2C higher.
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Offline DanS

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Re: VP2 6163 outside temp too high
« Reply #6 on: October 17, 2011, 07:28:48 AM »
Just out of curiosity, does the sun shine directly on the radiation shield when you are seeing this temperature difference?

Offline Fuzzy

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Re: VP2 6163 outside temp too high
« Reply #7 on: October 17, 2011, 07:31:27 AM »
When J check temperature is night or almost dark eventualy in full shadow
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Offline DanS

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Re: VP2 6163 outside temp too high
« Reply #8 on: October 17, 2011, 07:41:57 AM »
HI

J have problem with my VP2 beacause it is shows too high outside temperature. J observe station from few days. In POland last few mornings was little frozen about -1C maybe -2C. when J go at morning to my VP2 in my yard, FARS and rain collector is completly frozen but on console it is 0,5- 0,8C. What can be a problem? What can J do. J thought that davis VP2 is good station but my technoline ws3650 shows better temperature even cheap station for few dollars have better data. J`m very disappointed.


The reason I asked was with your opening post you mentioned mornings and I had the picture of sunshine possibly being a factor.

Online johnd

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Re: VP2 6163 outside temp too high
« Reply #9 on: October 17, 2011, 09:02:29 AM »
Everything frozen in the yard does not imply you are assuming things correctly. Has to be some more facts rather than impressions.

Agreed.

What height is the VP2 ISS at - could be an important factor?

What's the difference in exposure between the work and home locations? It's commonplace for home environments to be more enclosed and generally slightly warmer than an industrial work environment - certainly enough to see a 1-2degC difference in minimum temps. I run a VP2 in a relatively open but semi-urban back garden and also another system just a few km away in a much more open field environment. On a cold night, it's typical for the field minimum to be 1-2 degC colder, sometimes a 3-4 degC differential. And the accuracy of both systems has been checked as within the 0.5degC spec for the VP2.
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Offline Fuzzy

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Re: VP2 6163 outside temp too high
« Reply #10 on: October 17, 2011, 09:08:22 AM »
Johnd

It`s misunderstanding. J take this thermohigrometr from factory to my yard and J make a measurement 10cm from Davis FARS. J do it 10 times.
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Online johnd

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Re: VP2 6163 outside temp too high
« Reply #11 on: October 17, 2011, 09:19:28 AM »
It`s misunderstanding. J take this thermohigrometr from factory to my yard and J make a measurement 10cm from Davis FARS. J do it 10 times.

OK. The situation is starting to become clearer.

Next question then would be to ask you please to describe in a little more detail how you make the measurement with the reference thermometer. Is the ref sensor element also in an aspirated radiation shield, for example, or a bare sensor probe?

A difference in measured temperature of around 1 degC is not large and might be explained either by the nature of the reference probe or just by opposing errors - eg the VP2 might be reading say 0.5C high and the reference sensor low. I understand that the ref one was calibrated but to what accuracy and how long ago?
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Littleport, Ely, Cambs UK

Sorry, but I can't help with individual issues by email. Please post your issue in the relevant forum section here & I will comment there if I have anything useful to add.

Offline Fuzzy

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Re: VP2 6163 outside temp too high
« Reply #12 on: October 17, 2011, 09:37:13 AM »
J give you link how lokk this thermohigrometr

http://www.label.pl/po/rek704.dtrh.html

calibration was one year ago
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Online johnd

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Re: VP2 6163 outside temp too high
« Reply #13 on: October 17, 2011, 10:47:09 AM »
J give you link how lokk this thermohigrometr...


OK, again that helps a little. But how did you make the measurement? How long was the sensor left outside for? Was the sensor vertical or horizontal - it coud make a difference?

I suspect that your reference thermometer is not a unit that's really designed for measuring outside minimum temperatures - it doesn't really have any substantive radiation shield (which stops the sensor element getting too cold at night, as well as too hot in full sun). My best guess is that if left outside for any significant length of time then the body of the sensor unit will cool down below the ambient air temperature and the temperature sensor will be reporting the temperature of itself rather than the surrounding air and so will tend to under-record true air temperature by at least a little.

But I repeat: You're trying to understand an apparent discrepancy of only around 1 degC or slightly more between two thermometer of quite different types, which you're aiming to use to estimate air temperature, which your reference thermometer AFAICS is not designed to do. Just 1 degC is quite a small difference but it just so happens in this particular case that it's straddling freezing point, which perhaps gives it more significance in your mind that it really deserves.
Prodata Weather Systems
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Sorry, but I can't help with individual issues by email. Please post your issue in the relevant forum section here & I will comment there if I have anything useful to add.

Offline kered

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Re: VP2 6163 outside temp too high
« Reply #14 on: October 20, 2011, 06:29:11 AM »
Why do you think that there's necessarily anything wrong? As you make more careful measurements, so it reveals more about the physics of what we call temperature as it relates to different materials.

I can't make out from your post whether your FARS fan was spinning or not, but assuming that it was then the VP2 would certainly have been measuring air temperature, whereas the rain gauge material would probably have cooled below air temperature on what I'm guessing was probably a clear (ie non-cloudy) night, because of loss of heat radiated (ultimately) into space. Put another way, the rain gauge and the surrounding air were not in temperature equilibrium and your VP2 was measuring what it was designed to measure, ie _air_ temperature.

None of this rules out absolutely that there might be a temperature error on the VP2, but radiation cooling of surfaces is a more likely explanation.

BTW, unwetted surfaces in temperature equilibrium cannot experience wind chill
.

I know that but what i said was "Maybe its correct but just feels colder, ie windchill " as in wind in the face comming out of a warmer enviroment
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