Author Topic: Humidity readings too high w/ new VP2  (Read 3190 times)

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Offline dtcatTX

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Humidity readings too high w/ new VP2
« on: September 30, 2008, 06:57:35 PM »
Hello.  I recently purchased a Vantage Pro 2.  It arrived in the mail about 2 weeks ago.  The shipping box was a little banged up, so I was wondering if the unit or console was damaged.  Physically, there is no damage.  So, I then wandered about the accurateness of the equipment.  Basically, I have consistently been getting much higher humidity readings than my three other smaller thermometer-hygrometer units - higher by 10%.  I also found a fire station near my house that uses a Davis Vantage Pro, and I brought my unit and console to compare.  Again, my humidity readings were higher by 4-5%.  The fire station's unit was up on a pole above the one story building, and I was only able to place my unit on a 6 foot ladder.  There was cement all around, so this was not ideal conditions for getting an accurate reading...  However, the environments were pretty constant.

Is my concern valid?  Does anyone have suggestions or input?  I do have a 30 day return with this station..  Should I send it back?

Thanks, Debbie

Offline Weather Display

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Re: Humidity readings too high w/ new VP2
« Reply #1 on: September 30, 2008, 07:15:19 PM »
I would recommend using the dew point reading instead when doing comparisons
(as humidity is relative)
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Offline mmorris

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Re: Humidity readings too high w/ new VP2
« Reply #2 on: September 30, 2008, 08:01:53 PM »
Probably if the unit turn on and light up and if the rain bucket is work as that is the only part of the unit that has any moving parts along with the wind dir and speed. As far as the other reading being right or the same as your other units you own I can't say that they are right either and maybe the fire station is a little off who knows.
Probably if you don't like it you should send it back as you will always think something is wrong. If you need a unit that is NIST-TRACEABLE then if you look in the catalog on Page 20 there is a paragraph in yellow that explains how you go about getting NIST-Traceable Sensors.

As I have a brand new VP2 plus well maybe 6 month old unit and mine seem to be accurate enough for me for an off the shelf factory made weather station. I also owned a wmII for many year and when I had both units running at the same time the reading didn't match at every moment of the day it was not worth it to send the VP back. And now that I only have the VP running I don't see any difference at all..
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Offline mackbig

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Re: Humidity readings too high w/ new VP2
« Reply #3 on: September 30, 2008, 08:37:52 PM »
In its simplest form dewpoint is just a function of temperature and relative humidity.  So if the comparable stations have the same temperature reading, the dewpoints would be off as well if the humidity reading is wrong.

Andrew

I would recommend using the dew point reading instead when doing comparisons
(as humidity is relative)

Andrew - Davis VP2+ 6163, serial weatherlink, wireless anemometer, running Weather Display.  Boltek PCI Stormtracker, Astrogenic Nexstorm, Strikestar - UNI, CWOP CW8618, GrLevel3, (Station 2 OS WMR968, VWS 13.01p09), Windows 7-64

Offline floodcaster

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Re: Humidity readings too high w/ new VP2
« Reply #4 on: September 30, 2008, 08:45:27 PM »
Can you compare dewpoints with a sling psychrometer? My readings appeared high after getting my VP2+ so I purchased a sling for comparison and they matched closely. I have a few small lakes not far and my readings accurately represent my micro climate compared to the nearest automated awos about 7 miles away.

Are you other units different brands and are all located near the VP2? Siting of the equipment can make a big difference. The VP2 may be more accurate than your other sensors. The moisture is normally higher closer to the ground (under calm or light wind conditions with little mixing of the air) so perhaps your test at the fire station demonstrated this(?).

I do applaud you for your interest in getting the most accurate readings possible. Best of luck.

Bill


Offline andro700

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Re: Humidity readings too high w/ new VP2
« Reply #5 on: September 30, 2008, 09:00:11 PM »
My humidity is reading high as well according to CWOP. I am in a rural area and the airport sits on top of a hill and are in the wide open. I am about 2 blocks for the Yellowstone River. Another giveaway is when my humidity is high my grass and other things are wet with dew so I think my readings are ok.

Offline racenet

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Re: Humidity readings too high w/ new VP2
« Reply #6 on: September 30, 2008, 09:01:17 PM »
As already stated, if the unit itself isn't smashed, it powers up and all moving parts work fine, I wouldn't worry about it. Put it up and enjoy monitoring the weather.




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Offline SLOweather

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Re: Humidity readings too high w/ new VP2
« Reply #7 on: September 30, 2008, 09:03:48 PM »
From the Davis site, VP2 humidity specs:

Resolution and Units . . . . . . . . . . . . . . . . . . . . . . . . 1%
Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 to 100% RH
Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±5%

That's at full scale, so RH can vary by a potential of ±6 full % and still be OK. The fire station can be off by the same amount, so your station and theirs could, in theory, disagree by 10-12% and both be in spec.

I wouldn't fret it. RH is measured by a capacitive sensor, and compensated by the temp sensor (it takes both to make the RH calc). So, you need to make sure that the temps match to compare RH directly. As GBW suggested, dew point would be a better parameter to compare, as it's calculated from RH and Temp and should be the same for slightly differing temps in the same area.

If you really care, buy, borrow or make a dry bulb/wet bulb psychrometer, and check it that way.

Here's a C based explanation and table:
http://www.nasa.gov/centers/langley/pdf/245887main_MeteorologyTeacherRes-Ch11.r3.pdf

And an F based one:
http://www.erl.dot.state.ia.us/Apr_2008/IM/content/378.pdf

Offline dtcatTX

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Re: Humidity readings too high w/ new VP2
« Reply #8 on: October 03, 2008, 05:19:26 PM »
Thanks to everyone who responded.  I did not realize that the humidity readings had a plus/minus 5% accuracy.  I would have thought my readings would be more right on target with the Vantage Pro 2...

Anyway, I found a few weather stations near where I work (via Wunderground), and I went and set up my thermometer/hygrometer/raingauge on a 4 foot ladder either next door or across the street and compared my reading to theirs.  Mine were pretty much right on target with an Oregon Scientific and another Vantage Pro 2.  I think I was worrying for nothing.

Thanks again!   Debbie

From the Davis site, VP2 humidity specs:

Resolution and Units . . . . . . . . . . . . . . . . . . . . . . . . 1%
Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 to 100% RH
Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±5%

That's at full scale, so RH can vary by a potential of ±6 full % and still be OK. The fire station can be off by the same amount, so your station and theirs could, in theory, disagree by 10-12% and both be in spec.

I wouldn't fret it. RH is measured by a capacitive sensor, and compensated by the temp sensor (it takes both to make the RH calc). So, you need to make sure that the temps match to compare RH directly. As GBW suggested, dew point would be a better parameter to compare, as it's calculated from RH and Temp and should be the same for slightly differing temps in the same area.

If you really care, buy, borrow or make a dry bulb/wet bulb psychrometer, and check it that way.

Here's a C based explanation and table:
http://www.nasa.gov/centers/langley/pdf/245887main_MeteorologyTeacherRes-Ch11.r3.pdf

And an F based one:
http://www.erl.dot.state.ia.us/Apr_2008/IM/content/378.pdf

 

anything