Humidity/dew Point Reading Too High?

[Evening thunder]

Hi, I have noticed that my Davis Vantage Pro2 the dew point and humidity are often a bit higher than local stations and the MetO one at Exeter Airport which I look at here. (There seems to be 2 stations there, the normal 'airport' one you see on XCWeather and this one which can be a bit different)

This seems to happen more when the dew point is not too close to the temperature, so yesterday with relatively high humidity this may not so be apparent, though there can still be a 1C or so difference. I have noticed often 1-2C, sometimes more like 3C difference. There is also a new Davis Vantage Vue on Weather Underground literally about a mile from me that also shows 1-2C lower than mine.

For example when I checked at 3pm on Wednesday Exeter's temp/dew was 22.4/12.0C where as mine was 22.4/14.5C (the temp happens to be exactly the same). I've also made checks through days and mine is consistently at least 1C, often 2C higher. The biggest difference I saw was Exeter's dew point at 8.4C, and mine at 12.9C, a 4.5C difference. This was also when the wind was a NW'erly (blowing from Exeter Airport towards me) and there wasn't a local phenomena like a sea breeze.

When this happens it can also fluctuate rapidly, sometimes by 2-3C. Could this occur naturally with pockets of more moist air?

I've probably rambled on enough now so being slightly obsessed with accuracy, what I am wondering here is, is there any way to check the readings or calibrate/fix this? I'm assuming there is not a 'set' inaccuracy throughout its range so you can't just apply a blanket 6% humidity correction for example. This of course means my high/low humidity and dew point records would be off, as would my (maybe not such a) record high dew point of 20.0C on the 27th of June, so I want to be reasonably accurate here.

I replaced the temp/hum sensor in August 2009 when it went faulty, so any issue would date back to then at the earliest or it may well have 'drifted' at a later date.

I could buy a replacement temp/hum sensor but I'm not sure if it's worth the money for this and I definitely want to make sure there’s no easier fixes first. This post has ended up a bit longer than I expected! if you get this far It would be interesting if anyone has ideas? Thanks for any replies.

Edited August 14, 2011 by Stormmad26

[Evening thunder]

I guess no one has any ideas they can help with? Though I guess there may be no real solutions except getting a new one.

[Paul]

It could be linked to the placement, is it near walls, fences, trees, bushes or anything?

[reef]

It could be that its correct. Airports by their very nature are places with lots of concrete and less greenery around for evaporation into the air. Theres also the microclimatic nature of a garden to take into account.

Those issues aside, as you mention the error margin on most non-met grade stations is generally +/-5-7% in the middle of the humidity range and higher at the top and bottom. I wouldnt take any notice of the Vantage Vue readings nearby either, as due to their design they do tend to show higher temps / lower humidity during the day when the sunshine is out as it heats the casing.

On my VP2 I do struggle to record extremely low humidities (Ive never seen below 30%) and rarely does it go above 96%, however on the whole I find it tracks local official stations reasonably well.

[4wd]

You can calibrate the settings at the console with the Vue, presumably this is also possible with VP2?

If you don't have the manual I expect it can be downloaded from davis.net

Temperature and humidity can vary surprisingly depending on local micro climate though.

But if it consitently reads a bit higher than others nearby in all conditions it is suspicious.

[Evening thunder]

Thank you for the replies above. I don't think it's placed anywhere that should affect it really, it's on a pole nearly a metre above a fence bordering the garden and open fields, but there's free air flow.

Thanks reef - That's true what you say about airports/gardens and the Vantage Vue - I have tried to make sure it's not a day with local factors like a sea breeze and it's been cloudy quite often with a decent breeze (not off the sea). I've occasionally noticed a 10% or so humidity difference but usually it's a bit lower. I tend to record up to 98/99% and my lowest this year is about 39%.

Thanks 4wd, I know you can calibrate it by setting an offset i.e 5%, but I don't think the error is the same for different humidities, like at the moment it's 94% seems in line with others, and I'd never record above 93-94% with a 5% offset for example. Apart from high humidities >90% it does tend to constantly read higher.

With your replies in mind I think getting a new sensor may be a bit over the top for now, and I'll have to see how it goes and live with it probably being a bit high.

Edited August 20, 2011 by Stormmad26