[wxqc] Vantage-Pro Dew Point Calibration Question

Thu Nov 16 08:08:03 CST 2006

Hmm, what would be interesting is if the fan would switch on when the wind
was recorded under 2mph or so. The fan then would only run when the air was
too still to mix in the sensor compartment correctly. Mixing of air can
produce a tad of heat through friction and heat generated by the motor, but
I am not sure if it would be to any degree to affect readings. It is
probably very minor.

On the side of humidity sensors though, I don't know of any that work well
below 10% and above 95% humidity. In the extreme parts of their sensing
ranges they become largely non-linear and much effort has to be taken from
the embedded code of the console to extrapolate what the sensor is
reporting. The difference in voltage output from a sensor between 95% and
100% may be very small, in the order of only a few bits difference on the
output of an A/D converter. In other words I am surprised we get any valid
measurement between 95% and 100% on the VP, but it seems they have done a
good job in attacking that problem.

-LS

  _____  

From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Lance
Sent: Thursday, November 16, 2006 3:05 AM
To: wxqc at lists.gladstonefamily.net
Subject: Re: [wxqc] Vantage-Pro Dew Point Calibration Question

Hi Evan

Interesting idea about the fan aspiration. I too have a 24hr fan aspirated
unit. If the air weren't fully saturated across a large area, the movement
of air across the sensor in pockets of varying levels, might not ever allow
the sensor to achieve 100%. As Steve mentioned, he had extreme fog for an
extended period before reporting 100%.
It sure would be nice to have a remote on/off switch for that fan, it would
be interesting to see if there is really any difference.

Lance
CW1020
www.mountainbase.com

Evan Bookbinder wrote: 

In the ASOS realm, if the visibility is 6 miles or less, HZ or BR will 
be appended to the observation. ASOS cannot determine smoke and does not 
discriminate visibility reduction purely based on precipitation 
intensity (heavy rain or snow).

BR (mist -- visibility reduced by water vapor/droplets) will be used 
when the dewpoint depression is 5F or less and HZ (Haze -- visibility 
reduced by particulates) will be used for dewpoint depressions of 6F or 
above. FG (dense fog) will be used when visibilities of 1/2SM or less 
are observed.

Personally, I have never seen my RH sensor go above 95%, even in 0SM 
visibility in fog last Tuesday (our first fog event in over a year). I 
think part of the problem may be the fan aspirator which is inducing a 
circulation although the air in a radiation fog event is typically dead 
calm -- but since my dwpt/RH are almost always within 1F/1% of the 
nearby ASOS, I haven't considered it a problem. Some other 
manufacturers' humidity sensors are FAR less stringent and won't even 
report values below 20% or above 90%.

Evan

Dave Helms wrote:

In METAR reporting procedures, the general rule is you can report fog as 
"present weather" (e.g. restricting horizontal visibility to less than 7 
miles) with dew point depressions as great as 7 F degrees. As you found, 
it is not uncommon to observe fog where complete saturation is not 
reached.   I poked around hoping to find some statistics from an ASOS 
station on the frequency fog (types BR, MI, and FG) is reported as a 
function dew point depression and/or RH, but didn't find a source.  Any, 
no big deal to have fog and not fully saturate.  Then again, you should 
watch to see if your humidity sensor can saturate at all or too much as 
a potential problems.

Dave
CW0351

Steve Hatchett wrote:

We had a fog event that lasted quite a few days. It was extremely foggy, and
everything was dripping wet. I was convinced it could get no foggier or more
saturated than it was. But my VP2 refused to go above 99%. I was somewhat
disappointed, and literally on the verge of bumping the calibration up by a
percent when it proceeded to get even foggier and more saturated than I
thought possible and the station began reporting 100% humidity. It stayed
that way for several hours until a system moved through and ended the
inversion. Now I'm glad I didn't do the calibration adjustment.

Steve

-----Original Message-----
From: Keith Miller [mailto:kdmiller at oldsgmail.com]
Sent: Tuesday, November 14, 2006 11:49 PM
To: lwood at mountainbase.com; 'Discussion of weather data quality issues'
Subject: Re: [wxqc] Vantage-Pro Dew Point Calibration Question

-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net]On Behalf Of Lance
Sent: Wednesday, November 15, 2006 1:34 AM
To: wxqc at lists.gladstonefamily.net
Subject: Re: [wxqc] Vantage-Pro Dew Point Calibration Question

I have a question for the group.  Can you be socked in with
fog and not have 100% humidity? I have never seen it on my station,
maybe 98%. I am already getting hit for being too high and don't
want to adjust it up.

I've seen 100% http://weather.stadhaugh.com/record.php Although,
typically, I see 98% or 99% during a rainfall event...

Keith
--
CW5250 => http://weather.stadhaugh.com

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