Topic: VP2 dewpoint running high

[zachelliottwx]

Today I noticed my max heat index seemed really high. It's been hot, but not THAT hot. so i compared it to another site close to mine on Wunderground, and my Dewpoint was about 5 degrees higher than theres. I checked Gladstoneweather for my MADIS rating, and my dewpoint error is average 2 degrees. Temperature was less than a degree. Humidity was average -7.5% error with a 4.7% standard deviation. It's not good. Any ideas of why my dewpoint would be reading high and my humidity? Compared to the other sites, it's not sited any different. It is over native vegetation, but it's mowed regularly and it's been dry, so Evapotranspiration should not be locally higher. Could this be a sensor error? Is there anything I can do to the sensor, or should I just add a multiplier in wview to make the value closer to reality?

[dendrite]

Is this a change from the readings you normally expect or is it the first time you've looked and compared? A 2F dew error really isn't a lot for a home station...especially if you're using an older SHT11. If you've had the sensor for awhile it may be time to upgrade it to one of the newer SHT31s. Last I peeked they were only $45 at Scaled Instruments.

Are you E3101 on CWOP?

[zachelliottwx]

Yea i'm E3101. I looked at the Gladstone data and it appears the error has pretty much always been the same per the CSV over that past. I think it's really the first time i've looked. I've had the station for 3 years. I just noticed that my heat index seemed high so I decided to look. Everything else is great so I thought it was weird it was just humidity that was off.

[zachelliottwx]

Looking at the last 52 weeks, it appears my average dewpoint is actually within 2 degrees no problem. Perhaps i'm just overreacting a bit.

[dendrite]

Don't beat yourself too much comparing your data to other MADIS stations. Analyzing your MADIS error is almost another whole branch of science.

I ran about 2F warm for years until switching to the new SHT31 sensor. Now I'm under 1F consistently with lower std devs. More importantly, I'm more in line with the local ASOS stations during the afternoon. If it continues to bother you I'd definitely recommend the new sensor...you could always keep your current one as a backup in the case the new one eventually fails.

[dendrite]

Here's mine going back to Jan. Apr 16 is when I swapped the SHT15 out for the SHT31. There's a clear, consistent jump in the graph there around that date where my dewpoint began running cooler by about 1F. I used to think my exra few degrees was due to evapotranspiration too, but maybe most of it is just sensor accuracy. It'd be interesting to see if you could notice a difference between an SHT35 and SHT31 on the graph, but alas, we may be waiting awhile for that.

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[CW2274]

Don't beat yourself too much comparing your data to other MADIS stations. Analyzing your MADIS error is almost another whole branch of science.

Absolutely. My PWS is as good as it's ever been but now my Gladestone comparisons are as bad as they've ever been. Speaks volumes.

[ValentineWeather]

I'm believing in the SHT31 dp's after 2 hours heavy fog I was 98-99% humidity while the ASOS was 93% and fog was just as heavy. No better calibration test than that.

[WxLover16]

I'm believing in the SHT31 dp's after 2 hours heavy fog I was 98-99% humidity while the ASOS was 93% and fog was just as heavy. No better calibration test than that.

I've always noticed that most of the ASOS's in North GA almost never have humdity above about 93%, only in extreme circumstances. I wonder why that is? I know these ASOS's have much more expensive equipment, but are they really that much more accurate than a VP2 SHT31?

[CW2274]

I'm believing in the SHT31 dp's after 2 hours heavy fog I was 98-99% humidity while the ASOS was 93% and fog was just as heavy. No better calibration test than that.

I've always noticed that most of the ASOS's in North GA almost never have humdity above about 93%, only in extreme circumstances. I wonder why that is? I know these ASOS's have much more expensive equipment, but are they really that much more accurate than a VP2 SHT31?

My experience is that in extreme high and low humidities all sensors are compromised to a degree, including ASOS's. The SHT31 is by far the best at those extremes, including temp, not just humidity. Just about all sensors are good in the "sweet spot", but the 31 is as close to an ASOS as you can get with proper sitting, ect., ect., least I say sometimes better.

[WxLover16]

I'm believing in the SHT31 dp's after 2 hours heavy fog I was 98-99% humidity while the ASOS was 93% and fog was just as heavy. No better calibration test than that.

I've always noticed that most of the ASOS's in North GA almost never have humdity above about 93%, only in extreme circumstances. I wonder why that is? I know these ASOS's have much more expensive equipment, but are they really that much more accurate than a VP2 SHT31?

My experience is that in extreme high and low humidities all sensors are compromised to a degree, including ASOS's. The SHT31 is by far the best at those extremes, including temp, not just humidity. Just about all sensors are good in the "sweet spot", but the 31 is as close to an ASOS as you can get with proper sitting, ect., ect., least I say sometimes better.

Thanks for this response and all other responses you've helped me out with. You're a wealth of knowledge on this stuff and it's all much appreciated! I've learned alot from you!

[CW2274]

I'm believing in the SHT31 dp's after 2 hours heavy fog I was 98-99% humidity while the ASOS was 93% and fog was just as heavy. No better calibration test than that.

I've always noticed that most of the ASOS's in North GA almost never have humdity above about 93%, only in extreme circumstances. I wonder why that is? I know these ASOS's have much more expensive equipment, but are they really that much more accurate than a VP2 SHT31?

My experience is that in extreme high and low humidities all sensors are compromised to a degree, including ASOS's. The SHT31 is by far the best at those extremes, including temp, not just humidity. Just about all sensors are good in the "sweet spot", but the 31 is as close to an ASOS as you can get with proper sitting, ect., ect., least I say sometimes better.

Thanks for this response and all other responses you've helped me out with. You're a wealth of knowledge on this stuff and it's all much appreciated! I've learned alot from you!

That's what this forum is all about.