Author Topic: Adjusting T/RH offsets for enclosures  (Read 637 times)

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Offline kobuki

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Adjusting T/RH offsets for enclosures
« on: July 05, 2017, 12:25:40 PM »
This forum might not be the best place to ask this, but it's directly related to Davis stations and I'm hoping that weather experts/meteorologists might give some hints or a common, working solution.

So, I'm using my own receiver with WeeWx with my VP2 ISS and anemometer transmitter kit. I also have Davis Weather Envoy. I observed that the T/H sensor inside my small sensor box reads about 2.22C (4F) higher than the internal temp of the Envoy, and RH reads 4..8% lower. I applied a simple -2.22C (-4F) offset to temp and they're now are 0.1C apart at most, any time. Good, we're within sensor accuracy and even better. However, applying a simple positive offset to the RH doesn't work, the difference fluctuates too much to be able call it an accurate value. It's closer to reality but too far yet.

Looking for a solution, I made an attempt co actually calculate the RH from the offsetted temperature (Tnew) and a dew point calculated from the original temperature (T). In short, the algorithm:

1. calculate Tnew from the original temp T by adding the offset
2. calculate dew point DP using T
3. calculate adjusted RH RHnew using Tnew and DP

The exact calculation method is detailed on this page.

Now RHnew follows the Envoy's RH value within 0..2%. This should be acceptable since both sensors have a datasheet accuracy of 3%, but worst case the error could be 5%. While this still looks somewhat acceptable, I have a limited way to test it in indoors environment, where temperature swings aren't too great, at most 10C or less. So I'm not 100% convinced this is the Right Way.

Any hints?
« Last Edit: July 05, 2017, 12:27:40 PM by kobuki »

Offline alexstaar

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Re: Adjusting T/RH offsets for enclosures
« Reply #1 on: July 06, 2017, 01:20:46 AM »
Your method of calculating dewpoint to find the new (actual) RH given the measured and actual temperatures and measured RH should be valid and correct, assuming you are not dealing with condensation or evaporation of water vapor (i.e. RH = 100%, etc.). I'm assuming that this setup is inside, so that shouldn't be a problem. This would be an elementary thermodynamics problem for someone learning thermodynamics, so it's pretty simple. Good critical thinking skills! =D>

Sources: an Atmospheric Science degree ;)
-Alex

Davis 6153 VP2 with 24-hour FARS/
Davis 6250 Vantage Vue
WU: KNJMOUNT68/KSCGREER81
CWOP: FW5135/EW6390
www.uah-mcthornmoracres-weather.com

Offline kobuki

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Re: Adjusting T/RH offsets for enclosures
« Reply #2 on: July 06, 2017, 03:17:38 AM »
Thanks for the confirmation, alexstaar!

It might be elementary, but I don't have a degree in Atmospheric Science ;) I'm interested in the calculation details though, so I'll dig into that later.