<p>I am attempting to calculate wet bulb temperatures needed for agricultural curing processes. I am trying to do this by inferring the wet-bulb value from relative humidity and dry bulb temperature.</p>
<p>The problem is this: My measured relative humidity using a shtetl 30 sensor is about 45% when, according to the 4 degree difference between dry (28 deg) and wet bulb (24 deg) , it should be reading 75%.</p>
<p>What am I missing? Why is my relative humidity sensor getting it so wrong?</p>
<p>I tried another sensor brand (albeit with the same RH method of measurement) and likewise got humidity in the region of 45% when it should have read 75%.</p>
<p>Has anyone else experienced this issue? If so, how did you resolve it?</p>
<p>Many thanks in advance!</p>

CC BY-SA 4.0
2021-06-09T09:51:00.683Z
I can confirm the relative humidity for the temperatures you specified should be around 75%. My spreadsheet gives a value of 70%. Do the sensors need to be calibrated prior to using them?
Fred
CC BY-SA 4.0
2021-06-09T14:57:04.393Z
@Fred thanks for the response - the SHT30 claims to be calibrated "out of the box" so I dont' understand what's going wrong. I find it hard to believe that it's 30% out or that there is just a fixed amount that I have to adjust it by. Thanks for confirming the RH I should expect.
Hayden Eastwood
CC BY-SA 4.0
2021-06-10T08:50:43.990Z
these may be of interest but may not be directly helpful here: [How do the pros measure humidity?](https://earthscience.stackexchange.com/q/19152) and [How is relative humidity determined from a wet and dry bulb readings?](https://earthscience.stackexchange.com/q/19151) and [How does a whirling hygrometer work, how does it differ from static dry/wet bulb thermometer, and how to obtain humidity from its readings?](https://earthscience.stackexchange.com/q/21198) and [How is the specific heat of a wet bulb determined?](https://earthscience.stackexchange.com/q/15349)
uhoh
CC BY-SA 4.0
2021-06-10T08:51:08.053Z
and [How would I use data to find specific humidity and mixing ratio?](https://earthscience.stackexchange.com/q/13511)
uhoh
<humidity><environmental-sensors>
Incorrect relative humidity with SHT30 when comparing to dry and wet bulb calculation
Hayden Eastwood
# Incorrect relative humidity with SHT30 when comparing to dry and wet bulb calculation

(Asked by: username_0 on 2021-06-09)

I am attempting to calculate wet bulb temperatures needed for agricultural curing processes. I am trying to do this by inferring the wet-bulb value from relative humidity and dry bulb temperature.

The problem is this: My measured relative humidity using a shtetl 30 sensor is about 45% when, according to the 4 degree difference between dry (28 deg) and wet bulb (24 deg) , it should be reading 75%.

What am I missing? Why is my relative humidity sensor getting it so wrong?

I tried another sensor brand (albeit with the same RH method of measurement) and likewise got humidity in the region of 45% when it should have read 75%.

Has anyone else experienced this issue? If so, how did you resolve it?

Many thanks in advance!

**Tags:** humidity, environmental-sensors

> **Comments:**
> **username_1**: I can confirm the relative humidity for the temperatures you specified should be around 75%. My spreadsheet gives a value of 70%. Do the sensors need to be calibrated prior to using them?  
> **username_0**: @username_1 thanks for the response - the SHT30 claims to be calibrated "out of the box" so I dont' understand what's going wrong. I find it hard to believe that it's 30% out or that there is just a fixed amount that I have to adjust it by. Thanks for confirming the RH I should expect.  
> **username_2**: these may be of interest but may not be directly helpful here: [How do the pros measure humidity?](https://earthscience.stackexchange.com/q/19152) and [How is relative humidity determined from a wet and dry bulb readings?](https://earthscience.stackexchange.com/q/19151) and [How does a whirling hygrometer work, how does it differ from static dry/wet bulb thermometer, and how to obtain humidity from its readings?](https://earthscience.stackexchange.com/q/21198) and [How is the specific heat of a wet bulb determined?](https://earthscience.stackexchange.com/q/15349)  
> **username_2**: and [How would I use data to find specific humidity and mixing ratio?](https://earthscience.stackexchange.com/q/13511)  

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## 0 answers

