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Humidity accuracy

Thu Sep 15, 2016 8:10 am

I have seen lots of posts and advice over Temperature accuracy, I have my sensehat on a ribbon cable extension....

I have some reason to suspect that my humidity reading is too high. Around 15%. However I cannot prove it. I have cheap humidity instruments to compare the values, but they can't agree, either.
It is working, i.e. when our wonderful country does its thing of torrential downpours, the humidity increases, I just think it is too high for indoors. getting paranoid about damp....whatever happened to warm dry summers?

Can anyone suggest a cheap / accurate way of checking and calibrating a humidity instrument?


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Re: Humidity accuracy

Thu Sep 15, 2016 10:08 am

This is not very reliable, but you could try to keep it outside for a while, record humidity for that period, and then compare to the nearest station of your local weather service if that data is available. This will at least give you a rough idea of whether the trend is correct - you'll probably not get an exact match though, due to local effects, even *if* your sensor is accurate.

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Re: Humidity accuracy

Thu Sep 15, 2016 10:25 am

Also see a not very scientific comparison between the Si7021, DHT22, and DHT11.


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Re: Humidity accuracy

Thu Sep 15, 2016 10:47 am

Wow! Thanks for your answers, There is a lot to take in.... maybe a short delay while my brain warms up....

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Re: Humidity accuracy

Thu Sep 15, 2016 12:13 pm

Simple and cheap solution to calibrate humidity measuring systems to make special salt solution and above the closed volume over the solution a given relativ humidity will be formed. Try google or physical chemistry books for salt containts and concentrations.
on a Raspberry B+ with whezzy, two Zero with Jessie Light

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Re: Humidity accuracy

Fri Sep 16, 2016 8:35 am

Just proves that everything in computing ( everything else ?) is knowing what question and who to ask.....found these.... ... /salt-test ... 17c0afd309

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Re: Humidity accuracy

Fri Sep 16, 2016 10:11 am

Just in case you need it after that ... /
For the places to put calibrations

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Re: Humidity accuracy

Tue Sep 27, 2016 11:16 am

There is an easier way to make a calibration curve and that is to use glycerol solutions. You can buy glycerol from the drug store. Basically what you do is make different concentrations of glycerol/water. If you use something like a mason jar with a weatherproof gland in the top, you can position you sensor above the liquid and after a few hours, the humidity in the jar will equilibrate to a value depending on the glycerol/water blend. Do that for a few different concentrations and you have a calibration curve you can apply to your raw measurement.

Try this link for the humidity values at different concentrations. ... 2.full.pdf

The process is also documented in ASTM D5032 but you have to pay for that.

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