richrarobi
Posts: 271
Joined: Sun Feb 08, 2015 1:13 pm

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?

RichR

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dasmanul
Posts: 502
Joined: Wed Sep 30, 2015 10:20 am
Location: Frankfurt, Germany

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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joan
Posts: 14352
Joined: Thu Jul 05, 2012 5:09 pm
Location: UK

Re: Humidity accuracy

Thu Sep 15, 2016 10:25 am

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

viewtopic.php?p=952398#p952398

richrarobi
Posts: 271
Joined: Sun Feb 08, 2015 1:13 pm

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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brekee12
Posts: 335
Joined: Wed Feb 03, 2016 3:36 pm
Location: HU

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.
Brekee12
on a Raspberry B+ with whezzy, two Zero with Jessie Light

richrarobi
Posts: 271
Joined: Sun Feb 08, 2015 1:13 pm

Re: Humidity accuracy

Fri Sep 16, 2016 8:35 am

Thanks!!!
Just proves that everything in computing ( everything else ?) is knowing what question and who to ask.....found these....

http://www.cigarsinternational.com/ciga ... /salt-test

https://ambientweather.wikispaces.com/H ... 17c0afd309

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bensimmo
Posts: 4182
Joined: Sun Dec 28, 2014 3:02 pm
Location: East Yorkshire

Re: Humidity accuracy

Fri Sep 16, 2016 10:11 am

Just in case you need it after that
https://github.com/raspberrypi/document ... /README.md
For the places to put calibrations

scienceintheshed
Posts: 6
Joined: Mon Sep 26, 2016 11:05 pm

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.

http://horttech.ashspublications.org/co ... 2.full.pdf

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

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