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  • Author Author: koudelad
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Testing a range of a humidity sensor

koudelad

Hello,

 

Recently, I've been playing with various sensors. I've been interested in measuring air temperature, humidity and pressure. (Didn't get a CO2 and dust sensor yet.)

 

I thought I could post just a simple idea how to test a range of a humidity sensor. For measuring humidity, I tried SHT21, SHT31 and HTS221 sensors.

 

The pictures below contain a reference design with SHT21.

 

  • Very low relative humidity

What is needed: a small plastic bag with working sealing, silica gel (often supplied with shoes).

 

Result:

image

(It is possible to get even smaller relative humidity, but I didn't want to wait longer than a few minutes.) I also tried to let the bag opened and the silica gel has almost no effect. This means packaging anything with silica gel inside is useful only if the whole package is airtight.

 

  • Very high relative humidity

What is needed: a bathroom (the smaller and less ventilated, the better results) and someone having a shower / a bath.

 

Result:

image

(I didn't want to let the water condensate on the sensor, so I stopped at this value.)

 

SHT21 and SHT31 typically output +/- 2 % RH difference when placed next to each other, they can get as close as +/- 0,5 % RH in home environment after a while (nothing is moving around and there is almost none air movement), From an engineering point of view, humidity accuracy is typically +/-2 % RH for this sensor, which means the 2 decimal digits are only for reference. (Typical temperature accuracy is +/- 0,3 centigrade, so the same applies here.) However, seeing the data interpreted to the LSB gives a nice overview about the stability of output values and the noise present.

 

David

 

Note 2: I also tried 2 pieces of HTS221 sensor and was surprised by the results. One gave the output of 72 % RH , while SHT21/31 showed 59 % (which is about 22 % error). At higher humidity, the difference was even bigger: HTS221 gave output of 117 % RH, while SHT21/31 showed 91 % (which is about 28 % error). The other sensor gave output of 66 % RH while SHT21/31 showed 60 % (which is about 10 % error). Both sensors should have been calibrated and places on ST's reference designs, so I asked a question on ST community hoping to get an explanation of this difference by an ST engineer. I will post some update if I get any response. (I don't want to start a duplicate discussion here.)

  • Great post.

     

    Yes, using a shower is a great way to move into a very humid environment.

     

    For dry environments, you could also use your oven at low temperatures.  You can put your sensors under the heating elements, which should give you the driest location in the oven.

     

    As @John Wiltrout stated, you can reheat the little moisture packs to make them absorbent again.

    In my early days at the laser lab I had to do this weekly to keep our more sensitive crystals dry.

     

    DAB

  • Hi David,

    If those little bags of silica Gel are left out they can become saturated and may no longer work. Putting them in a very dry environment for a while or heating them can rejuvenate them. Heating can be problematic as the bags can burn. Several weeks in a freezer where the dew point is very low may do the trick.

    John