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Some time ago I purchased one of these very cheap hygrometer/temp modules made in China. They come for less than 5 EUR. I know, the precision is not very good, but I thought, why not give it a try. Somewhere I read they have an accuracy of +/- 5% for humidity which would be acceptable for some use cases.

The device seemed to work well enough for some time but then it unfortunately got wet. The display went blank so I immediately removed the batteries and disassembled the module. To my surprise there's very few parts and they are not glued or anything. The display's pink contact bar is simply pressed against the contacts on the little board when put into the case.

I've dried all the parts and put them back together and it still works! But now the humidity is way off. Before it was in a range of 40-60% which are reasonable values. Now it only shows between 20%-40% which is far from realistic values. I've also done the calibration test using wet salt in a sealed bag. It should give 75% after some hours. The module only showed 40%.

The humidity sensor looks fine. I can hardly imagine how some drops of water could seriously damage it. I've bent it up a little on the image below:

So given the very simple board it made me curious how it works. I guess that there's a small microcontroller under the black pad. My assumption is, that it may use some autocalibration algorithm, which somehow got messed up. If that's correct then there should be a way to recalibrate the device.

Does someone have an idea, how these modules usually work and how they are calibrated during production? And is there maybe some trick to recalibrate this module?

Maybe someone can also identify the sensor and whether they don't like direct contact with water.

UPDATE: As answered below, the sensor could be a HR202L. If you compare the sensor image above to a HR202L you'll notice, that my sensor has a little black spot, right on the top of the black winding tracks, that connects both tracks. I've had a closer look and thought, it might be some dirt - so I removed it with a scalpel. This was a bad idea, as now the display shows even lower values < 20%. So probably time to buy a new module.

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    \$\begingroup\$ By the way, as a bit of trivia, the pink contact bar is called an "elastomeric connector", also sometimes known by the trademark "Zebra". Kind of a neat structure, they can even be designed as matrices of conductors. \$\endgroup\$ – uint128_t Sep 15 '16 at 21:00
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You have ruined the humidity sensor. You could destroy it even by touching it by fingers. You can replace it by new one, just search "10pcs humidity sensor" on ebay. It looks like HR202L, but I can't guarantee it.

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  • \$\begingroup\$ Oh, that sounds bad. Can you elaborate a little more? Why is it so sensible that you can't even touch it? And is there really no way to recover it? Maybe clean it with pure alcohol or something? \$\endgroup\$ – Michael Härtl Sep 15 '16 at 18:10
  • \$\begingroup\$ IMHO that sensor is covered by some thin film which is essential for proper sensing. If you destroy or contaminate that layer then do not expect precise measurements. \$\endgroup\$ – Chupacabras Sep 15 '16 at 18:41
  • \$\begingroup\$ Interesting, thanks. I've also updated my question, as my sensor differed in a little detail (a black spot on top of the black lines). Still I'd be interested if you have a source with more details. I also wonder, how you would solder the sensor if it's so extremely delicate that you can't even touch it. \$\endgroup\$ – Michael Härtl Sep 15 '16 at 19:06
  • \$\begingroup\$ I have no links. I read about it a long time ago. Just try to google. \$\endgroup\$ – Chupacabras Sep 15 '16 at 19:12

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