a mysterious sensor

Jan Panteltje · 2017-04-30T18:19:05+00:00

Hi, been doing some sensor stuff. I connect those to a PIC micro and an POE ethernet module, and send UDP data that way. Today was the turn for the relative humidity and temperature sensor DHP11. It is supposed to be within a few percent accuracy for RH. However with 60 % RH it displays 20 %.... I have 2 and both do the same, software is good and checksum of communication is good. So started googling, seems MANY people have the same problem, nobody solved it, some people just wrote some correction math... That is what I am doing now too (and ordered an other sensor type). But, playing with it, trying to find out what I did wrong or what was wrong with those sensors: I noticed that if I put my finger on it the reading went way up from 20 % to over 60 %. Heat? no, also if I put my finger 5 mm away next to it... Tried a light bulb (Edison type), hot, no effect on RH. Tried a hot piece of metal: no effect on RH. Tried a dark screen, tried a high brightness CREE flashlight: no effect. Tried radiation sources (uranium marbles, thorium rods): no effect. Tried a wet piece of cloth next to it, way up it goes. But but, my fingers are DRY. So dry I need to wet those to open a sandwich bag... So WTF is going on ? Interesting physics, it seems to sense blood THROUGH the skin at a distance?? (theory 1). But HOW come these sensors are wrong for so many people ? It says in the data sheet that there is an internal lookup table in the on board micro it has for calibration (not use accessible AFAIK). Now I remember a discussion here years ago about customs using Xrays to check things, erasing chips, could have happened and would explain why so many people have this problem (theory 2), especially as datasheet mentions: 5) Light Affect long time exposure to strong sunlight and ultraviolet may debase DHT's performance. UV, Xray? But a sensor that can detect blood through the skin? Or has our body a moisture halo around it all the time? Anybody here played with the DHT11? Any...