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a mysterious sensor

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 application for this effect?

Reply to
Jan Panteltje
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fingers are far from dry.

NT

Reply to
tabbypurr

Jan you might check the data sheet for response time. Most of the ones I've seen are very slow.

Hul

Jan Panteltje wrote:

Reply to
Hul Tytus

Welcome back, Jan. We've missed you.

Skin has liquid water on one side and relatively dry air on the other. The concentration gradient drives a lot of water vapour flow from the skin to the air.

Cheers

Phil Hobbs

Reply to
pcdhobbs

I seem to rember those sensor needing to be reconditioned if they have been stored wrong. Some like a 2 hour bake at 100c

Reply to
Lasse Langwadt Christensen

+1
Reply to
Cursitor Doom

I've been using Phys_Chem Scinetific humidity sensors in my environmantal monitoring system for 20+ years. They seem to work quite well, and although I have not rechecked the calibration, it seems to still be reasonable. These are capacitive sensors, and need a circuit built out of an Analog Devices audio chip.

Jon

Reply to
Jon Elson

Welcome back!

Have you tried checking the calibration of the device? One fairly straightforward/cheap way is to create a slurry of a salt and water. The particular salt (in a small enclosed space) forces a particular humidity. Do a web search and you'll find a few examples.

Reply to
Frank Miles

+1

Thank you,

--
Don Kuenz KB7RPU
Reply to
Don Kuenz

+2
Reply to
BM2335

On a sunny day (Sun, 30 Apr 2017 15:04:11 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

Thank you. Yes, could be 'sweat fog' to keep body temperature constant.

It triggered memories of Kirlian photography:

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look at the picture of the finger tip. Sure moisture would do something in HV environment.

Reply to
Jan Panteltje

On a sunny day (Mon, 1 May 2017 03:06:28 -0000 (UTC)) it happened Don Kuenz wrote in :

Thank (or blame) Bil Sloman, he emailed me a while back and reminded me of SED.

Reply to
Jan Panteltje

On a sunny day (Sun, 30 Apr 2017 15:46:44 -0700 (PDT)) it happened Lasse Langwadt Christensen wrote in :

Well datasheet says

(1) Operating conditions Applying the DHT11 sensor beyond its working range stated in this datasheet can result in 3%RH signal shift/discrepancy. The DHT11 sensor can recover to the calibrated status gradually when it gets back to the normal operating condition and works within its range. Please refer to (3) of this section to accelerate its recovery. Please be aware that operating the DHT11 sensor in the non-normal working conditions will accelerate sensor's aging process.

(2) Attention to chemical materials Vapor from chemical materials may interfere with DHT's sensitive-elements and debase its sensitivity. A high degree of chemical contamination can permanently damage the sensor.

(3) Restoration process when (1) & (2) happen Step one: Keep the DHT sensor at the condition of Temperature 50~60Celsius, humidity 70%RH for 5 hours. (4) Temperature Affect

This refers to a 3% error... Mine have a 300 % error (20 % versus 60 %) Not sure baking would help.

I tried running it outside last night with my compensation math:

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seems usable, seems to match the local weather station (few miles away).

Need to fix the display algo for leading zero... (position). red is temperature, blue is humidity, once every hour.

Reply to
Jan Panteltje

On a sunny day (Sun, 30 Apr 2017 21:27:53 -0500) it happened Jon Elson wrote in :

Interesting, frequency change based? Or leakage based?

Reply to
Jan Panteltje

On a sunny day (Mon, 1 May 2017 02:53:06 -0000 (UTC)) it happened Frank Miles wrote in :

Yes, clearly the calibration is wrong I did the wet salt test etc to calibrate my old 'analog' mechanical humidity sensor years ago, that one is still OK.

Then the idea came to put a camera on that, and scan the image circular in ever bigger circles, correlation of brightness changes in the circles then gives the position of the hands,... It has 2 hands, one moves, and one you can set as reference,. Difference in color can tell which one is which.. But the 2 $ sensor is faster, cheaper, smaller, and better, But if it MUST be ;-)

Reply to
Jan Panteltje

Jan! I never measured RH, but it should be a strong function of temperature. (I think temperature is the "relative" part in relative humidity.)

Find a different sensor? George H.

Reply to
George Herold

On a sunny day (Mon, 1 May 2017 05:11:37 -0700 (PDT)) it happened George Herold wrote in :

Indeed, that is why the temp sensor is in the same unit, and its micro does the correction, or at least it should.

This is the DHT11, I now ordered 2 AM2302, supposed to be equal to the DHT22.

AM2302 has a RH range of 0-99.9% in steps of .1 % DHT11 of 20-90% in steps of 1%. Software and hardware interface seem the same, no need for new code AFAIK. Both are 'factory calibrated'. Of course that does not work if customs xrays those and erases the on board micro's calibration constants (my guess what happened).

Reply to
Jan Panteltje

Oh, boy, this requires deep digging in the old memory banks. I THINK it uses constant frequency, and measures the capacitance by how much AC is coupled to a resistive load. I THINK that's how it works. I built these from an app note almost 30 years ago, and they've just been perking along since. I think these sensors are the same type as used in a number of VCRs to shut them down if close to dew conditions.

Jon

Reply to
Jon Elson

On a sunny day (Mon, 01 May 2017 14:13:49 -0500) it happened Jon Elson wrote in :

OK, datasheet of the new sensors I ordered (google AM2302.pdf many links) says it is capacitive too. This DHT11 I am having problems with, says it uses a resistive type. The fact that yours works so well for so long gives me hope the new sensors will be OK. The DHT11 that gave 20 % RH now gives 48 % RH while it is 60 % RH, fast aging, was outside all night and it rained :-) Just wrote the PIC asm code for that AM2302, slightly different from the DHT11, so am ready when those arrive.

Ha, Sony Umatic helical scan, I remember in Rome in a sports hall needed to record, and the d*mn thing would not. used an electric heater blower to get it working just in time :-) Now that was long ago too.

Reply to
Jan Panteltje

Seems electrostatic to me.

Mark L. Fergerson

Reply to
alien8752

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