Author Topic: Humidity sensors - pros and cons  (Read 980 times)

Online John@PC

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Humidity sensors - pros and cons
« on: July 16, 2013, 09:15:43 AM »
I've used several different types of humidity sensors (all digital) and pretty much took them for granted.  That was until my last sensor (an Acurite 613)crapped out in the cave.  This was the second sensor that I had to die on me so I thought I'd do some investigating.  I opened up the Acurite and found the sensor (the white chip in the middle of the picture below).  Thought it may have gotten coated with whatever was floating around in the cave and gave it a light alcohol wipe.  It only made the reading more off  :P so I replaced the whole unit with a new Acurite I had on hand.  Came across this excellent article and from the pictures recognized that what the Acurite sensor was a resistive sensor (as opposed to capacitive or thermal conducting per the article).  I also found that resistive sensors rely on a coating material of a conductive salt or polymer which explains that the alcohol wipe is probably the worst thing to do.  That said, I'm not sure any of the three digital technologies would be completely free from failing because of the conditions in a cave fridge.  I guess you can resort to analog (poor accuracy) or a wet-bulb / dry-bulb (and run the RH calculations), but when they work the digitals are inexpensive and good accuracy. 

The article says one good thing about the resistive sensor is that it is field replaceable so I ordered 10 from Ebay.  I really like the Acurite and have several so I figured it would be worth $8 to have a lifetime supply (if they work!).  I know some of you on the forum have this same model based on your cave pictures, so if you experience sensor failure I would be happy to drop one of the chips in the mail to you.

If I do order a new sensor I'm going to get one with a remote sensor.  Found this one that looked interesting; sensor wire is only 18" but it does have dew point display.   Curious if anyone out their has a sensor or method they think is the "best".


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Offline BobE102330

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Re: Humidity sensors - pros and cons
« Reply #1 on: July 16, 2013, 10:37:15 AM »
Darn you! You got my DIY electronics juices flowing.  A sensor in each mini cave with a switch to select them...  Better yet, build a circuit to control stepper motors to control each minicave humidity in real time.  Another project in the queue. 

I have Meade and Accurite remote sensors now.  Both have gone to constant readings of 99 or 100% RH a couple of times.  Taking them out of the cave for a few days allowed them to dry out and give readings that made sense, although I haven't calibrated. 

Offline FRANCOIS

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Re: Humidity sensors - pros and cons
« Reply #2 on: July 16, 2013, 03:01:49 PM »
We use a dry bulb/wet bulb set up and I can tell you it's not perfect either.  They need to be cleaned and the water supply and or fan is always getting covered in mould etc, throwing off readings.  There really is no perfect technology for it.  They all require servicing of some kind.

Online John@PC

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Re: Humidity sensors - pros and cons
« Reply #3 on: July 16, 2013, 05:29:26 PM »
I have Meade and Accurite remote sensors now.  Both have gone to constant readings of 99 or 100% RH a couple of times.  Taking them out of the cave for a few days allowed them to dry out and give readings that made sense, although I haven't calibrated.
That's what happened to my Acurite: went to constant reading of 99%.  I pulled it out for a few days but it still registered higher so that's when I opened it up and dealt it a fatal blow with the alcohol wipe.  It will be interesting if the$1 resistive sensor replacement works.
We use a dry bulb/wet bulb set up and I can tell you it's not perfect either.  They need to be cleaned and the water supply and or fan is always getting covered in mould etc, throwing off readings.  There really is no perfect technology for it.  They all require servicing of some kind.
   Maybe the best solution is to buy 3 of the Acurite digital sensors for $39 from Amazon, and average all readings.  If one gives outlier readings solder in another sensor chip.  I actually have three of the 613's on hand so I may try that method.   

Offline FRANCOIS

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Re: Humidity sensors - pros and cons
« Reply #4 on: July 16, 2013, 06:28:13 PM »
Unfotunately we use the sensor readings to control our humidifiers, so an off reading is very bad.  Our solution has just been to more aggressively maintain them.


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Online John@PC

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Re: Humidity sensors - pros and cons
« Reply #5 on: July 20, 2013, 03:40:34 PM »
Ok, I'm mad now  >:(.  My replacement Acurite was in the cave less than a week when I noticed an unusual increase in RH reading.  I happened to have a couple more new ones so I got those and put all three in the oven at 100F.  The one I pulled out of the cave stayed 8% or so above the new ones. 

One thing you may not know (I didn't until I did some checking) is that it's very easy to check humidity sensor accuracy.  If you put the sensor in a closed environment (plastic bag) with a small amount of saturated salt solution the RH% reading should eventually be 75%.  I had three Acurite sensors on hand and one analog; the first picture shows that two of the digital sensors equalized right on target and the third (the one I pulled out of the cave) was still about 8% high.  The analog gauge was also about 8% high.  I then put all four sensors in a bag of silica gel, and all three Acurite's settled at 16% and the analog was still higher.

I used a gallon zip-lock bag.  Put in a couple of tablespoons of water and added salt until no more would dissolve.  It takes at least 24 hr. to equalize, but I found that by putting in piece of paper towel into the solution the equalization occurs much faster - in a few hours.

My sensors are not re-calibratable.  But from now on if I suspect a problem I'll repeat the salt check.   However much "off" the sensor is I'll add (or subtract) that same amount.