Humidity meters and their sensors. 

I used these in my first job making moisture instrumentation in 1999. The sensor needs a higher frequency driving wave at higher humidity to stop polarisation and a much lower frequency at very low humidity. I developed a simple bit of code that smoothly varied the frequency from 10kHz down to 100Hz based on what the sensor was reading, hugely extending the accurate range of the sensor.

Hi Clive, just dropping in to say how useful this video is 4 years later. I've been researching humidity sensors for a 3D printer filament drying project and was nearly going to pull the trigger on a couple of $8 already in the nice plastic case with the display jobbies, when I remembered I had a few sensors lying around from a previous arduino project. I wondered how they worked, and the words " hygrometer eBay " brought me straight here. I'm fairly certain if I type anything I'm remotely interested in with the word eBay after it I'll end up with result number 1 being a video of yours.... Top quality content as always mate, stay safe during this quarantine, and as always... keep the explosion containment pie dish handy.

How the hell do you know these awesome little tidbits like the humidity in a bag trick? That kind of stuff is awesome to know and I really have to thank you for it. It's why I subscribe to this channel -infotainment of the highest caliber!

I purchased 10 of these little devices, they were sold as ºC (celsius) only. I wanted to use them mainly for the humidity function. As with everything I get, I take it apart first. I discovered there is a set of solder points inside that will set it to ºF (fahrenheit) temperature readings. Bonus! :)

I really do appreciate videos like this, Clive. I know it seems like such a simple thing but this is a good unbiased educational video for a product that has worked very well in my life. Thanks for the work you do!

Very useful video I bought a pair of these with the sensors on flying leads and put them in a wall mounted box on a cavity wall I've been having damp problems with. One panel is reading the ambient humidity (and temperature), the other the sensor goes through a hole in the wall so I can measure humidity in the cavity and compare them against each other. Now I can make sure the two units are calibrated the same.

Thanks for showing us Clive, I didn't really know how these things work but I'm glad you did this video.

Why would anyone need more than 6 of tho... oh wait, you're Clive, never underestimate Clive.

put mine through the washing machine. had a significant affect on its accuracy.

Thank you for such a great video! I was wondering how those sensors work. I have very poor knowledge of physics but at a amateur level I understood how it works. Thanks a lot!

You answered every question I had about these units and more. Much appreciated.

Thank you so much Mr. Clive for this video, me and the wife are looking for new ones for the green houses that dont just crap out in the sunlight and moisture so this is extremely helpful ! you would think a moisture meter would have some protection from the elements but apparently not

Every time I look at your channel there's something interesting and timely

I never knew that you could check the calibration like that. I have a DHT22 (digital) humidity sensor connected to my Raspberry Pi weather station that I suspect is read way off so could test it with this method. Thanks for the tip!

Great tip about testing. I now know which of my hygrometers to trust. Thank you Clive!

Great for homemade humidors, and I know know heaps more about them. Clive, you're a star.

Just subbed to Patreon Clive, keep doing what you're doing mate.

Great video and great channel. Subbed for sure. Had several of these under cover that drifted due to very fine silica dust (middle east weather). Now I know how and how not to clean these! Most appreciated.

Wonderful explanation. Thank you so much for sharing. I am also enthusiastic about hygrometers and since they show slightly different values I didn't know which one is correct. Now I know how to check. I'm really excited to try this out.

I use these in my guitar cases. Thanks for the calibration tips

Fantastic little thing, I'm ordering one for my server rack right now, I have my own temp sensor I rigged up using a dallas instruments one wire device but something to permanently install will be handy, though without the ability to datalog like mine. First one I found has a remote sensor, and I figure that will work nicely since I will be able to put it somewhere in the rack if I want to read the temp from somewhere else.

Excellent video, thanks! Also, thanks for explaining how to test the accuracy on these things. I have varying different humidity meters around, and I don't know if any of them are correct or not, so I will definitely get them checked.

Great video Clive! Very informative, I've always wondered how those cheap devices measured humidity.

hey Clive your knowledge is immence another ace video top class viewing .Keep it up sir .

Thanks for showing us how to test the accuracy i was a bit worried about that and now i have something to help me out!

This video was the simple explanation I was looking for. Thank you!

Nice video and humidity sensor explanation. Great way to test the units. If they were adjustable, wish you would have shown how to bring them closer to the correct reading.

Thanks big man for another quality video to keep us exciting party folk entertained, this Friday night.

very cool, thank you for showing me this Clive!

ahey! the first product that you've dismantled that I own, good to see whats inside 'er.

Always learning something....Thank you!

Thank you. This is of particular relevance to me right now.

Great job. Thorough and even authoritative

Thanks, bigclive! Learnt something new today. AC, not DC for RH sensors. :)

I fitted one of these into a smart Roadster a few months ago. They suffer from leaks and the water turns to vapour and condenses on the main controller PCB for most of the electrics. Monitoring the humidity in the car was an obvious step.

You can use various salts with their own equilibrium relative humidities. To calibrate a low point at 33%, Magnesium Chloride is an easily accessible salt. You should use kosher salt or canning salt instead of table salt for your Sodium Chloride slurry, though. Common table salt contains anti-caking agents that can throw things off.

Last winter, I bought a few of these round analog humidity meters from ebay, because my apartment is always too dry and I was using a humidifyer for the first time. When they arrived, they all showed a different value, so I had to clip open and manually rotate the dial in order to get a somewhat accurate reading (but in fact, I'm happy if it shows me "around 20%" or "around 40%", with the former being almost unbearable for my respiratory system and the latter being the start of where it becomes tolerable.

Man, that is amazing. Humidity cal for cheap meters. Thank you so much for this!

i have a temperature meter in the same package, quite neat to keep an eye on temp inside refridgerator(it has a lead with the sensor on the end) Might get one of these for fun

Clive I thought they were capacitors of which the capacitance varies with humidity. I traced the circuit of one of these and I found this; There was a CMOS 555 on board of which the frequency varied as a function of the capacitance of the sensor. I could be wrong perhaps the varying resistance changed the frequency directly. But I remember that the sensor was located where normally a capacitor goes into the 555 oscillator circuit. I built and sold a dozen humidity display plus recorders to a pharmaceutical Co. about 6 years ago.

wow, an eye opening video. again so glad i subscribed.

Useful and interesting video, thank you.

If applying a DC voltage across these sensors polarizes them one way or the other.. could that be used as a cheap way to nudge some of the ones you have their closer to accurate?

Thank you for the video. I always wondered how some of these worked. This clarifies and teaches me a lot about them. I wonder how good is their response time and how does their different encasing can affect both the accuracy and response time of it.

I got this humitidy meter, but with external long probe model. Later I am going to test its accuracy according to your test. Thx.

I've often wondered how those work. V cool vid.

Awesome video! Thank you so much.

timed this visit to RU-vid quite well!

Humidity sensors require temperature sensors to get the relative humidity (absolute isn't really used much), you can probably tune the ones that are out by simply tweaking the resistance of the thermistor.

Great help to calibrate/check the hygrometer, which turns out to be essential in covid-19 prevention. That is, keeping a room above 40 %RH and preferable below 50 %RH.

I own a few of these. Great help for tropical plants and exotic animals.

So, i) do you know if the above LCD ones suffice? If yes do you know for how long? ii) Do you know if the only analog one in the video is the correct one, "AC"? a) Do you know if it's good enough? If yes do you know for how long?

I have always wanted to know how these worked, thanks.

A great fan of your videos. Please make a video about the humidity sensors that comes with common xh-m452 temperature humidity controller used in egg incubators. Currently most use this "asair am2120" and it doesn't works for more then 2 weeks.

Hi, I have the same LCD devices. At the shows the constant 10% humidity. Is the sensor defective or does it have to be calibrated? I have already put airtight in a box and with salt. not much has happened.

Interesting, thanks Clive. Are there other salts you can use that would have different humidity equalisation points? I wonder if the offset you noticed on some of the devices is linear.

Thanks Clive. Had to find out if these are reliable enough to monitor the humidity for my sneks.

Clive, On the polarisation of the sensor from using DC, have you ever tried to measure the resistance of silver loaded conductive paint while it's drying? You could make a video of that but you'd have to do several stripes at hourly intervals before measuring the resistances.

Hi, I have bought two hygrometers. When I do the salt Calibration test they have a difference of 10 between them, one reads 76 the other 86, however when they are out the salt test and in the same spot in my room they read more or less the same? Do you know why this might be? Thanks

So the million dollar question remains unanswered - did those ones you tested drive the sensors in AC or DC...? :)

Hey Clive. I had basic science stuff in school but never really understood the differences between Volts, Watts, Amps etc. Any chance you could do an informational video explaining this like you'd explain it to a 5 year old?

I had an old 480Watt window a/c, so I decided to use the condenser to dry my bedroom. My humidity sensor Implied 47% humidity at 27.7C/82F. I had it in operation overnight, 5 gallons of water later and the sensor still reads 47% Humidity. I think my hair is a far better measurement of humidity.

I learned so much in 7 mins thank my good sir..

Am I getting it right that for the little container with salt and water the idea is to take so much of both that in the container there will be a saturated solution of salt in water pluss excess salt that then will act on the surrounding air?

Very helpful thank you!

It would be great if you could check with an oscilloscope whether these cheap all in one modules correctly use ac on the sensor.

Hi Clive. Is there any chance to reuse the HR202L after getting it wet? Bc I wanna try it into some project and it requires to wash it on a daily basis... in case it's not possible, do you know another idea that could work with it? GREETINGS FROM ARGENTINA!

How does the BME280 humidity / pressure / temperature sensor work? I have one and it's advertised to be very accurate. I can confirm that it's very fast and that it has a high resolution (which - I'm aware - is not the same as accuracy).

Impedance according to the data sheet is 31.0k@60%RH excited at 1Khz. If these are put in as one leg of a 31.0K "bridge" you can drive them with a 1khz sine wave and then send the output in to a op amp buffer and then rectify it from there. The op amp can have enough gain to compensate for the diode loss and then it can be read directly with a DMM or ADC input. It means ordering expensive 0.1% resistors though and perhaps "balancing" out an offset.

I really like the HIH-4000. It's a capacitive sensor that does a similar job. The resistive salt based devices age. Capacitive sensors do not. They also output a 0-5v analog signal and don't need to be driven with hard to deal with sine waves. Something to look into...

I thought that they generally worked as capacitors. I know the ones I've seen have open air separating the plates and the changing humidity in the air changes it dielectric properties, therefore changing the capacitance which can then be measured.

The last humidity meter thing I had was a cheap one which came with a thermometer and a clock in a 3-gauge plastic housing, never worked right, especially after I made the spring pop out... :P

hey clive nice video here. how much salt/water do you have to add and in what ratio? 1:1? I tried it and after one day all the meters (i bought three of them) showed 99%...

1:49 I'd say the variation in humidity reading is not so much an error in the humidity reading, but rather the calculation it performs based on the temperature. You might mention it later, but I'm commenting in real time; relative humidity is based on the capacity of air to hold water vapor and that changes based on temperature. The meters in the tub are ranging from a low of 18.3C to a high of 19.9C and show a difference in relative humidity of 4%. I think it comes down to the accuracy of the temperature probes, but there is another 5m7s to this video and I could be totally incorrect.

BTW most have a small area with 2 solder point that can be shorted for use with Fahrenheit , actually has a C/F denoted on the board

If only you made this video 2 months ago clive! I used these little things to make a weather-station and I spent days trying to get it to work correctly, ended up getting the digital sensor modules (DHT modules) and using those instead. It wasn't until later I found that its not very good to drive them using DC.

One note: The bottom of the line Acurite humidity/ temperature meter I use seems correct for humidity and degrees C in whole numbers. However if I switch temperature display to degrees F it is obvious that it only displays whole numbers in C, because as the degrees change in F they jump 1 or 2 degrees F. The omitted F numbers never show up on the display. Living in the USA and living in higher whole number resolution Fahrenheit , I just do not have a good feel for Centigrade.

bigclivedotcom, why do you assume that the humidity offset of the "non-accurate" sensors is a linear function, and only of humidity itself? ta

I know that this is an old video, but still relavent. I recently bought five of these on ebay. They were cheap, but I expected them to be fairly accurate. However two had an error of -7% and one of -8%. As they work on resistance, do you think that they could be recalibrated by soldering a resistor across them? I have quite a few around so I wouldn't cost me anything. But before I start it would be nice to know if someone has done this and what values to add. This would only work if the voltage needed to be lowered. I don't know of any way to increase the voltage if that is required. I need to raise the displayed value by 7%. (I think that the resistance will decreace with an increase in humidity, so adding a high value resistor in parallel has a chance of working and raising the displayed value.)

I have these things all through the house,all acceptably accurate. Any discrepancy I just take the average. Very useful little devices.

Boveda brand sells salt packs with different humidity settings including a 75% and 33% humidity for calibrating humidity meters...

I prefer the digital ones (SHT) from Sensirion. I was using the same method to calibrate my humidor meters.

How can you be sure that the measurement error (I mean the +/- 2% you're gonna add to the results) is actually caused by the "y intercept" and not the "slope parameter"? I know that we're talking about very cheap stuff and I dont expect it to be reliable and accurate, but...

Brilliant vdeo. Do you know how long it usually takes for a hygrometer to stabilise, for instance if you move it from one place to another?

At that price, it would worth ordering 10 and doing the calibration test to cherry pick the most accurate ones. Even the more expensive ones I bought a while back vary several percent from each other when left placed side by side overnight. Great tip about the salt water based calibration. As for distilled water, dehumidifier water makes a good substitute as we use it in the clothes iron, steam cleaner, etc that normal tap water would otherwise leave gunk once evaporated. Going by Amazon, it's surprising how many people fall for the snake oil single-use moisture traps claiming to dehumidify one's home considering they only collect 1-2 litres per refill (which takes several weeks and expensive to buy), compared to 2 to 4 litres per day for a typical electric dehumidifier.

Interesting... I have heard exactly the same thing, but _without_ using salt results in a humidity of 50% in a sealed container. I never tried this to find out if it is true though.

Handy, Big guy..

It's interesting how some of these indoor humidity/hygrometer meter/monitors differ in price.. do they all use the same sensors inside? I thought of using my phone, but found that most newer don't have temperature and humidity sensors.. but there are a few that do: Samsung Galaxy Note 3, AGM X2, Galaxy S4, Motorola Moto X, and the Moto X 2nd Generation.

Whats better the analog one or the digital? Im planning to buy the analog one because it doesnt have battery ,but I dont know how acurate it is and how it works inside..

the zoomed pic of the humidity sensors looks like the ones that used to be fixed to the heads in vhs machines

I'm not really clear on how much salt you should use, though. Obviously 1 grain of salt in a cup won't do much... Should I just pour in salt until it can't dissolve? Or will too much salt also skew the results?

what humidity level is bad for storing electronics? not powered on, but would like to know if I pull the plugs on sth like speakers or a tv and only have them powered on for a short time over a day is bad if its sth like 70-80%? I plan to store stuff in the attic and got it boarded up with osb but there will always be parts that leak to the outside.

Just got one as a Christmas gift thinking about putting it in my car

hi how accurate weather clocks (AKA atomic clock ) are for humidity reading,? do you know the margin of error.. I want and need to buy one for humidity level. Thanks in advance. great upload

Unfortunately you left without telling us how the cheap sensors measure the resistance. :( Have you had a look at it? Is it a square (how precisely is the duty cycle of 50% reached) or have they got some kinda analogue oscillator?!

i have the same unit it used to read 75 and then the battery started to run down I changes it and it now reads 78 with 2 new batteries, I don't know how to change it, I suppose i assume it's +3 as my Bovida packs are 75

Dear Mr Clive, if you have the time, could you test the various resistors and pads to figure out how to get the temperature to display in Fahrenheit? I see the LCD does have "F" next to the "C", and this LCD does look purpose made for this hygrometer. I'd love if I could meaningfully read the temperature in the U.S! I bought 5. Would buy 20 more if they read in °F.

You should try out the strain gauges mate. I've used a lot of them for aircraft structural tests. The linear ones are on ebay now for cheap as chips prices and rather similar to these sensors. Great video! Cheers.

if you touch them you can heat them up a bit with the soldering iron while in use until they read right again

thanks clive