Calibrating a cheap digital Chinese hygrometer

This article describes an method that could be used to get reasonably correct read-outs on a digital hygrometer even if it has no provisions for calibration at all. This was mostly an experiment but it has proven to work for the few cases where I tried it. Don't expect miracles from this method, but it may keep the hygrometer readout pretty reliable around the point where it will normally be used.

Introduction

Very cheap combined digital thermometer and hygrometer units can be bought from the usual Chinese marketplaces. The thermometer part of these things is usually reasonably accurate, but the hygrometer part is a game of roulette. Especially if you allowed the sensor to become truly wet by placing the meter in a room with 100% humidity, the risk is high that the calibration will be way off. A proper hygrometer should have a calibration button or button combo that has to be pressed after leaving the unit in an enclosed space with a saturated salt solution, which will stabilise at 75% RH after many hours. These cheap things however rarely have such button.

In the hopes that the device shown in the above photo would have some secret button combo to reset the measurement to 75%, I have tried inserting the battery while holding down buttons in various combinations, or mashing the buttons while the unit was already on, but without any success. I doubt whether devices like this contain any EEPROM at all, the measurement tables are probably hard-coded in ROM, and it is assumed that the humidity sensor will be accurate enough.

Hence the only way to correct the read-out, is to modify the signal coming from the sensor itself. There are two possible approaches:

1. Add resistors to either increase or reduce the electrical impedance of the sensor. This is probably a bad idea however, because it will change the slope of the humidity response curve. The meter will only be correct around a very limited set point, and deviate increasingly when moving away from this set point. It would take some fancy op-amp circuitry to truly shift the impedance curve without altering its slope, and this is way too much effort for a cheap device like this: just buy a better unit for the money that would go into buying parts and spending time on building this circuit!
2. Physically alter the sensor itself. These sensors are simple: it is a substrate sandwiched between two metal plates, or with two intertwined grids of metal ‘fingers’ at one side. The substrate contains some mildly hygroscopic material that will absorb moisture from the air, and the more humid the substrate, the better it conducts electricity and the lower the impedance. Therefore if we can alter the degree to which the substrate wants to absorb water, we can shift the impedance value. I'm not sure whether this won't also alter the slope of the impedance curve, but from my initial tests this seems pretty OK when using the right substances.

I have tried approach 2 with reasonable success. While in theory you could use only a saturated salt solution to check the correctness of your adjustments at 75% RH, it is better to also rely on a true calibrated hygrometer to verify that the cheap meter gives more or less correct readings around the humidity level where you will use it the most.

The method (updated 2020/11/25)

A warning in advance: to get good results, you will need a ton of patience. This is not something you can get right in a few hours unless you are extremely lucky. This takes multiple iterations of a cycle that takes at least 2 days. If time is money, buying a good and reliable meter is much more economical.

I have encountered two styles of sensors and they react somewhat differently to manipulation, but in the end the following strategy seems to work best. We'll start out by applying too much of a substance that is hygroscopic, in other words that likes to absorb water. Then we'll gradually wipe away some of this substance until the read-out is correct. The tricky thing is that wiping the sensor will somehow disturb it in such a way that one needs to wait several days until it stabilises.

Initially I used only hand sanitiser gel as the hygroscopic substance. Such gels will always contain something that attracts water, because otherwise the alcohol in the gel would dehydrate your skin. Typically it will be glycerine (glycerol) but it doesn't really matter what exactly it is. The concentration of this additive is also usually quite low in these gels, which makes it easier to apply small adjustments. The hand gel worked OK for the first type of sensor I tried to tweak. For the second type of sensor however, I had to use a calcium chloride (CaCl₂) solution, mixed with a bit of the sanitiser gel to make it more sticky. I obtained the CaCl₂ from the typical passive humidity absorbers that use a bag or brick of CaCl₂. This absorbs humidity from the air and gradually turns into a brine solution that drips down into a container. These devices are mostly useless as far as dehumidification goes, but they are a good source of CaCl₂. I simply took a drop of the brine solution, mixed it with a bit of the gel, and applied it to the sensor, wiping off the excess such that it wasn't entirely soaked and gave a read-out below 99%.

Next to the hygroscopic substance, you'll also need a classic hygrometer calibration box. This consists of a watertight sealed box, with inside it a small container filled with a saturated salt solution. Take a small cup like a shot glass, fill it with salt, then add water until the salt has become a wet sludge with the water just not spontaneously flowing out of it. This kind of salt sludge has the interesting property that when placing it in a sealed box, it will make the relative humidity inside that box gradually become almost exactly 75%.

So to recap, the things you need are:

• Some hygroscopic substance that sticks to things. What seems to work best is a mix of hand sanitiser and calcium chloride brine.
• A sealed box with inside it a small cup of saturated salt solution,
• a reasonably well-calibrated hygrometer,
• a lot of time.

Steps

The general strategy is to start with too high a read-out and then gradually wipe the sensor to bring it closer and closer to the correct value. The most tricky thing is not to wipe too much, because then you have to start all over again. Wiping the sensor disturbs it only a little. Re-applying the hygroscopic stuff brings it totally out of whack and it takes many iterations to make it stable after that. Therefore you will want to avoid bringing the sensor to a state of too low read-out. Wipe carefully!

1. Start by ensuring the sensor is clean. If there is any residue on it from a previous failed experiment, remove it with water or rubbing alcohol, whatever works best.
2. Cover the sensor uniformly with the hygroscopic stuff. Most likely this will cause the read-out to jump to 99%. Wipe off the excess until it drops below 99%.
3. Now rely on the calibrated hygrometer to continue wiping the sensor until it is near the actual humidity value. Needless to say, take care not to breathe humid air towards the general direction of either meter. This is the most tricky step. In my case I had to wipe until the read-out was 5 to 10% below the actual value. I can't really explain why, and it is possible you may need to aim for a different offset. This is one of the things that makes this process tedious. It seems the required offset became smaller with each cycle, so you should become more careful as the error on the read-out decreases with each cycle of this procedure.
4. Now put the hygrometer in the calibration box with the salt sludge and leave it alone for at least 36 hours. After that, see what value it settled at and remember this value, then take the hygrometer out of the box and let it settle again for at least 12 hours.
5. Now look at the final value and compare to the known humidity. Ideally you will want to do this at a humidity level where you want the hygrometer to be the most accurate.
6. If the read-out is very close to correct, resist the temptation to get it right to the last percent: most likely you will mess it up and will have to start all over. Just stop and be content with what you have. Remember, it is much harder to correct too low a read-out than too high.
7. Otherwise, if the value is still too high and it also was well above 75% in the box, go back to step 3 and wipe, but take care to make increadingly small corrections.
8. If on the other hand the value is too low, it depends. If it is way too low and the value in the box was also well below 75%, you have over-done the wiping and you will have to go back to step 2 and prepare yourself for several more days of going through the same cycle over and over. If it is just a bit too low but the read-out in the sludge box was very close to 75%, then most likely you will never get a perfect reading from this meter across the whole range. You could try to apply a different mix of hand gel and CaCl₂ in the hopes of getting a better overall response curve, but in the end you might just have to be content with slightly too low readings at lower humidities.

The 75% calibration box has a double purpose. First, it offers an initial test to see if the sensor has any chance of being close to correct. Second, it speeds up the stabilising of the sensor after it has been disturbed by the wiping. Again, I cannot really explain why it behaves like this, but in the end it does provide a good result.

As you can see, this method is tedious and involves a lot of trial and error. This is what makes it mostly useless, unless you're in the middle of a virus pandemic and are looking for things to kill your time and gain some usable measurement devices during this process…

Even if you can perfectly nail the calibration to 1% accuracy, of course you still shouldn't trust these things to be reliable. Also never use these simple hygrometers in environments where relative humidity get near to 100%, because in extreme cases, a drop of water may form on the sensor and disturb the hygroscopic layer. Just buy a good hygrometer that can be calibrated if accuracy and reliability is important.