Prevent mold! Hygrometer Calibration

I work with hygrometers frequently in my role as a home inspector. Consumer quality hygrometers and/or relative humidity meters are inexpensive and notorious for their inaccurate readings. That’s a shame because maintaining the proper relative humidity in your home is a good start in discouraging the growth of fungus or even mold. Mold can be difficult to identify and is often excluded from home inspection reports. However, if an inspector sees mold, they will usually mention it. Most experts recommend that the relative humidity in a home stay between 30% and 50%, and 60% is rarely a concern. You can go online and find hundreds of articles that explain the reasons for this and suggest optimal readings for your particular climate. You can also get that information from a university extension service in your area. Once you have that target percentage, customized for your climate and region, the simple procedures below will help ensure that the readings you get from your hygrometer are reasonable and accurate at all times.

Calibration of a hygrometer:

If you have a digital hygrometer or humidity meter and want to accurately calibrate it, without having to buy expensive salt calibration kits provided by the manufacturer, this is the easy solution. The physics behind this project is simple and reliable: different salts, when mixed with water to create a slime or mud, will generate a constant and predictable humidity.

Simplified scientific explanation:

A saturated solution at stable temperature and pressure has a fixed composition and a fixed vapor pressure. Thus, at constant temperature, no matter how much salt and water are present, the resulting relative humidity (RH) is fixed, as long as both water and solid phase are present. So unless the water dries up, or the salt gets so wet that it liquefies, a predetermined humidity can be produced.

We want a solution of ordinary salt mixed with water (preferably distilled water) to produce predictable moisture over a wide range of temperatures. The humidity created, with common salt (sodium chloride) and water, is 75.29% at an ideal temperature of 77 degrees Fahrenheit. The temperature of the room is not critical for our purposes. For example, the RH is quite stable even with wide variations: saline at 59 degrees Fahrenheit will produce 75.61% RH and at 86 degrees Fahrenheit the RH is 75.09%.

To calibrate the low end, again 33% moisture, magnesium chloride (a salt), and water are used. At the ideal temperature of 77 degrees Fahrenheit, this solution will produce an RH of 32.78%. At 59 degrees Fahrenheit it will produce an RH of 33.30% and at 86 degrees Fahrenheit it will produce an RH of 32.44%. Again, “room temperature” is not critical.

Detailed calibration procedures:

With most professional instruments, it is recommended to calibrate them to both a low point and a higher reference point. For convenience, most manufacturers have selected 75% and 33% RH as the default calibration standards. So, in order to calibrate our instruments, we need to be able to place the device in our own custom “humidity chamber”.

To create your 75% humidity chamber, put salt in a container and mix it with a little bit of water, but not too much. You want a moist mud, not soup. I made containers out of yogurt cartons. I cut the top off so they were about two inches tall, and cut out a recessed area so the hygrometer can rest with the sensor on top of the solution without it being in direct contact with the wet solution.

Place the hygrometer over the yogurt container and seal it in one or even two Ziploc brand bags. Having some air in the bag is unavoidable and it’s okay. This method should work with any hygrometer, including inexpensive mechanical hygrometers, which are typically only tested or calibrated to 75%. Once again, make the necessary adaptations to ensure that the instrument does not get wet; it must detect the RH and not the water. Typically with cheaper hygrometers you can’t actually calibrate the device by changing the settings, but you can take a reading at a known RH and from that calculate a correction factor. If you have a simple instrument, like this, just calibrate at 75%, get the correction factor for future reference, and work from there. It should be close enough for your purposes.

NOTE: If you have a professional electronic hygrometer, which has an integrated but accessible sensor, it can simplify the calibration procedure. Simply get a couple of plastic jars, like oysters or similar foods, and drill holes in the lids to fit the sensor on your instrument snugly. Label the jars 75% and 33% and put your salt mixtures in the jars. I still use the yogurt containers to hold the salt mixes and squeeze them in tightly, about 1/3 of the way into the jar, so that a moisture chamber forms near the top of the jar. Screw on the lids of the jars. If you have two hygrometers, place one on each jar lid. Otherwise, place your hygrometer on the lid of one jar and a piece of tape or a seal of some kind over the other so the RH settles. Once the proper RH has been created, in the same general time frame as described below, you can quickly check or recalibrate a hygrometer by inserting the sensor into either jar. Always give an instrument time to stabilize after moving it from one humidity chamber to another. This is the most accurate way to calibrate an instrument, if it can be done this way. Readings remain more stable than when using a plastic bag: If a bag is inadvertently compressed or the contents move, which is likely to happen if you have to calibrate the instrument instead of just looking at it, the stability of the camera Moisture content is affected and can result in calibration errors. As a result, that process needs to be done with caution and double-checked.

75% chemical solution:

Use pure salt, sodium chloride, without additives. Morton canning salt from a grocery store is such a salt and is inexpensive. Put a few tablespoons in the yogurt container and add distilled water to form a slurry. Put this in a Ziploc bag, with the hygrometer placed over the container, and let it sit for about 12 hours. That time is needed for the solution to stabilize. (I let it sit overnight.) Personally, I like to leave the hygrometer display on so I can see the readings through the bag as they change, and also that way I know when the solution has stabilized.

Most digital hygrometers need to be calibrated with the power or display turned off. So once the solution has set for 12 hours and the reading has obviously stabilized, I turn the unit off. Then I start with the manufacturer’s calibration procedures. Typically, this involves pressing, with a paper clip or similar object, a recessed button and other controls in a set order. In essence, you are “teaching” the instrument to “recognize” a set humidity the next time it is exposed to it. With the Ziploc bag, you can see the hygrometer reading and controls, so it’s easy to clip a small hole in the bag and calibrate the instrument without interfering with the relative humidity that has been created.

33% chemical solution:

You need Magnesium Chloride Hexahydrate. This isn’t as easy to get as regular salt, but it’s not as hard to find and can certainly be made a lot cheaper than buying salt calibration kits. Prices and availability change, but I buy small quantities of magnesium chloride hexahydrate, lab-grade flakes, on Ebay. You won’t be using much at one time, but hygrometers need to be calibrated twice a year, so they’ll be a worthwhile supply to keep on hand. It’s getting harder and harder to buy even simple chemicals, but you can find this one at online chemical supply houses. It is also used as a de-icer. (Don’t buy a magnesium + chloride supplement at a health food store – wrong product.) Mix Magnesium Chloride Hexahydrate with distilled water, in the same way as described above, and follow all the same procedures. You can start both bags, 33% and 75% at the same time, and put the instrument in one. This allows both solutions to stabilize at the same time and start producing the RH you need. After you have performed the first calibration, open and quickly place the hygrometer in the next bag. Give it time to stabilize. This can take anywhere from 40 minutes to six hours. You can tell when it’s ready for calibration because the reading stays the same for long periods of time. Complete the second calibration and you’re done for six months!

Note: If you want to test the overall accuracy of your instrument, other salts can produce many different RH levels. The procedures, in terms of mixing the salts and water and creating a controlled humidity chamber, are the same as those described above.


This calibration procedure can be used with any hygrometer, whether or not it can be calibrated, to determine its accuracy and mathematically correct for inconsistencies. For example, if the instrument reads 80% moisture in 75% salt solution, it is reading 6.0-7.0% too high and this should be taken into account when taking future readings. Typically, with a mechanical drive, only 75% reading is tested. Some people will test a hygrometer by wrapping it in a wet towel and after a few hours the reading should be around 98%. One problem with this is that if the instrument is set to read too high and shows a reading at the top of the scale, which seems reasonable when on a wet towel, the unit could be sensing 110% or even 120%. but that is not apparent because the reading is off the scale. Therefore, a subsequent reading showing 50% could actually be in error by 20 points. It is for this reason that the two lower calibration points, both clearly visible on the instrument screen, have been set by the manufacturers.

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