Prevent mold! Hygrometer Calibration
I work with hygrometers frequently in my role as a home inspector. Consumer grade hygrometers and/or relative humidity meters are inexpensive and notorious for inaccurate readings. That’s too bad because maintaining proper relative humidity in your home is a good start in discouraging fungus or even mold growth. 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 be kept between 30% and 50%, with 60% rarely being 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 allow you to 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 calibrate it accurately, 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 sludge or sludge, will generate consistent and predictable moisture.
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 relative humidity (RH) that is produced is fixed, as long as both water and the 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 humidity 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 room temperature is not critical for our purposes. For example, RH is fairly stable even with large 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% humidity, magnesium chloride (a salt), and water are used. At the ideal temperature of 77 degrees Fahrenheit, this solution will produce a RH of 32.78%. At 59 degrees Fahrenheit it will produce a RH of 33.30% and at 86 degrees Fahrenheit it will produce a RH of 32.44%. Again, “room temperature” is not critical.
Detailed Calibration Procedures:
With most professional instruments, it is recommended to calibrate at both a low point and a higher reference point. For convenience, most manufacturers have selected 75% and 33% RH as their 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 some water, but not too much. You want a wet sludge, not soup. I made containers out of yogurt cartons. I cut the top off so they were about two inches tall, and I cut 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 bags. Having some air in the bag is unavoidable and that’s okay. This method should work with any hygrometer, including inexpensive mechanical hygrometers, which are typically only 75% tested or calibrated. Once again, make the necessary adaptations to ensure that the instrument does not get wet; it should 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 to 75%, get the correction factor for future reference, and work from there. It should be close enough for your purposes.
NOTICE: If you have a professional electronic hygrometer, which has a built-in but accessible sensor, you can simplify the calibration procedure. Just get a couple of plastic jars, like oysters or similar foods, and drill holes in the lids to fit your instrument’s sensor snugly. Label the jars 75% and 33% and put your salt mixtures in the jars. I still use yogurt containers to hold salt mixes and squeeze them tight, 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. If not, put your hygrometer on one jar lid and a piece of tape or a seal of some kind on top of the other so the RH will stabilize. 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 shift, which is likely to happen if you have to calibrate the instrument rather than just look at it, the stability of the chamber humidity is affected and may result in calibration errors. As a result, that process should be done cautiously and double-checked.
75% chemical solution:
Use pure salt, sodium chloride, without additives. Morton’s 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 make a slurry. Put this in a Ziploc bag, with the hygrometer attached to 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 must be calibrated with power or display 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 is very 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 obtain as regular salt, but it’s not as hard to find and can certainly be made much cheaper than buying salt calibration kits. Prices and availability change, but I buy small amounts of magnesium chloride hexahydrate, lab-grade flakes, on Ebay. You won’t be using much at once, but hygrometers need to be calibrated twice a year, so they’ll be a worthwhile supply to have on hand. It’s getting harder to buy even simple chemicals, but you can find this at chemical supply houses online. It is also used as a de-icer. (Don’t buy a magnesium + chloride supplement from a health food store – wrong product). Mix Magnesium Chloride Hexahydrate with distilled water, in the same manner as described above, and follow all the same procedures. You can start both bags, 33% and 75% at the same time, and place 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 are 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.
POSTED SALT BATH HR AT 25°C
LITHIUM BROMIDE 6.37%;
LITHIUM CHLORIDE 11.30%;
POTASSIUM ACETATE 22.51%;
MAGNESIUM CHLORIDE 32.80%;
POTASSIUM CARBONATE 43.16%;
MAGNESIUM NITRATE 52.89%;
SODIUM BROMIDE 57.57%;
POTASSIUM IODIDE 68.86%;
SODIUM CHLORIDE 75.30%;
POTASSIUM CHLORIDE 84.34%;
POTASSIUM SULFATE 97.30%
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% humidity in the 75% saline solution, you are reading 6.0-7.0% too high and this should be taken into account when taking future readings. Typically with a mechanical unit, only the 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 displays a reading at the top of the scale, which seems reasonable when you are 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. So a subsequent reading that shows 50% might actually be 20 points in error. It is for this reason that the two lower calibration points, both clearly visible on the instrument’s screen, have been established by the manufacturers.
