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Prevent Mold! Hygrometer Calibration
I frequently work with hygrometers in my role as a home inspector. Consumer quality hygrometers and/or relative humidity meters are inexpensive and notorious for inaccurate readings. That’s too bad because maintaining the right relative humidity in your home is a good start to preventing mold growth or even mold growth. Mold can be difficult to identify and is usually omitted from home inspection reports. However, if an observer sees mold, he or she will usually mention it. Most experts recommend that indoor relative humidity be maintained between 30% and 50%, with 60% rarely being a cause for concern. You can go online and find hundreds of articles explaining the reasons for this and suggesting optimal readings for your particular climate. You can also get that information from the University Extension Service in your area. Once you have that target percentage, customized for your climate and region, the following simple procedures will allow you to ensure that the readings you get from your hygrometer are always reasonable and accurate.
To calibrate the hygrometer:
If you have a digital hygrometer or humidity meter and want to accurately calibrate it, without buying an expensive manufacturer-supplied salt calibration kit, here’s an easy solution. The physics behind this project is simple and reliable: different salts mixed with water to form a sludge or slurry will produce a uniform and predictable moisture content.
Simplified scientific explanation:
A saturated solution has a fixed composition and a fixed vapor pressure at constant temperature and pressure. Thus, at constant temperature, regardless of how much salt and how much water is present, the relative humidity (RH) is constant, as long as both water and solids are present. Therefore, as long as the water does not dry out, or the salt is wetted so much that it becomes a liquid, a predetermined humidity can be created.
It is convenient for us that a solution of common salt mixed with water (preferably distilled water) produces approximate humidity over a wide range of temperatures. A mixture of common salt (sodium chloride) and water has a moisture content of 75.29% at an ideal temperature of 77 degrees Fahrenheit. Room temperature is not critical for our purposes. For example, RH is constant even with large variations: a salt solution at 59 degrees Fahrenheit will produce 75.61% RH, and at 86 degrees Fahrenheit the RH is 75.09%.
To calibrate the lower end, 33% moisture, magnesium chloride (a salt) and water are again used. At an 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 a RH of 32.44%. Once again, “room temperature” is not critical.
Detailed calibration procedure:
As with most commercial instruments, it is recommended that they be calibrated at both a low point and a high reference point. For convenience, most manufacturers choose 75% and 33% RH as the default calibration standard. Therefore, in order to calibrate our equipment, 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 water – but not too much. You want a moist slurry, not a soup. I made containers out of yogurt containers. I cut the tops so that they were about two inches high and cut a recessed area so that the hygrometer could rest with the sensor on the solution without coming into direct contact with the wet solution.
Place the hygrometer in a container of yogurt and seal it in a Ziploc bag or two. It is inevitable and good to have some air in the bag. This method works with any hygrometer, including inexpensive mechanical hygrometers, which are typically only checked or calibrated at 75%. Again, make whatever accommodations are necessary to ensure the instrument doesn’t get wet — it should perceive RH, not water. Generally, with inexpensive hygrometers, you cannot calibrate the device by changing the setting but you can take a reading at a known RH and calculate a correction factor from that. If you have a simple instrument, say, just calibrate it to 75%, get the correction factor for future reference, and work from there. That should be close enough for your purposes.
Tip: If you have a commercial electronic hygrometer with a built-in but accessible sensor, you can simplify the calibration process. Simply get two plastic jars, such as oysters or similar, and drill holes in the lids so that they provide a snug fit for the sensor on your instrument. Label the jars 75% and 33% and place the salt mixture in the jars. I still use yogurt containers to hold the salt mixture and jam tightly about 1/3 of the way up the jar, creating a humidity chamber at the top of the jar. Screw the lids on the jars. If you have two hygrometers, place one in the lid of each jar. If not, put your hygrometer in a jar lid and put a piece of tape or some other type of seal over it to stabilize the RH. Once the correct RH is established, you can quickly check or recalibrate the hygrometer by inserting the sensor into one of the two jars, within the same general time period described below. Always allow time for an instrument to stabilize after moving it from one humidity chamber to another. This is the most accurate way for you to calibrate the instrument, if it can be done that way. The readings remain more stable than when using a plastic bag: if a bag is inadvertently compressed or its contents moved, if you want to calibrate the instrument instead of just looking at it, the humidity chamber is stable. are affected and may cause calibration errors. Consequently, that process must be carefully and double-checked.
Chemistry 75% Pass:
Use pure salt, sodium chloride — no additives. Morton canning salt from the grocery store is one such salt, and it’s inexpensive. Place a few spoonfuls in a curd container and add distilled water to prepare the curd. Place it in a Ziploc bag, place it on a hygrometer container, and let it rest for about 12 hours. This is how long it takes for the solution to stabilize. (I let it rest overnight.) Personally, I like to leave the hygrometer display on so I can see the readings from the bag as they change, and also know when the solution has stabilized.
With most digital hygrometers, they need to be calibrated with power or the display off. So, once the solution has set for 12 hours and the reading is clearly stable, I turn off the unit. Then I start with the manufacturer’s calibration procedure. Typically this involves pushing in a paperclip or similar object, a recessed button, and other controls in a set order. In essence, you are “teaching” the instrument to “recognize” the set humidity the next time it is exposed to it. With Ziploc bags, you can see the hygrometer readings and controls, so it’s a simple matter of punching a small hole in the bag with a paper clip and calibrating the instrument without interfering with the relative humidity created.
Chemistry 33% Solution:
You need magnesium chloride hexahydrate. It’s not as easy to get as regular salt, but it’s not as difficult to find, and it can certainly be done much cheaper than buying a salt calibration kit. Prices and availability vary but I buy magnesium chloride hexahydrate, lab grade flakes in small quantities on Ebay. You won’t use too much at one time, but hygrometers should be calibrated twice a year to make them a worthwhile supply to have on hand. Even simple chemicals are becoming difficult to buy, but you can find them 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, as described above, and follow all the same procedures. You can start both 33% and 75% bags at the same time and place an instrument in one. This allows both solutions to stabilize at the same time and start producing the RH we need. After you have done the first calibration, open and, quickly, place the hygrometer in the next bag. Allow time to settle. 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 your six months are done!
Note: If you want to test the overall accuracy of your instrument, other salts can produce many different RH levels. The procedures for mixing salts and water and creating a controlled humidity chamber are the same as previously described.
Salt bath illuminated at RH 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 Sulphate 97.30%
This calibration procedure can be used with any hygrometer, calibrated or not, to determine its accuracy, so that one can mathematically correct for discrepancies. For example, if the instrument reads 80% humidity in a 75% salt solution, it may be reading 6.0-7.0% too high and should be taken into account when taking future readings. Normally, with a mechanical unit, only 75% reading is tested. Some people will check the hygrometer by wrapping it in a wet towel and after a few hours, the reading should be around 98%. One of the problems with this is that if the instrument is set to read too high and it shows a reading at the top of the scale — which sounds reasonable when in a wet towel — the unit is really 110% or even 120% but it’s not obvious because the reading is off scale. Therefore, a later reading that indicates 50% may actually be 20 points lower. For this reason, two lower calibration points, both easily visible on the instrument’s display, are installed by the manufacturer.
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