I work as a house inspector often with a hygrometer. Consumer quality hygrometers and/or relative hygrometers are inexpensive and notorious, and readings are inaccurate. This is too bad, because maintaining proper relative humidity in the home is a good starting point to stop the growth of fungi and even mold. Molds can be difficult to identify and are usually not included in the home inspection report. However, if the inspector sees mold, he or she usually mentions it. Most experts recommend that the relative humidity in the home be between 30% and 50%, of which 60% is of little concern. You can find hundreds of articles online, explain why, and suggest the best reading for your specific climate. You can also get this information from the University Extension Service in your area. Once you have a target percentage tailored to your climate and region, the following simple procedure will allow you to ensure that the readings obtained from the hygrometer are always reasonable and accurate.
Calibrate the hygrometer:
If you have a digital hygrometer or hygrometer and want to calibrate it accurately without having to buy an expensive manufacturer's salt calibration kit, this is a simple solution. The physical properties behind the project are simple and reliable: different salts, when mixed with water to produce sludge or slurry, produce consistent and predictable humidity.
Simplified scientific explanation:
The saturated solution at a stable temperature and pressure has a fixed composition and a fixed vapor pressure. Therefore, at a constant temperature, the relative humidity [RH] produced is fixed regardless of the amount of salt and how much water is present, as long as water and a solid phase are present. Therefore, unless the water dries or the salt is wetted to liquefy, a predetermined humidity is generated.
For our convenience, it is convenient that a solution of a common salt mixed with water, preferably distilled water, produces a predictable humidity over a wide range of temperatures. The humidity produced using ordinary salt [sodium chloride] and water is 75.29% at a desired temperature of 77 degrees Fahrenheit. The temperature of the room is not important for our purposes. For example, RH is very stable even if the change is large: a 59 degree Fahrenheit salt solution will produce 75.61% RH, while a Fahrenheit 86 degree relative humidity of 75.09%.
In order to calibrate the lower end, 33% humidity, magnesium chloride [salt] and water were used again. At an ideal temperature of 77 degrees Fahrenheit, the solution will produce 32.78% RH. At 59 degrees Fahrenheit, it will produce a relative humidity of 33.30%, and at 86 degrees Fahrenheit it will produce a relative humidity of 32.44%. Again, "room temperature" is not important.
Detailed calibration procedure:
For most professional instruments, it is recommended to calibrate at the low and high reference points. For convenience, most manufacturers choose 75% and 33% RH as the default calibration standards. Therefore, in order to calibrate our instruments, we need to be able to place the equipment in our own custom "humidity chamber".
To make a 75% humidity chamber, put the salt in the container and mix with some water - but not too much. You want a moist sludge instead of a soup. I made a container from a yogurt carton. I cut the top off so they are about 2 inches tall and cut a recessed area so the hygrometer can place the sensor on the solution without direct contact with the wet solution.
Place the hygrometer on the yogurt container and seal it in one or even two Ziploc bags. Some air in the bag is inevitable, very good. This method is applicable to any hygrometer, including inexpensive mechanical hygrometers, which are typically only tested or calibrated at 75%. Again, make any necessary adjustments to make sure the instrument is not getting wet - it must sense RH instead of water. Normally, with a cheaper hygrometer, you can't actually calibrate the device by changing the settings, but you can read and calculate the correction factor from the known RH. If you have a simple instrument, for example, just calibrate it to 75%, get the correction factor for future reference, and start working there. It should be close enough to your purpose.
Note: If you have a professional electronic hygrometer that has a built-in but accessible sensor, you can simplify the calibration process. Just get some plastic cans, such as oysters or similar food, and drill holes in the lid so they can provide a comfortable fit for the sensors on your instrument. The jars were labeled 75% and 33% and the salt mixture was placed in the jar. I still use a yogurt container to hold the salt mixture and clog them tightly, about 1/3 into the tank, thus forming a humidity chamber near the top of the tank. Screw the lid onto the can. If you have two hygrometers, put one for each can lid. If not, place the hygrometer on a can lid and a piece of tape or some type of gasket so that the RH will stabilize. Once the appropriate RH has been created, you can quickly check or recalibrate the hygrometer by inserting the sensor into either of the two tanks within the same general timeframe described below. After moving the instrument from one humidity chamber to another, always give the instrument some time to stabilize. If you can do this, this is the most accurate way to calibrate the instrument. The reading is more stable than when using a plastic bag: if the bag is inadvertently compressed or the contents are displaced, this may happen if you need to calibrate the instrument instead of just observing it, the stability of the humidity chamber is affected and may result in Calibration error. Therefore, the process must be carried out with care and double checking.
Chemical 75% solution:
Use pure salt, sodium chloride - no additives. Morton canned salt from the grocery store is such a salt, which is cheap. A few tablespoons were placed in a yogurt container and distilled water was added to form a slurry. Place it in a Ziploc bag, place the hygrometer on the container and allow it to stand for about 12 hours. The solution takes a long time to stabilize. [I let it rest overnight.] Personally, I like to display the hygrometer on it so that I can see the readings through the bag because they change, and I know when the solution is stable.
For most digital hygrometers, they must be calibrated with the power or display turned off. Therefore, once the solution is set to 12 hours and the readings are clearly stable, I turn off the device. Then I started the manufacturer's calibration procedure. Typically, this involves pushing a recessed button and other controls in a set order with a paper clip or similar object. Essentially, you are "teaching" the instrument to "recognize" the set humidity the next time it is exposed to it. With the Ziploc package, you can see the hygrometer readings and controls, so using a paper clip to make a small hole in the bag and calibrating the instrument without disturbing the relative humidity that has been created is a simple matter.
Chemical 33% solution:
You need magnesium chloride hexahydrate. This is not as easy to obtain as ordinary salt, but it is not difficult to find, and it is definitely much cheaper than buying a salt calibration kit. Prices and availability have changed, but I bought a small amount of magnesium chloride hexahydrate, a laboratory quality sheet on Ebay. You won't use too much at once, but the hygrometer should be calibrated twice a year, so it's a worthwhile supply. It is getting harder and harder to buy even simple chemicals, but you can find them at online chemical suppliers. It is also used as a deicer. [Do not buy magnesium + chloride supplements at the health food store - the wrong product.] Mix magnesium chloride hexahydrate with distilled water in the same manner as above, and follow all the same steps. You can start both packages at the same time, 33% and 75%, and put the instrument in one. This allows both solutions to be stable at the same time and start producing the RH you need. After completing the first calibration, open and quickly place the hygrometer in the next bag. Give it time to be stable. This can take 40 minutes to 6 hours. You can know when calibration is possible because the readings remain unchanged for a long time. After completing the second calibration, you have completed six months!
Note: If you want to test the overall accuracy of the instrument, other salts can produce many different RH levels. The procedure is the same as previously described for mixing salt and water and for producing a controlled humidity chamber.
Salt bath rental RH at 25 ° C
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Lithium bromide 6.37%;
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Lithium chloride 11.30%;
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Potassium acetate 22.51%;
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Magnesium chloride 32.80%;
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Potassium carbonate 43.16%;
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Magnesium nitrate 52.89%;
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Sodium bromide 57.57%;
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Potassium iodide 68.86%;
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Sodium chloride 75.30%;
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Potassium chloride 84.34%;
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Potassium sulfate 97.30%
This calibration procedure can be used with any hygrometer, whether or not it can be calibrated to determine its accuracy so that inconsistencies can be corrected mathematically. For example, if the instrument reads 80% humidity in a 75% salt solution, the reading is as high as 6.0-7.0%, and this should be taken into account in future readings. Typically, when using a mechanical unit, only the test reading is 75%. Some people wrap the hygrometer with a wet towel for testing. 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 it shows a reading on the top of the scale - it looks reasonable when it is in a wet towel - the unit may actually sense 110% or even 120% but......
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