Prevent Mold! Hygrometer Calibration
- September 25, 2017
- Posted by: marlenedubois
- Category: CPR Training
I work with hygrometers frequently in my role as a home inspector. Consumer quality hygrometers along with/or relative humidity gauges are inexpensive along with infamous for inaccurate readings. which is actually too bad because maintaining the proper relative humidity at your home is actually a not bad start in discouraging fungal growth or even mold. Mold may be hard to identify along with the item is actually, typically, excluded on home inspection reports. However, if an inspector sees mold he or she will normally mention the item. Most experts recommend which relative humidity in a home be maintained between 30% along with 50%, with 60% seldom being a cause for concern. You can go online along with find hundreds of articles explaining the reasons with This particular along with suggesting optimum readings for your particular climate. You could also get which information coming from a university extension service in your area. Once you have which target percentage, customized for your climate along with region, the simple procedures below will allow you to make sure which the readings you get coming from your hygrometer are reasonable along with accurate at all times.
Calibrating a Hygrometer:
If you have a digital hygrometer or humidity gauge along with wish to accurately calibrate the item, without having to purchase expensive any-supplied salt calibration kits, here is actually the easy solution. The physics behind This particular project is actually simple along with reliable: Different salts, when mixed with water to create a sludge or slurry, will generate a consistent along with predictable humidity.
Simplified scientific explanation:
A saturated solution at a stable temperature along with pressure carries a fixed composition along which has a fixed vapor pressure. Thus, at constant temperature, no matter how much salt along with how much water are present, the (RH) relative humidity which is actually produced is actually fixed, just as long as both the water along with the solid phase are present. So, unless the water dries up, or the salt is actually made so wet which the item liquefies, a predetermined humidity can be produced.
the item is actually convenient for us which a solution of ordinary salt mixed with water (preferably distilled water) produces a predictable humidity over a wide range of temperatures. The humidity created, with ordinary salt (Sodium Chloride) along with water, is actually 75.29% at an ideal temperature of 77 degrees Fahrenheit. The temperature of the room is actually not critical for our purposes. For example, the RH is actually quite stable even with large variations: Salt solution at 59 degrees Fahrenheit will produce 75.61% RH along with at 86 degrees Fahrenheit the RH is actually 75.09%.
To calibrate the lower end, 33% humidity, Magnesium Chloride (a salt) along with water is actually used again. At the ideal temperature of 77 degrees Fahrenheit, This particular solution will produce an RH of 32.78%. At 59 degrees Fahrenheit the item will produce an RH of 33.30% along with at 86 degrees Fahrenheit the item will produce 32.44% RH. Once more, “room temperature” is actually not critical.
Detailed calibration procedures:
With most professional instruments, the item is actually recommended which they be calibrated at both a low point along which has a higher reference point. For convenience, most manufacturers have selected 75% along with 33% RH as the default calibration standards. So, 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 along with mix the item with some water – although not too much. You want a damp sludge, not soup. I made containers coming from yogurt cartons. I cut the tops off, so they are about two inches high, along with cut a recessed area so the hygrometer can rest with the sensor over the solution without the item being in direct contact with the wet solution.
Put the hygrometer across the yogurt container along with seal the item in one, or even two, Ziploc bags. Having some air inside the bag is actually unavoidable along with fine. This particular method should work with any hygrometer, including the inexpensive mechanical hygrometers, which are typically only tested or calibrated at 75%. Again, make any accommodations required to make sure the instrument does not get wet — the item must sense the RH along with not water. Normally, with the cheaper hygrometers, you cannot actually calibrate the device by changing the setting although you can take a reading in a known RH along with coming from which calculate a correction factor. If you have a simple instrument, such as This particular, just calibrate the item at 75%, get the correction factor for future reference, along with work coming from there. the item should be close enough for your purposes.
NOTICE: If you have a professional electronic hygrometer, which carries a built-in although accessible sensor, you can simplify the calibration procedure. Merely obtain a couple plastic jars, such as oysters or similar foods come in, along with drill holes inside the lids so they provide a snug fit for the sensor on your instrument. Label the jars 75% along with 33% along with put your salt mixtures inside the jars. I still use the yogurt containers to hold the salt mixtures along with jam them in tight, about 1/3 of the way into the jar, so a humidity chamber is actually formed near the top of the jar. Screw the lids on the jars. If you have two hygrometers, put one in each jar lid. If not, put your hygrometer in one jar lid along which has a piece of tape or a seal of some type over the different one so the RH will stabilize. Once the proper RH has been created, inside the same general time-frame described below, you can quickly check or re-calibrate a hygrometer by inserting the sensor in either of the two jars. Always give an instrument some time to stabilize, after moving the item coming from one humidity chamber to another. This particular is actually the most accurate way for you to calibrate an instrument, if the item can be done This particular way. The readings stay more stable than they do when a plastic bag is actually used: If a bag is actually inadvertently compressed or the contents shifted, which is actually likely to happen if you have to calibrate the instrument instead of merely viewing the item, stability of the humidity chamber is actually affected along with which can result in calibration errors. As a result, which process must be performed cautiously along with double-checked.
Chemistry 75% solution:
Use pure salt, sodium chloride — no additives. Morton canning salt coming from a grocery store is actually such a salt along with the item is actually inexpensive. Put a few tablespoons inside the yogurt container along with add distilled water to form a slurry. Put This particular in a Ziploc bag, with the hygrometer positioned over the container, along with let the item rest for about 12 hours. the item takes which long for the solution to stabilize. (I let the item rest overnight.) Personally, I like to leave the hygrometer display on so I can view readings through the bag, as they change, along with also which way I know when the solution has stabilized.
With most digital hygrometers, they must be calibrated with the power or display turned off. So, once the solution has set for 12 hours along with the reading has obviously stabilized, I turn the unit off. Then I commence with the any’s calibration procedures. Typically This particular involves pushing in, which has a paperclip or a similar object, a recessed button along with different controls in a set order. In essence, you are “teaching” the instrument to “recognize” a set humidity the next time the item is actually exposed to the item. With the Ziploc bag, you can see the hygrometer reading along with the controls so the item is actually a simple matter to punch a tiny hole inside the bag with the paper clip along with calibrate the instrument without interfering with the relative humidity which has been created.
Chemistry 33% solution:
You need Magnesium Chloride Hexahydrate. This particular is actually not as easy to obtain as regular salt, although the item is actually not which difficult to find along with the item certainly can be done much cheaper than purchasing salt calibration kits. Prices along with availability change although I purchase little quantities of Magnesium Chloride Hexahydrate, lab quality flakes, on Ebay. You will not use much at a time, although hygrometers should be calibrated two times per year so the item will be a worthwhile supply to have on hand. the item is actually becoming harder to buy even simple chemicals, although you can find This particular one at online chemical supply houses. the item is actually, also, used as a de-icer. (Do not buy a magnesium + chloride supplement at a health food store – wrong product.) Mix the Magnesium Chloride Hexahydrate with distilled water, the same way as was described above, along with follow all the same procedures. You can start both bags, 33% along with 75% at the same time, along with place the instrument in one. This particular allows both solutions to stabilize at the same time along with to begin producing the RH you need. After you have done the first calibration, open along with, quickly, put the hygrometer inside the next bag. Give the item time to stabilize. This particular can take coming from 40 minutes to six hours. You can tell when the item is actually ready for calibration because the reading stays the same for long periods of time. Complete the second calibration along with you are done for six months!
Note: If you wish to test the overall accuracy of your instrument, different salts can produce many different RH levels. The procedures, as far as mixing the salts along with water along with creating a controlled humidity chamber, are the same as previously described.
SALT BATH PUBLISHED RH 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%
the item is actually possible to use This particular calibration procedure with any hygrometer, whether the item can be calibrated or not, to determine the item’s accuracy so one can mathematically correct for inconsistencies. For example, if the instrument is actually reading 80% humidity inside the 75% salt solution, the item is actually reading 6.0- 7.0% too high along with This particular should be taken into account when future readings are taken. Typically, which has a mechanical unit, the item is actually only the 75% reading which is actually tested. Some people will test a hygrometer by wrapping the item in a wet towel along with, after a few hours, the reading should be around 98%. One problem with This particular is actually, if the instrument is actually set to read too high, along with the item shows a reading at the top of the scale — which seems reasonable when the item is actually in a wet towel — the unit might actually be sensing 110% or even 0% although which is actually not apparent because the reading is actually off the scale. Therefore, a later reading which shows 50% might actually be off by 20 points. the item is actually, with This particular reason, which the two lower calibration points, both readily apparent on the instrument’s display, have been established by manufacturers.