# Hygrometers: They're accurate, and why they're not



## Boa249 (Jan 10, 2014)

I found this gem in an Amazon.com review, and thought it was worth sharing. This was for a digital hygrometer costing less than $20:


I laugh when I read reviews stating that a particular hygrometer is "accurate" or "not accurate." I know these reviewers can not really tell one way or the other. Even laboratory calibrators have problems with determining accuracy. But here I'm going to call the Caliber III reasonably "accurate" and explain why. I do critical environmental surveys as part of my professional work and use a variety of expensive analog and digital instruments, thermometer wet/dry bulb psychrometers, multi-hair recording hygrothermographs, some instruments with NIST certification (but not mirror instruments). I've bought several "cheap" analog and digital hygrometers costing under $150.00 (including the Caliber III) to play with and check for suitable monitoring use by my daughters who humidify and dehumidify their living and storage spaces for violins and violas. I give this little instrument 5 stars for being reasonably "accurate" for its low price and small size - certainly accurate enough for musical instrument storage, cigar humidors, and the most simple basic room environmental monitoring. However, don't expect that "accurate" means what the instrument displays is what the environment really is. And do not expect these inexpensive instruments to all read the same when placed side-by-side. And do not trust bag/salts solution verification or for "calibration." Even laboratory closed environmental chambers used for calibration purposes are difficult to manage. All the inexpensive instruments I purchased and all my costly professional instruments costing into the thousands do not give exactly the same readings when placed together in my laboratory controlled climate chamber. Do not expect a lot of precision in humidity measurement for a number of factors too complicated to go into here but related to airborne particles and various gaseous molecules in the air. RH is notoriously difficult to pinpoint with accuracy and a lot of work has gone into trying for critical industrial operations. For most instruments costing under around $150.00 that are displaying, let's say, 50% RH, just believe the actual RH is anywhere in the range 46-54% - at best. Accept that limitation. Its hard to do, though, when you see that single digital number displayed. But you need to think in ranges only. And just like the mechanical/analog instruments, even the better ones with multiple strands of human hair, the potential range of digital instrument reading variations from actual RH will widen as your environment comes to either extreme from 50% RH, though not as much with digital as with mechanical/analog. This is important for the cigar keepers. If your better low cost digital unit is reading 65% RH, it really can be within the range of 50-75% RH but there is the "probability" (and that's all you can count on) that it is around 55-65% RH, and not likely 70% RH due to the upper end resistance to accurate measurements of all instruments. Forget using mechanical/analog "dial" instruments for humidors. The only ones reasonably accurate would not fit in most humidors. Using these at the extreme of 65% RH, if that is your target, is asking too much of them as the further from 50% RH the environment, the more resistant these are to reading the actual. However, for larger humidor cases, I have found the Oakton 357 to be good enough (I have three)and can give you some confidence that if its displaying 65% RH, its probably 60-70 RH. For instruments up to $1,000, a 50% RH reading probably represents actual RH of 48-52% RH. Do not expect more. One other thing you must know about digital hygrometers, and this is critical to recognize, they are fast(the more you pay for the sensor within it, the faster its response) and they quickly read their local environments which includes your body heat, skin moisture, breath moisture, and varying temperature and humidity in local air currents you may not even feel. You are a walking warm cloud. Certainly don't hold one and expect it to match another instrument sitting next to it on a shelf! The heat of your hand warms the plastic case raising the temperature reading and thus also drastically affecting RH. Don't even stand within three feet of a sensitive and fast digital instrument and expect it to match the display of one four feet from you. Also, this digital instrument and others like it will detect and display numbers representing something close to the real differences in the environment that can exist just inches apart. That's why two of the same instruments next to each other may not match - its not bad quality but high sensitivity. This speed and accuracy of even inexpensive hygrometers is probably the sole cause of people complaining about "inaccuracy" of digital hygrometers and the frustration they experience. The differences they observe in the readings can be really due to accurate sensitivities. Though digital relative humidity monitors tend to be fast, and the more you pay the faster they are, for many purposes the less fast the better because their relative slowness tends to eliminate the small local differences in environments that make people think their instruments are going "haywire." For more information related to how difficult it is to really get accurate RH readings and difficulty in calibration, you must read "The Trouble with Humidity" under the tab "Publications" on the Veriteq.com website. If you want just "a little more accuracy", you can get "a little more accuracy" by paying a few extra hundred dollars, like over $500 for a Vaisala HM34. For the price, and considering how nearly impossible it is to get a true measurement of RH, maybe its good enough for you just to know, for example, "well, the RH is likely to be somewhere between 45 and 55 according to this instrument because its displaying 50."


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## Tom (Jan 10, 2014)

I've got no disagreement with any of that.

Within 10-15% points is close enough for my purposes.

Thanks for posting. Hope this adds some clarity for some people.


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## wellington (Jan 10, 2014)

I have found that most things give you a + or - of accuracy. 
You can apply the same train of thought too most things. Good post. Thanks for sharing.


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