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kind of annoyed they are 10%+ out with each other. One is new, calibrated one year ago, the other about 6 yrs old. Would you trust the new one and calibrate the old one to it, or send them both out for calibration?

No. I wouldn't even trust the new one until it's calibrated in your shop at the general temperature and barometric pressure in your area. Get or make yourself a wet/dry thermometer, draw air across it until it stabilizes, take the readings, go to http://easycalculation.com/weather/dewp ... ulator.php and plug in the numbers. You can get the pressure from Yahoo Weather or Accu Weather. Do it several times over a period of a couple of weeks and adjust the Hygrometer till it maintains correct readings. Mine, after going through that process stays within 1 or 2 % all the time. When I called the tech support at Abbeon, they told me to adjust it and to do it a number of times till it was correct.

P.S. When taking the readings take them as closely as possible to the 1/2 degree. Small variables make a big difference. Choosing lab thermometers with easy to read scales helps a lot. I find it easier to use Centigrade readings on the thermometers, but either will work.

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Waddy

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Another simple method of calibrating is using moistened salt and placing in an airtight bag along with the hygrometer. Leave for at least 6 hours. The humidity will stabilize at 75%. Note the difference that your hygrometer reads and adjust it by that amount.

Most of the Abbeon Hygrometers can be calibrated. There are a couple of other brands that can be too. Most of the better ones use the "hair" method. Abbeon uses a synthetic hair in thiers, maybe they all do for consistency, I don't know. Go here: http://www.ambientweather.com/hygrometers.html. They seem to have pretty good prices on the better ones. I bought the HTAB178 because they were out of the two black faced ones which I liked better, but now I like it fine. I was initially dismayed that it wasn't accurate, but after calling them, and hearing that it didn't seem to bother them, and since getting it dialed in, I'm happy. I don't think the age is an issue on accuracy. Abbeon will calibrate them for a price, but it's easily done once you have the set up. I check mine weekly, but have not touched the calibration after I got it set right.

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Waddy

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grumpy did some experimentation with using dewpoint measurements to determine relative humidity for calibration. He like it better than the wetbulb/drybulb method, IIRC.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

I use salt solutions to test for RH. They are extremely stable over the temperature range and phenomenally accurate and easy to do. The salt I recommend is KCO3 (Potassium Carbonate) which in an aqueous solution has a 43% RH above the solution. Rather than describe the process, here is a link:

http://www.kingofthehouse.com/hygrometer/

The important thing is to have a wet slurry -- you don't want a ton of water, just enough for the KCO3 to be extremely wet.

I use a Caliber III hygrometer as my test mule -- and then reference my other hygrometers from that one. There are many ways to calibrate but I personally like this method the best.

I like this idea as I don't think there's any guarantee as to the linearity in the response of these hygrometers. Personally I have a psychrometer that I use to test my caliber iii (which was pretty much dead nuts on right out the box) but the idea of calibrating at 75% and expecting it to work at around 40% always seemed rather….er…."hopeful" to me. Not saying that it absolutely can't work but how would you know if it doesn't?

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Jason-

I think it costs $25 a unit to have Abbeon calibrate for you to about 5% accuracy. If you do this, tell them to calibrate at 42% RH or whatever you're comfortable with in your shop environment.

I use a $12 sling psychrometer sp? (wet/dry bulb method) to get the reading in the shop and then adjust my 2 Abbeon hygrometers with a small flat screwdriver to match. Do small adjustments on the hygrometer until it reads the same. Right now, mine haven't been calibrated for 2 years and read 4% RH apart. I need to test with the psychrometer to see how far off they are from the true RH in my shop.

The only problem I see with using the wet table salt (Sodium Chloride) method is it is being set at 75% RH - the farther away from 75% RH you go, the less accurate the hygrometer will be. You need to set the hygrometer in the humidity level you want to build in. That way the hygrometer will be the most accurate at that particular % RH.

Like Toonces (Simon) says, the best solution to use would be the Potassium Carbonate salt solution which gives a 43% RH at 25C. I'm sure the various salt methods are more accurate than the psychrometer, but I'm curious just how much more accurate. I'm no expert at this stuff, but my methods have worked well for me. Personally, I would calibrate them myself and save the $$.

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John Lewis
Wannabe builder owned by 2 crazy dachshunds

I have two of the Abbeon hygrometers. One I bought new with "guaranteed calibration." It arrived way off. At the time, I was using a psychrometer for calibration. Once I calibrated it, it has held fine over a few years. I am very happy with both of my Abbeons. Nothing else I have tried, other than the psychrometer, has been as reliable. The problem with the psychrometer is that a difference in a temperature reading of less than a degree can make a humidity difference of several percent. I got some potassium carbonate from a make your own soap company and have been using that for calibration. It's easy and accurate. StewMac has a new analog hygrometer that is much cheaper than the Abbeon. I will be interested in reports on that one.

Most of the less expensive, analog, hygrometers use a metal spring for the sensor. This works, but is less accurate and dependable. Many of those can be calibrated, but they also have to be exposed to nearly 100% humidity from time to time to continue working well. I don't understand that, but that's what the instructions say.

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Waddy

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Sound Clips of most of my guitars

Thanks guys, live in Canada, so shipping to Calif for calibration wasnt going to work, but have a sling psychrometer coming.