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Digital hygrometers

Kiln operators discuss options for measuring conditions in their lumber drying and storage facilities. July 26, 2000

Is a digital hygrometer accurate enough for reading wet bulb, or should I get a sling-type psychrometer?

For kilns and predryers, I've always depended on a high-quality sling psychrometer to cross reference readings and such.

For around the offices, we use desktop-type ColeParmer instruments, and they work just fine.

For casual spot checks throughout my drying system, I also use a regular wet bulb/dry bulb hygrometer that I got from Lignomat USA, Ltd., in Portland, OR. It's a relatively simple, aluminum-bodied device that easily slides into the sticker slots, and it's equipped with two high-quality thermometers. I get accurate readings very quickly with this thing.

I'm setting up my first Nyle DH kiln chamber. Do you get better readings if you take them in the pile as you mentioned?

I always use a $30 digital hygrometer that I got from Radio Shack to check conditions in a kiln or predryer (or even in a factory or storage facility). Frequent checks with a sling show that this instrument is extremely accurate (within 1/2 degree F and 2 percent relative humidity), and I have had it for eight years.

It's fast, durable (can be dropped), accurate (more accurate than a sling, with 2-degree-F divisions on the bulbs), and, if you have two or three, easy to check for calibration.

We always measure the RH and temperature of the air as it enters the pile of lumber -- not at intermediate spots, or when it exits.
Gene Wengert, technical advisor

If you get an instrument that's accurate to .5 degrees F and 2 percent RH for $30, you should get back out and buy a lottery ticket!

Instruments that are good to plus or minus 3 percent (between 10 and 95 percent RH) and plus or minus 1 degree F (between 0 and 100 degrees) cost around $300.

And I know from experience that some of the more expensive ones don't deliver the accuracy promised.

If you don't mind commenting once more about your hygrometer, could you post a model number or more details? I don't have a Radio Shack catalog.

It sounds like an excellent device.

I continually check the Radio Shack device against a psychrometer, so I know it works very well. Model number is 63-855.

What I do is buy five, put them in a plastic bag, and then see if they all read the same. Those that do not I return. At least all of the ones I have agree with each other. These also have a memory (high and low) for both RH and tmperature. It is amazing what the new electronic RH devices can do.

Incidentally, I have a $300 electronic instrument from ColeParmer in Chicago that will give me the same readings -- the expensive one is faster and comes on a longer probe. But for work in the kilns and with wood, the Radio Shack one is great.

Here are the technical aspects: At room temperature, a one-degree, wet-bulb depression change will account for 4 percent RH change (80 degrees F dry-bulb and 70 degrees wet-bulb, 10-degreee depression, is 61 percent RH; an 11-degree depression is 57 percent RH.)

So, to talk about measuring RH to more than 2 or 3 percent RH accuracy is better than most thermometers can do -- a 1-degree instrument has a 1/2-degree error possible on both bulbs, so the depression is only within 1 degree F.

Don't you need 600 feet per minute (fpm) air velocity across the wick to provide the evaporative cooling necessary to determine the actual wet-bulb temperature?

Yes, indeed, 600 fpm is best. In many kilns and predryers, I have seen the readings change (for the better -- more stable and quicker response) when I added a fan blowing onto the wet bulb.

Also, I have seen many that have 2-degree F divisions, which is not satisfactory.

Two more things about RH meters that weren't mentioned: If you walk into a warm, wet kiln with a cool, dry sensor, it gets covered with condensation and stops working for a while. And, if you go from kiln to kiln, the temperature compensation circuitry can give erroneous readings (at least with the ones I've used).

The little Radio Shack sensor can be put into your pocket when you leave a kiln, so it will not cool the way the bigger, more expensive instruments do. The temperature effect is a problem for all devices, however. Even psychrometers take time to adjust to a new environment.

This is why I buy five units for $150 (or less when on sale). I can leave them in the predryer, storage shed, kiln, etc. as long as needed. Note that the best range is under 130 degrees F and under 95 percent RH.