Is a digital hygrometer accurate enough for reading wet bulb, or should I get a sling-type psychrometer?
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.
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
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.
It sounds like an excellent device.
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.
Also, I have seen many that have 2-degree F divisions, which is not satisfactory.
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.