Automatic Water-Fill Dry-Wet Bulb RH Metering

      Advice on the fine points of keeping a relative humidity monitoring setup supplied with water for the wet bulb. June 13, 2014

Does anyone know an automatic water fill dry-wet bulb sensor for measuring relative humidity (or EMC)?

Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
I have seen many that use a float system like a toilet does (see for example, SII Dry Kilns). I have seen many that use a needle value (or similar) so that the water flow can be regulated. I have seen some (too many) that have the water pouring in so fast that the drain pipe cannot handle the excess. When evaluating a water box, also consider the importance of air flow across the bulb. I have seen some boxes that are so large that the restrict air flow substantially. Specifically, I would guess that every large kiln company has a regulated water box available for sale.

From the original questioner:
I'm going to build one myself, so I would like to find any draw/pictures in internet to inspire myself. I'm a little confused about dimension of wet bulb, air flow around it and cotton thickness.

From Gene Wengert, forum technical advisor:
Although the ideal shape and air flow has been mathematically described, from a practical point of view it is not important. You do want a bulb that has a longer time lag so that the temperature is not bouncing up and down like crazy. Many wet-bulbs are pencil size. The wicking can be diaper cloth, but always wash it first to eliminate the sizing used on new fabric. Air flow should be about 600 fpm across the bulb. Using an external fan is often a good idea.

From Gene Wengert, forum technical advisor:
Remember that there must be very good air flow across the wet-bulb. The water box cannot be so large as to restrict air flow - consider both fan directions. The WB sensor itself is what must have the air flow and it cannot be too close to the water either. I have seen many large water boxes used and then when we put an auxiliary fan to blow on the bulb there will be a significant drop in WB temperature.

From contributor P:
I've read varying statements about required air flow across the wet bulb (anything from 300 fpm to 1000 fpm).

From Gene Wengert, forum technical advisor:
The key is to use the same air flow as the original studies used. If they had used different air flow, then the tables would have been different. The air flow was 600 fpm. It is likely cited in the original work done in about 1909. I saw a theoretical discussion of this in a book called Transport Phenomena by Byrd, Stuart (or Stewart) and Lightfoot.

From contributor P:
Good old BSL! I looked it up in my old first edition first, but the second edition had a little more useful information. From my reading of it the fluid velocity needs to be "high enough that the thermometer readings are unaffected by radiation and by heat conduction along the thermometer stems, but not so high that viscous dissipation heating effects become significant." For glass thermometers these assumptions are true in the 30-100 ft/s range, which is considered the low mass transfer rate range.

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