The dog days of summer: humidity-measuring devices.

The dog days of summer will shortly be upon us, and with them will come the semiannual string of complaints about indoor air quality: everything from "I can't breathe" (which, as far as we can tell, lasts a maximum of about three minutes) to "mushrooms are growing from the carpeting" (a startling revelation, considering no one uses sterile, composted horse manure as a floor covering). Although highly humid conditions are generally the culprit, the subjectivity of interpretation can often turn an inspector's halcyon day into a nightmare. We have found that a bit of objectivity, as seen through the "eyes" of a humidity-measuring instrument and perhaps coupled with a bit of science theater, can reassure even the most vociferous complainant - or at least buy some time until relief can be mustered.

Humidity-measuring instruments come in a variety of shapes and operate in a variety of ways. For the most part, the larger, digital, battery-operated units cost about $200 (add an extra $100 or so for programmable instruments and those with dew-point measurement capability). The reliable, old-fashioned, hand-operated sling psychrometer costs about $100 (the lazy-person's unit with a battery-operated fan costs about $50 more), and the newest type of device, the digital pen-style pocket hygrometer, costs about $50. We have found that all units do a credible job, but those with the greatest utility are also the lightest in weight and most portable. Keep in mind that the well-equipped sanitarian takes along numerous tools when responding to an indoor air quality complaint. This arsenal can consist of several air-flow-measuring devices in addition to a carbon dioxide monitor, an ultraviolet (UV) lamp for detecting molds on fabrics and other porous surfaces, a dial thermometer, and anything else appropriate to the situation. After working with the whole gamut of humidity-measuring devices, we tend to favor the sling psychrometer and the pen-style pocket hygrometer - and these types also happen to be the least expensive.

Before we dismiss any of the other devices, we would like to say a little about them. (Since they are more appropriate for winter season investigations,

we will be reviewing them in more depth in an upcoming issue). The digital thermohygrometer is about the size of a standard sound level meter and usually comes in a hard-sided carrying case. This device has the advantage of a sensor probe that can get into the access areas of a ventilation system and between diffuser vanes to measure humidity. The digital thermohygrometer has a two percent margin of error within a humidity range of 10 percent to 95 percent. All units of this type feature large LCD readouts, [degrees] F/ [degrees] C switches, minimum/maximum memory, and automatic power-off. All use standard batteries that last about a year with moderate use.

We have found that measuring comfort levels in the summer months does not require a sensor probe. The goal is simply to turn a subjective observation into an objective measurement - humid versus not so humid - and with a degree of relative accuracy. To do so, we need to be where the humidity generates the effect, and for this purpose, we have found that portability outweighs sophistication. The unit that gets the greatest workout in our institutional practice is the pen-style pocket thermohygrometer. This little generic(*) button-battery-powered instrument displays temperature and humidity simultaneously in an LCD window. With reset, hold, auto-off, and [degrees] F/[degrees] C switch, this pocket unit provides the information essential for determining unhealthy dampness levels. It measures humidity from two percent to 98 percent and temperatures from 32 [degrees] F to 122 [degrees] F (0 [degrees] C to 50 [degrees] C); both measurements have a margin of error of three percent. Optimally, measurements are taken where air movement is detectable, since humidity contact on the protected sensor head provides the capacitance that translates into temperature and percent humidity readings. It takes a little over two minutes of exposure for the unit to stabilize and yield a credible result. When measuring humidity levels in refrigerated storage and food-processing rooms, allow the unit to come to ambient conditions before taking a reading. On rare occasions, we have questioned the accuracy of these units and have used a sling psychrometer to confirm the readings. We cannot pinpoint which environmental or operating conditions make these pocket hygrometers go a bit squirrelly; our best advice is to let your intuition be the judge. Even with this glitch, these handy, moderately priced, rugged instruments are quite useful in the field. The unit we evaluated is about three years old and is still going strong on the same set of batteries. What you lose in battery economy you make up in longevity. As a final note, these pocket thermohygrometers are factory set and do not require field calibration, although periodically comparing the readings with a sling psychrometer is prudent.

The sling psychrometer is about as old as the sealed glass thermometer itself. We have an elegant, English-manufactured, brass, psychrometer that probably was made at the last turn of the century - it still works! Simple in operation and very low maintenance, these measuring devices have been the standard since their development. Our sling psychrometer consists of two parallel, frame-mounted, 30 [degrees] F to 110 [degrees] F, 4-inch stem glass thermometers (with 1 [degrees] gradations); one of the thermometers has its bulb covered with a water-saturated cloth wick. We prefer to use demineralized water to prevent salt buildup. The unit is designed to whirl about, and after one and a half to two minutes of continuous whirling, the difference between the wet and dry bulb readings gives you the temperature of evaporation. The wet and dry bulb readings are compared to a psychometric chart that provides the relative humidity by percentage (%RH). For accuracy, read the wet bulb first and as quickly as possible. By the way, these units are inherently accurate if the liquid in the thermometers is not separated. Also, while barometric pressure has some effect on accuracy, its impact generally is not sufficient to cause errant readings.

For several reasons, our personal unit of choice is the Bacharach(**) Sling Psychrometer. This psychrometer is probably the most portable of all the devices. It is held in a high-impact plastic cylinder that protects the thermometers, contains the water reservoir, and serves as a slide rule chart; also, the outer holding tube, which is connected to the inner body by a plug-and-swivel assembly, can be used as the whirling handle. Lightweight, compact, and sturdy, the entire self-contained unit measures 1 inch by 8 inches and weighs just over 3 ounces. To use this instrument, simply slide the outer holding case to extend beyond the inner frame to which the thermometers are attached. The outer case becomes the whirling handle. When the unit is reassembled, the outer holding tube and the inner body form two slide-rule scales from which to read the relative humidity, so carrying a separate psychometric chart is not necessary. The Bacharach instrument comes with mercury-filled thermometers or, for the safety conscious, red-spirit-filled thermometers. In the company's instructional guide, all parts of the unit are listed, and all parts can be replaced if broken or worn out. No need to stock up on extra parts since the Bacharach folks are the fastest in the business in getting out an order. The units are warrantied for one year, but everyone we know who has a Bacharach sling psychrometer has had it forever without trouble. The design has not changed over the years. Our unit is about 15 years old and sees regular use. Although some of the numerals on the holding tube are getting faint, the unit is still as serviceable as the day it was purchased.

As a final note - and to explain the cryptic allusion to theater in the first paragraph - the sling psychrometer provides the best science theatrics of any investigation. Everyone who sees the twirling is fascinated by it, and many onlookers will volunteer to do the whirling. Of course, this always provides an excellent opportunity for a bit of education, and, more important, it demonstrates to complainants that "something" is being done. If it looks like a fan, it's got to be working - now doesn't that feel better already?

* The pen-type pocket hygrometer is available through most scientific instrument catalogs.

** Bacharach, Inc. 625 Alpha Drive Pittsburgh, PA 15238-2878 [phone]