A superbly accurate motorized psychrometer.

Two years ago, we wrote about the pen-type and sling psychrometers. If you recall, we rather liked them, because they could give us relatively accurate readings in a short time; they are compact, portable, and surprisingly durable and inexpensive. At least one of these tools remains a mainstay in our arsenal of instruments. We have kept the pen-type electronic psychrometer, but we discarded the sling psychrometer when our last one broke, resulting in a minor mercury spill. (Thank goodness it was not one of us who did the breaking.) Our reasoning for discarding the sling psychrometer is simple. We felt that we did not want to trade a relative-humidity measurement for an environmental disaster. Consider, if you will, that the mere mention of "mercury" and "spill" in the same sentence sends some folks packing to the next county and brings on a cast of characters in Level A protective gear. Not a pretty sight. As a side note: We tried the spirit-filled thermometer units but found that they were exceedingly difficult to read under low-light conditions.

This left us in a quandary. We still required the accuracy afforded by wet- and dry-bulb thermometers for validating our electronic units and for situations in which the measurement of humidity is detailed in a standard. Those of us who work in universities and other facilities engaged in animal research know that the measurement of moisture in the air is an exacting science. In acceptable trade standards, the percent relative humidity in vivaria is specifically detailed. These standards require a wet- and dry-bulb thermometer measurement with an airflow velocity of 10 feet per second or greater. While the sling psychrometer certainly meets the spirit of the standards, each one of us spins the unit at our own pace--the variability determined by our enthusiasm or lack thereof. We therefore needed a unit that would be safe, portable, and accurate and, more important, that would meet standards like those of the National Institutes of Health, the U.S. Department of Agriculture, the National Research Council, and the American Association for Laboratory Animal Science.

After doing some research on the measurement of humidity--and particularly on measuring units that would meet our needs--we settled on the Psychro-Dyne, a battery-operated, motor-driven, wet-and-dry-bulb thermometer psychrometer manufactured by Industrial Instruments & Supplies.* Our decision was predicated on the following: First, the difference between the wet- and dry-hulb measurements is the most accurate means of measuring the quantity of moisture in the air.

Second, the theory of wet- and dry-bulb thermometers is based on the evaporative cooling effect of the wet bulb. The drier the surrounding air, the more rapidly water will evaporate. The evaporation causes the wet bulb to depress in temperature until a balance occurs between the cooling effect and the normal heat gain from the ambient condition. The psychrometric formula relates this observed temperature depression and the temperature of the dry bulb to the quantity of moisture in the air. It can express this quantity as relative humidity, absolute humidity, dew point, or vapor pressure. An accurate measurement of the true wet- and dry-bulb temperature does, however, require adequate ventilation across the wet bulb to remove ambient moisture buildup. The Psychro-Dyne is designed to provide airflow of 15 feet per second throughout a reasonably long battery life.

Finally, we wanted a unit with wet and dry bulbs that would be easy to read and would be protected from damage. Our unit has nicely demarked mercury thermometers, and it is backlit in red for easy reading; this is particularly useful in low-light conditions. The capstone that clinched our decision was the reference charts included in the instructions that come with the unit. Based on the psychometric formula, these psychyrometric charts, tables, and calculators are provided as a convenience and can be used to rapidly determine relative and absolute humidity, dew point, or vapor pressure. In addition, the unit itself has a psychrometric chart on its cover and a humidity calculator slide rule in the carrying case.

The entire unit, including the Psychro-Dyne and its carrying case, measures 8 by 12 by 3 inches and weighs approximately 3 pounds when fully loaded with three D-cell batteries and a plastic bottle of distilled water. An extra bulb and an extra wet-bulb wick are provided. All components of the unit, including the thermometers, battery, and fan, are exposed when the hinged cover with magnetic latch is opened. There is even a place to store the distilled water bottle. The D-cell batteries and lamp are easy to install and remove. The thermometers are held in place by rubber grommets and retaining clips, so that split mercury columns are likewise easily remedied (instructions are included).

To operate the unit, first saturate the wick of the wet-bulb thermometer with distilled water. By the way, use only distilled water to prevent salting of the wick fabric. Close the cover and hold the instrument in a horizontal plane facing the graduated thermometers. Avoid strong light on the wick end to prevent heating from radiant energy. Next, turn on the control switch, which starts the fan motor and aspiration. As the fan operates, the wet-bulb temperature will begin to rapidly depress. A reading should not be taken until the depression completely stops. This step may take from one to two minutes, depending on the dryness of the air. Read the temperature on both the wet- and dry-bulb thermometers, and use the charts, the tables, or a calculator to get the value desired.

The price of this unit varies with the distributor. We have seen it as high as $350; ours cost about $250. Comparative shopping is worth it--and so is the Psychro-Dyne.

Inspection Tip of the Month

We monitor and, when necessary, replace all batteries in our measuring tools on a quarterly basis, except when the units are in constant use. Then alkaline batteries are replaced daily and button batteries monthly. An inexpensive pocket-sized multimeter, available through RadioShack, is the best tool for routine checking of batteries before an instrument goes into the field.

*Industrial Instruments & Supplies, P.O. Box 416, County Line Industrial Park, Southampton, PA 18966. Telephone: [phone], extension 257.

Robert W. Powitz, Ph.D., M.P.H., R.S., D.A.A.S.

James J. Balsamo, Jr., M.S., M.P.H., R.S., D.A.A.S.

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Please direct specific questions to: Tools for EH, NEHA, 720 S. Colorado Blvd., Suite 970-S. Denver, CO 80246

This article is provided by NEHA for informational purposes only. It is designed to better inform our members about topical and relevant goods and services available to the environmental health professional. Opinions expressed about any product or service in this column either expressly or implied are solely and completely those of the author/s and do not necessarily represent the views or opinions of the National Environmental Health Association.