"Depression" explained

      How to determine relative humidity with two types of temperature readings. June 20, 2000

Q.
In Gene's answer to another question, he asked, "Do you achieve a 10-degree F depression within 6 hours." Can you explain what that means?

A.
To measure the relative humidity (RH) of the air, the traditional method, since about 1909, has been to measure the air temperature with a "regular" thermometer and also to measure the temperature of a wet (distilled water) muslin wick with brisk air flow across the wick (600 fpm). The temperature was measured with a thermometer that had a large bulb or reservoir of mercury at the bottom. Maybe you have seen one of these types of thermometers.

Regular air temperature is often called the dry-bulb temperature, because the bulb of the thermometer is dry. We often shorten this to just "dry-bulb" (DB) and omit the word temperature. On the other hand, the wet-wick temperature is called the wet-bulb temperature, or just "wet-bulb" (WB).

If you know the DB and WB, you can get the RH by looking it up in a table. The difference between the DB and WB is called the wet-bulb "depression," or just "the depression."

As an example, if the DB equals 110 degrees F and the depression is 10 degrees F, then the WB equals 100; the RH equals 70 percent; and the equilibrium moisture content (EMC) equals 12 percent.

EMC is the moisture content of the air, expressed in terms of wood moisture content (MC). In other words, wood exposed to a 10-degree F depression at 110 degrees F DB will dry to 12 percent MC; the air is 12 percent EMC.
Gene Wengert, forum moderator



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