WET-BULB TEMPERATURE (WBT)– That temperature measured with a thermometer similar to that used for measuring DBT, except that a wet wick is fitted closely over the bulb (or sensor). A “natural” WBT is obtained with no additional movement of air over the wick other than that which occurs naturally. An “aspirated” WBT is obtained by increasing air movement over the wick with a fan, motorized psychrometer, or sling psychrometer. The “true” WBT environment is approximated when a moderate air flow is maintained and the bulb is shielded from radiant heat. Although the natural WBT depends on the DBT and the moisture content of the air, it does not provide a direct indication of the amount of water vapor in the air. The aspirated WBT is therefore of greater value in planning corrective action than the natural WBT. The term WBT will hereafter refer to aspirated WBT unless otherwise specified.
When the dry and wet-bulb temperatures are identical, the air is said to be saturated, and the relative humidity is considered to be 100 percent.
HUMIDITY—The quantity of water vapor mixed with other atmospheric gases.
ABSOLUTE HUMIDITY (AH)—The mass of water vapor present per unit volume of air.
RELATIVE HUMIDITY (RH)—The ratio of the actual amount of water in the air (absolute humidity) to the maximum quantity of water the air can hold at a given temperature.
DEW POINT—The temperature at which the absolute humidity reaches a maximum and the air becomes saturated with water vapor.
PSYCHROMETER—An instrument for measuring atmospheric humidity using two thermometers, one with a web bulb and one with a dry bulb, whirled manually or by motor to provide the moderate air flow necessary to obtain an aspirated WBT reading.
AIR MOVEMENT OR VELOCITY— Usually expressed in feet per minute (fpm) or cubic feet per minute (cfm). It is measured by various instruments depending on air flow velocity.
RADIANT HEAT—The transfer of thermal energy by wave motion from one object to another without warming of the intervening space.
VERNON GLOBE THERMOMETER— Consists of a 6-inch hollow copper sphere, with a wall 0.022 inches thick painted flat black on the outside, and containing a temperature sensor like that of an unshielded dry-bulb thermometer with the bulb, or its equivalent, at the center of the sphere.
WET-BULB GLOBE TEMPERATURE (WBGT) METER—A compact electronic instrument that independently measures dry-bulb, wetbulb, and globe temperatures.
INFRARED THERMOMETER—Used in measuring the temperature of infrared energy emitted from various sources.
EFFECTIVE TEMPERATURE (ET)—An index combining into a single value the effects of temperature, humidity, air velocity, and thermal radiation. Combinations of conditions that produce the same subjective feeling of warmth in reference to still saturated air are assigned the same effective temperature.
MEAN RADIANT TEMPERATURE (MRT) of a nonuniform environment (e.g., walls, ceilings, floors). The temperature of a uniform black enclosure in which a solid body or an occupant would exchange the same amount of radiant heat as in the given nonuniform environment. It is estimated from the globe thermometer reading and is useful in determining radiative heat transfer (net gain or loss) in human beings.
HEATING
Aboard ship the conventional approach for heating involves drawing fresh outside air over steam coils and discharging the heated air into various compartments where it is required. To avoid condensation of moisture on the airducts and to provide a flexible heating system, initially preheat outside air to 42° to 50°F DBT. The air is then heated to the desired delivery zone temperature and distributed to the various compartments and spaces within that zone. A “heating zone” is defined as a group of adjacent spaces with approximately the same heating requirements.
A zone temperatureof 70°F DBT is required aboard surface vessels for berthing, dressing, lounge, messing, medical, dental, office, and control spaces. No effort is made to control the moisture level in these spaces during cold weather; therefore, Medical Department personnel should 5-3
