MIL-DTL-917F(SH)
4.6.3 Measurement of ambient temperature for heat runs.
4.6.3.1 Effective ambient temperature. The effective ambient temperature to be employed in temperature rise
calculations shall be the mean value of at least three sets of ambient temperature readings taken at equal time
intervals throughout the last quarter of the heat run with stable ambient conditions prevailing. For tests of duration
shorter than 2 hours, readings which cover the last half-hour or the entire test, whichever is shorter, shall be
employed. The ambient temperature conditions will be considered sufficiently uniform and stable when the
maximum difference between similar measurements at different locations does not exceed 9 °F (5 °C) and when the
variation between successive readings at the same location does not exceed 1.8 °F (1 °C), the rate of temperature
change does not exceed 7.2 °F (4 °C) per hour, and the difference in readings of thermometers in air and in oil-filled
cups (in locations where both are required) (see 6.2 and 4.6.3.3.1c) does not exceed 3.6 °F (2 °C). Should these
stability criteria not be complied with during the last quarter of the temperature test, continuous duty heat runs shall
be continued until three successive sets of readings are obtained that do meet the criteria. Should the stability
criteria not be met for heat runs of limited duration (that is, tests at the short-time duty, intermittent duty, and
overload ratings), corrective measurements shall be applied and the test shall be repeated until the required
conditions are met.
4.6.3.2 Ambient temperature limits. The heat run ambient temperature shall be between 50 and 122 °F (10 and
50 °C). It shall be assumed that the temperature rise is the same for all ambient temperatures between the limits of
50 and 122 °F (10 and 50 °C). No heat runs shall be undertaken on equipment which has recently been brought
from a place varying in temperature by 15 °F (8 °C) or more from the room in which the test is to be made.
Equipment shall be stored or installed in the test area for at least 24 hours if the originating location varies in
temperature by 15 °F (8 °C) or more from the room in which the test is to be made.
4.6.3.3 Method of temperature measurement. Temperature shall be measured as specified in 4.6.1 through
4.6.3.3.1 Equipment other than that cooled by water, oil, or forced air from separate sources. Method of
temperature measurement for equipment other than that cooled by water, oil, or forced air from separate source shall
be as follows:
a. The equipment under test shall be protected from drafts other than those produced by the integral cooling
fans in the case of fan-cooled equipment. The equipment under test shall also be protected from heat radiation from
outside sources.
b. The ambient temperature shall be measured at four locations around and level with the center of the
equipment. For most equipment, the thermometers may be placed at a horizontal distance from the equipment of
3 to 6 feet. In cases where the heat radiation from the equipment under test is negligible, or where an ambient
temperature more nearly representative of the conditions of test would be obtained, the thermometers may be placed
12 inches from the equipment.
c. For equipment weighing 500 pounds or more each, the ambient temperature thermometers shall be inserted
in heavy oil-filled cups of not less than 1 inch in external diameter and 2 inches in height. Where oil-filled cups are
used, each shall be accompanied by an air thermometer to provide an indication as to whether variations in the
cooling air temperature are maintained within acceptable limits. Only those temperature readings from
thermometers in the oil-filled cups shall be averaged to determine the effective ambient temperature. Likewise, only
these readings shall be used to determine the incremental change and the rate of change of temperature when
checking compliance with the stability criteria.
4.6.3.3.2 Equipment cooled by forced air from separate source. Ambient temperatures shall be measured by
locating the ambient-temperature thermometers at the air intake of the equipment. If this location causes an
appreciable radiation error, a compromise location shall be chosen. The number and spacing of thermometers or
thermocouples shall ensure the temperature indication is a representative average.
4.6.3.3.3 Equipment cooled by water. Ambient temperature shall be measured as specified in the individual
equipment specification (see 6.2).
4.6.3.3.4 Equipment cooled by oil. Ambient temperature shall be measured as specified in the individual
equipment specification (see 6.2).
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