General Tools & Instruments IRT657 Manuel De L'utilisateur page 10

To explain the relationship between D:S ratio and
measurement accuracy, consider how the IRT657 would be
used to measure the temperature of a small AC motor
suspected of overheating. The motor measures
2
approximately 1 ft x 1 ft, so it has an area of 1 ft . If the
IRT657 is used to make the measurement from 24 ft.
away, the reading will have a large error. At this distance,
2
the target area is 2 ft . Therefore, the IRT657 will measure
not just the temperature of the motor, but also the
temperature of the physical surroundings in its field of
view, and average the two readings.
How inaccurate would the measurement be? If the motor's
operating temperature is 200°F and the background
temperature is 75°F, and the motor's area is half the target
area at the measurement distance, the following equation
gives the average temperature of the target area:
Tavg = (Tmotor + Tbackground) ÷ 2
Solving for Tavg, we get (200 + 75) ÷ 2 or 137.5°F., which
is what the IRT657 would display. In other words, trying to
measure the temperature of the motor from 24 ft. away
introduced an error of (200-137.5) ÷ 200, or 31% into the
measurement. In this case, the measured temperature was
31% below the motor's actual temperature because the
background is cooler than the motor.
To eliminate measurement error, the IRT must be moved
close enough so the motor is the only object in the target
area (see Fig. 4). For a motor with an area of 1 ft 2 and
using an IRT with a D:S ratio of 12:1, the optimum
measurement distance would be 12 ft.
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