General IRT105 Manuel De L'utilisateur page 15

To explain the relationship between D:S ratio and
measurement accuracy, consider how the IRT105 would be
used to measure the temperature of a small AC motor
suspected of overheating. The motor measures approximately
2
1 ft x 1 ft, so it has an area of 1 ft . If the IRT105 is used to
make the measurement from 12 ft. away, the reading will
2
have a large error. At this distance, the target area is 2 ft .
Therefore, the IRT105 will measure not just the temperature
of the motor, but also the temperature of the physical
surroundings in its field of view (see Fig. 5, top), 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 IRT105 would display. In other words, trying to
measure the temperature of the motor from 12 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.
15