General Tools and Instruments CIH20DL - Manual (Page 24)

APPENDIX
The field of view of the CIH20DL's IR thermometer
HOW TO MAKE ACCURATE IR MEASUREMENTS
The CIH20DL has a distance-to-spot (D:S) ratio of 8:1. This means that the target area
(spot) whose infrared radiation (temperature) is being measured increases in
diameter by 1 inch for every 8 inches you move away from the target. Conversely, the
diameter of the target area measured decreases by 1 inch for every 8 inches you
move closer to the target.
All IR thermometers (IRTs), including the CIH20DL, take the average temperature of all
objects within a circular target area (spot). Although the distance “D” in the D:S ratio
is defined as a linear value and the “S” defines the diameter of the spot (see above
figure), the critical parameter is the target area. Depending on the distance to the
target (the object whose temperature you want to measure), the target area may
include both the target and background objects near or behind the thermometer’s
field of view, which defines the target area or spot.
To explain the relationship between D:S ratio and measurement accuracy, consider
how the CIH20DL would be used to measure the temperature of a small AC motor
suspected of overheating. The motor measures approximately 1 ft. x 1 ft., so it has an
area of 1 ft 2 . If the CIH20DL is used to make the measurement from 24 ft. away, the
reading will have a large error. At this distance, the target area is 3 ft 2 . Therefore, the
CIH20DL 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 CIH20DL 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.
24
inches
mm 25@200
50@400
75@600
Numbers indicate spot size@distance
1@8
3@24
2@16
CIH20DL-FINAL-122111_awb 12/21/11 9:58 AM Page 24