APPENDIX
The field of view of the CIH30DL’s IR thermometer
HOW TO MAKE ACCURATE IR MEASUREMENTS
The CIH30DL has a distance-to-spot (D:S) ratio of 30:1. This means that the target
area (spot) whose infrared radiation (temperature) is being measured increases in
diameter by 1 inch for every 30 inches you move
away
from the target. Conversely,
the diameter of the target area measured decreases by 1 inch for every 30 inches
you move closer to the target.
All IR thermometers (IRTs), including the CIH30DL, 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 CIH30DL 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 CIH30DL is used to make the measurement from 90 ft. away, the
reading will have a large error. At this distance, the target area is 3 ft
2
. Therefore, the
CIH30DL 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 = ( Tbackground) ÷ 2
Solving for Tavg, we get (200 + 75) ÷ 2 or 137.5°F., which is what the CIH30DL
would display. In other words, trying to measure the temperature of the motor from
90 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@750
50@1500
75@2250
Numbers indicate spot size@distance
1@30
3@90
2@60
D:S=30:1
