How To Make Accurate Ir Measurements - General IRT850K Manuel De L'utilisateur

HOW TO MAKE ACCURATE IR MEASUREMENTS

The IRT850K has a distance-to-spot (D:S) ratio of 50:1. That means that the target area (spot)
whose infrared radiation (temperature) is being measured increases in diameter by 1 inch for
every 50 inches you move away from the target. Conversely, the diameter of the target area
measured decreases by 1 inch for every 50 inches you move closer to the target.
All IR thermometers (IRTs), including the IRT850K, 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 figure on previous page), 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
IRT850K 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
IRT850K is used to make the measurement from 100 ft. away, the reading will have a large error.
At this distance, the target area is 2 ft
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:
Solving for Tavg, we get (200 + 75) ÷ 2 or 137.5°F., which is what the IRT850K would display. In
other words, trying to measure the temperature of the motor from 100 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 figure below). For a motor with an area of 1 ft
with a D:S ratio of 50:1, the
optimum measurement distance
would be 50 ft.
For best results, the targeted
area (spot) should fall within the
target's boundaries, as in the
figure at right.
Measuring a motor's
temperature from the wrong
(top) and right (bottom)
distance
2 . Therefore, the IRT850K will measure not just the
Tavg = (Tmotor + Tbackground) ÷ 2
2 . If the
2 and using an IRT
wall
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