| #57 Measuring Objects Smaller Than the Field-of-View |
| For some non-contact temperature monitoring tasks, the object to be measured is too small to adequately fill the field-of-view of one of the IRt/c models. The monitoring can still be successfully performed if two conditions are met: |
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The object size and distance from the IRt/c are constant. |
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The area surrounding the object within the field-of-view of the IRt/c has a repeatable temperature. |
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| The signal produced by the IRt/c represents the average temperature within its view. Accordingly, the signal can be represented by the equation: |
| T = (TtAt / A) + ( TsAs / A ) |
| where T is the output signal, Tt the target object temperature, At the target object area, Ts the surroundings temperature, As the surroundings area as seen by the IRt/c, and A the total area seen by the IRt/c. |
| For example, to measure the temperature of a thin rubber strip 0.1" (2.5 mm) wide moving continuously 1" (25 mm) away from an IRt/c.2, at a temperature expected to be about 200°F (93°C), and a surrounding temperature at 80°F (27°C). At 1" (25 mm) distance, the IRt/c.2 spot size will be approximately 0.5" (13 mm). |
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| Computing the results for the equation gives: |
| T = (200)(.5)(.1)/[pi(.25)2]) + (80)[pi(.25)2 - (.5)(.1)]/[pi(25)2] |
| This result shows that the average signal will be 31°F (17°C) above the surroundings temperature, compared to an actual object temperature of 120°F (67°C) above surroundings, or approximately one-fourth, which is the ratio of object area to surroundings area measured. Therefore, if the surroundings are expected to be repeatable to 1°F (.6°C), the IRt/c signal will be repeatable to 4°F (2°C). For the final display on a controller, or other read-out device, calibrate in standard fashion by using the available offset adjustment. If the object is to be controlled over a wide range of temperatures, calibrating with a span adjustment will yield greater accuracy. |
| If the target temperature falls within the range of one of the LoE models, the LoE model should be used, even if the target is not metallic. Since a small target results in the same radiation mathematics as low emissivity, a LoE model will reduce errors due to size change and positioning by a factor of approximately 4. See Tech Note 59. |
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