![3. User adjustment errors](/images/new-backgrounds/143174/1431745x1.webp)
3. User adjustment errors
A setting of emissivity = 0.9 on an IR “gun” from one manufacturer will not necessarily match that of another IR “gun” from another manufacturer. There are no standards set in the industry on the precise measurement and meaning of “emissivity”.
Also, Quality Assurance programs should not rely upon any instrument that allows users to alter the instrument settings and to let it display whatever the user wishes.
Effect of Ambient Temperature on Target Reading for 100 °F (38 °C) Target with .8 Emissivity
Target T (F) |
| 4 | 16 | 27 | 38 | (C) |
110 |
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| Conventional Infrared | 43 | ||
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106 |
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102 |
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98 |
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94 |
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90 | 20 | 40 | 60 | 80 | 100 | 32 |
0 | 120 | |||||
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4. Background reflection errors
Even if emissivity is constant (see #2), there are still errors induced by changing ambient temperatures. For example, with emissivity = 0.9, ambient reflections account for 10% of the signal that the IR “gun” will see. If ambient changes, the IR “gun” will display a different target temperature, even if the target remains at the same temperature.
Figure 3.
5. Contact errors
Thermocouples, RTDs, thermistors and other contact devices only measure their own temperature. They do not measure surface temperature. Published “Accuracy” specifications are for the probes only, not the surfaces they must measure. Users must guarantee that the probes are brought to the same temperature as the surface. Can you guarantee that your probes are brought to the same temperature as the targets to be measured?
6. Friction heating errors
For moving surfaces, a contact probe is prone to frictional heating. The size of the error is dependent on the rough- ness of the surface, the speed, the coating on the probe, and so on. It is impossible to control all the variables.
Time Comparison Between
500 |
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| Contact Error 260 | |
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400 |
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| 200 | |
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300 |
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| 150 |
deg F |
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| deg C |
200 |
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| Contact Probes | 90 | |
100 |
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0 |
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0.001 | 0.01 | 0.1 | 1 | 10 | 100 | 1000 |
| Time from start of Measurement (sec) |
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Figure 4.
7. Heat sinking errors
For most
8. Time based errors
Contact temperature probes are slow. The temperature of a target can change more quickly than most probes, resulting in errors in real time measurement.
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