Chapter 5 Qualitative and Quantitative
Temperature Measurements
Most of the time thermal imager users are measuring apparent temperatures that are inextricably linked to qualitative inspections. Qualitative inspections focus on temperature differences as opposed to actual temperatures. The reason is that temperature differences are sufficient to indicate most abnormalities in electrical and mechanical equipments. In other words, qualitative inspections don’t aim at taking accurate temperature measurements. Their goal is to capture and identify different thermal patterns in a given thermal scene, which are indicative of potential failures and / or equipment malfunctioning. Since there is no intention to measure the actual or absolute value, the technician has no need to correct for target emissivity (which is set at “1” on qualitative inspections), reflected temperature from the environment (which is not enabled in the unit on qualitative inspections) or account for target spot size and distance from the target (quite often the target of interest is always compared to a similar target in the same thermal scene). Also, the operator does not need to adjust or account for atmospheric attenuation, angle of incidence and other interfering factors.
On the other hand, quantitative inspections are intended to accurately measure the actual temperatures of specific areas in electrical or mechanical equipment. Although less common than qualitative inspections, quantitative inspections are sometimes required. One good example would be measuring electric motor temperatures: in this case, the absolute temperature value is required since it is closely linked to the useful life of the motor. To accurately measure temperature using infrared temperature measurement technology, the customer should be aware of the key concepts and factors that can radically interfere with the quality of the quantitative temperature measurement.
The following reviews each of these factors.
Distance to Target (Spot) Ratio
The optical system of an infrared sensor collects the infrared energy from a circular measurement spot and focuses it on the detector. Optical resolution is defined by the ratio of the distance from the instrument to the object, compared to the size of the spot being measured (D:S ratio). The larger the ratio factor the better the instrument's resolution, and the smaller the spot size that can be measured from a greater distance.
All
See Figure 29 for an illustration of the D:S ratio and the optical chart for the ThermoView
Ti30 unit.
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