Monday, September 29, 2014

TIPS: How do you measure temperature of an object with unknown emissivity?

Emissivity is the measure of effectiveness of a material in emitting thermal radiation. Objects or materials with high emissivity will produce a more accurate temperature measurement through thermal imager (with the right compensation and settings in the thermal camera). On the contrary, objects or materials with low emissivity have high reflectivity, making temperature measurement more challenging.
That said, it is very important to know the emissivity of the targeted object or material that we are measuring, to obtain an accurate temperature reading using thermal imagers. Most thermal imagers in the market today do come with a pre-defined emissivity table for common items that can be found around us.

Figure 1: Emissivity table found in U5855A TrueIR thermal imager
Together with this table, users also have the flexibility to input the emissivity values from 0.1 to 1.0. However, when the emissivity value or material of the object is not known, these steps can be taken to determine its emissivity

  1. Clean up the surface of the targeted object to remove dusts or foreign materials.
  2. Apply blackbody paint or electrical tape with known emissivity on the surface of the targeted object. See Figure 2.

  3. Figure 2: Object with unknown emissivity

  4. If the targeted object is radiating heat, do allow some time for the surface of blackbody paint or tape to reach equilibrium with the targeted object. 
  5. Perform the reflected temperature calibration (RT cal) and set the known emissivity (Ɛ) value of the blackbody paint or tape on your thermal imager. Measure the temperature of area (A) whereby the blackbody paint or tape was applied. Ensure the thermal imager is focused correctly and perpendicular to the targeted object to minimize emittance effect. Record the average temperature of that focused area.
  6. Measure the temperature of area (B) as shown in Figure 2. Manually lower the emissivity (Ɛ) value on the thermal imager until the temperature of area (B) is equal to the temperature measured at area (A). The emissivity reading indicated now will be the emissivity value of the targeted object.
All in all, by knowing the emissivity of the object or material we will be able to get the right temperature measurement, or better known as a quantitative measurement; measurement of data that can be put into numbers. However, another method is to do a qualitative measurement; where data is purely comparative and not numerical. For example, when checking a three-phase electrical system, it is merely comparing the hotspot among the three phases. 

Figure 3: IR image of a three-phase circuit breaker taken using U5855A TrueIR thermal imager

There are advantages and disadvantages to both qualitative and quantitative measurements, depending on whether or not further statistical analysis is needed or just a need to know the general feel of how the target object is.

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