A thermal imaging camera only reads the electromagnetic radiation it receives in a specific range of wavelengths. To display this reading the camera makes several calculations to convert data into actual temperature, and one that is vitally important is emissivity.
Emissivity is efficiency with which an object emits infrared radiation at a given temperature by comparison with a black body at the same temperature and with the same surroundings. Values range from 1.0 for lampblack down to 0,02 for polished silver. And in most cases the pyrometer/camera readings need to be adjusted to take this emissivity into account.
If its value is incorrect, the actual temperature will be exponentially different to the display temperature; the Stefan-Boltzmann law explains the science behind this. Worse still the margin increases in line with temperature so the results are not just different, but seriously skewed. A phase imbalance that may seem to be just a few degrees can actually be upwards of 30°C.
The temperature of the steel and the tape should be the same, but a 4°C error due to emissivity is observed.
The visual set-up of the pyrometer/camera would be the only other way of determining the severity of the fault. But as any experienced thermographer will tell you, the visual component is not a reliable alternative. Depending on the level, span and range on the pyrometer/camera it is still very easy to miss a severe problem. It is therefore vital that users understand emissivity and how to set-up a thermal imaging camera accordingly. Understanding the basics of infrared temperature measurement is easy; just send an e-mail to [email protected] with your details and the company will e-mail back a booklet covering the basics.
The safe practice of using infrared windows on switchgear and other inspection points, rather than removing covers whilst equipment is live, is becoming more of a requirement and can introduce errors in readings if not done correctly.
With windows the infrared passes through a very specific material and some intensity is lost. Almost all cameras and pyrometers have a transmissivity setting that compensates for this. Also different materials react in different ways. The polymer based windows have a fixed transmissivity that is compensated for on installation, whilst crystal based windows vary throughout their lifetime as they are hydroscopic and absorb moisture which affects their transmisivity.
The coffee cup test is a simple way to check the transmission of any infrared window. Use it before installation and during regular IR window maintenance.
1. Pour hot water into a cup and place a target of known emissivity on the side, such as a strip of electrical tape or an IR-ID label.
2. Set the camera’s transmission to 1.
3. Measure the temperature of the target without the IR window.
4. Place the window in front of the camera and complete the measurement again.
5. Use the camera or the reporting software to change the transmission coefficient of the image until the adjusted temperature (taken through the window) and the original temperature are the same.
6. Record the transmission rate on the IR window label and in the report template for future reference.
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