FEATURE ELECTRICAL SAFETY


ACCURACY CONSIDERATIONS for infrared inspections


When undertaking infrared inspection of critical electrical assets it is important to know that measurements are accurate


A


typical plant will be full of equipment that requires periodic


“Even slight errors in emissivity


compensation can lead to significant errors in


temperature and Delta T


infrared (IR) inspection. The challenge is getting an accurate indication of equipment health. A thermal imaging camera is only able to read the electromagnetic radiation it receives in a specific range of wavelengths. To display this reading the camera will make several calculations to convert data into actual temperature and emissivity is an important part of this calculation. Indeed,


(difference in temperature) calculations...”


according to Martin Robinson, CEO


or IRISS, properly compensating for the various emissivity values


of all the components one encounters on the factory floor is possibly the most critical factor in performing accurate and meaningful inspections.


EMISSIVITY Emissivity is the efficiency with which an object emits infrared radiation at a given temperature by comparison with a black body at the same temperature with the same surroundings. Values can range from 1.0 for lampblack down to 0.02 for


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polished silver. In most cases the camera readings need to be adjusted to take this emissivity into account. “Even slight errors in emissivity


compensation can lead to significant errors in temperature and Delta T (difference in temperature) calculations,” said Robinson. “ Electrical cabinets are a good example, as they may contain materials with emissivity values ranging from 0.07 to 0.95.” If the emissivity value is incorrect, the


actual temperature will be exponentially different to the display temperature. Stefan-Boltzmann’s Law explains the science behind this. Worse still, the margin increases in line with temperature so the results are not just different but can be seriously skewed. A phase imbalance that may seem to be just a few degrees can actually be upwards of 30°C. The only other way of determining the severity of the fault would be to alter the visual set-up of the camera. However, as any experienced thermographer will tell you, the visual component is not a reliable alternative. Depending on the level, span and range on the camera it is still very easy to miss a problem.


MAKING A SIMPLE CHECK Infrared windows will also have an effect on the accuracy of temperature readings. A simple check of the transmission rate of an infrared window before initial installation and during regular IR window maintenance for windows can be


achieved with the following simple test: 1. Pour warm or hot water into a cup and place a target of known emissivity on the side of the cup such as electrical tape or an IR-ID label. 2. Set the camera’s transmission to 1.0. 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. It is also important to consider that not


all infrared windows are the same. IRISS, for example, uses the same grade and thickness of polymer for all its window products – whether opaque or clear. This means that if the camera has been set- up correctly for one IRISS window then the user knows it is set for all IRISS windows. The products also have fixed and stable transmission and are immune to the ambient temperature outside the enclosure, length of time in the field, relative humidity and barometric pressure.


IRISS www.iriss.com T: 01245 399713


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