Plant & Works Engineering November 2022

Focus on: Thermal Imaging Maintenance Matters November 2022 www.pwemag.co.uk <<Plant & Works Engineering | 13 OGI camera has an imaging sensor that is integrated with a cryocooler that lowers the sensor temperature to cryogenic temperatures. This reduction in sensor temperature is necessary to reduce the noise to a level below that of the signal from the scene being imaged. Cryocoolers have moving parts made to extremely close mechanical tolerances that wear out over time, as well as helium gas that slowly works its way past gas seals. Eventually a rebuild for the cryocooler is required after 10,000-13,000 hours of operation. Cameras with cooled detectors have a filter that is attached to the detector. This design prevents any stray radiation exchange between the filter and the detector which allows for better image sensitivity. This increase in image sensitivity could cause the imager to visualise certain gases more effectively and even allow the OGI camera to meet regulatory standards like the US EPA’s OOOOa or other requirements. Uncooled OGI cameras use a microbolometer detector that does not require the additional parts necessary to cool a detector. These are often made of vanadium oxide (VOx) or amorphous silicone (a-Si) and are responsive in the 7-14 ?m range. They are much easier to manufacture than cooled cameras but lack the sensitivity, or Noise Equivalent Temperature Difference (NETD), which makes it more difficult to visualise smaller gas leaks. NETD is a figure of merit which represents the minimum temperate difference a camera can resolve. Figure 6 shows the effects of sensitivity for cooled and uncooled detectors. A better NETD would result in a cooled OGI camera detecting gas at least five times better than uncooled. A similar standard used to determine how well an OGI camera can detect gas is Noise Equivalent Concentration Length (NECL) which determines how much gas can be detected over a defined pathlength. As an example, the NECL of a FLIR GF320 cooled OGI camera (3-5 ?m detector) for methane detection is less than 20 ppm*m whereas the NECL of an uncooled solution (7-14 ?m detector) is more than 100 ppm*m. Another consideration with uncooled OGI cameras is the filter. Some cameras are not filtered in the longwave spectra meaning they are just a wide-open detector using unique analytics to visualise a gas. FLIR’s patented High Sensitivity Mode (HSM) is an example of a camera utilising software and analytics to enhance the visualisation of gas. Some cameras have more targeted filters built into the camera system. These could be associated with the lens, between the camera and lens, or engineered in several ways. With uncooled filtering, you lose thermal sensitivity due to limiting the radiation that reaches the camera’s detector. This would result in a higher NETD but could present a better image related to gas imaging. As the spectral filter width is narrowed to focus on specific gases, the radiation from the scene decreases while the noise of the detector remains the same and the reflected radiation from the filter increases. This results in creating a much higher quality image related to gas imaging but decreases the camera’s thermal sensitivity for temperature measurement (radiometry). When you have a cold filter, as in a cooled OGI camera, this phenomenon is avoided since there are very small amounts of radiation from reflections. Whether you are looking to save money, meet regulatory standards, increase worker safety, or simply to be a good environmental steward, the options are greater than ever and can sometimes be confusing. Many factors can go into a decision to choose an OGI camera beyond price. L E A R N I N G B Y D O I N G TECHNICAL TRAINING SOLUTIONS ELECTRICAL • INSTRUMENTATION • MECHANICAL technicaltrainingsolutions.co.uk tech.training@zen.co.uk 01634 731 470 An engineering skills training company providing top-quality hands-on courses across the UK and overseas. A range of industrially-focused courses are available on electrical, mechanical and instrumentation skills. Est 1980 IF YOU’RE READING THIS, THEN SO ARE 30,000+ ENGINEERS IMAGINE THE IMPACT YOUR ADVERTISEMENT COULD HAVE Contact Damien Oxlee at PWE Tel: 01732 370342