However, if a single point of temperature measurement is useful, a thermographic image reveals much more. A thermograph uses 76,800 temperature sensors (the 320 x 240 pixel array of a microbolometer) to electronically create an image based on temperature readings. An isotherm color palette is then applied to render a visual image with cooler temperatures represented as blues, violets and blacks, and the hottest ranges as reds, yellows and white.
Uncooled microbolometer detectors made portable thermographic imaging affordable, truly portable, and practical for all kinds of predictive maintenance applications, such as finding overheated bearings, leaking furnaces, roof moisture detection and failing electrical components. The cost savings provided by predictive maintenance with thermography is well known among plant engineers. It is estimated that every dollar spent on IR imagers can provide a return of $4 or more in avoided downtime. Because of the evolution of technology, the cost of IR cameras has come down nearly 50% in the last four years. As a result, the return on investment for predictive maintenance using IR imaging technologies is increasing dramatically.
The principal difference between these two imaging technologies is that microbolometer cameras are ideal for the 8-14 micron long-wave infrared range, while the cryogenically cooled detectors work in the 3-5 micron mid-wave band. The 8-14 micron band is ideal for general-purpose plant thermography because it can be calibrated to read much larger temperature bands (e.g., 0-500°C) with ±2°C accuracy, and it is immune to problems from moisture, fog and smoke. However, when measuring glass temperatures with infrared in the 8-14 micron band, reflectance can seriously skew readings. Glass also changes its properties in the process, and a small change in emissivity causes greater errors in the 8-14 micron range. Glass temperatures are ideally measured in the 5-micron spectral band because glass is a nearly perfect emitter of IR-that is, it absorbs and emits 100% of the radiation that corresponds to its characteristic temperature, resulting in the most accurate surface temperature measurements. Applications that require looking through flames are best handled in the 3.9-micron band.
That has all changed as a result of patented advances in detector, filtration and software technologies.* These developments allow the long-wave camera to accurately measure temperatures and produce excellent images in the specific mid-wave infrared bands of
5 microns for glass and 3.9 microns for "through-flame" applications. Several
8-14 micron cameras are now available that can be configured for one of these optional mid-wave bands. The fixed-mount cameras are designed for remote monitoring and process control applications on the factory floor, and can handle ambient temperatures up to 100°C with optional cooling. Even the portable cameras are sealed to IP54, equipped with a Firewire® interface, and can tolerate ambient temperatures to 50°C, allowing them to be used for fixed process monitoring or taken off line for predictive maintenance use.
*These advances were developed and are patented by Mikron Infrared, Oakland, N.J.
One of the most common causes of unplanned downtime is failure of electrical connections and components locked away-for safety reasons-in protective cabinets. Until now, the options for performing an infrared scan of switchgear have not been appealing. Removing or opening electrical panels for infrared scanning always poses a risk when high energy is involved. Human error, equipment failure and other factors can lead to an arc flash explosion, which can be fatal for the inspector and anyone nearby. As a result, specialized personal protective equipment and barricading are needed, but the heavy protective suits, gloves, hoods and face shields often required make this work challenging even in a cool environment.
Closed-door inspection is safer, allowing the thermographer to work with reduced protective equipment and without the help of an electrician. Previously, however, closed-door inspection required the installation of costly and fragile IR-transparent windows (or sight glasses), which created serious limitations. IR sight glasses are made of various crystalline materials, all of which reduce to some degree the amount of IR energy getting to the detector. Some windows transmit IR energy only in the wavelength of 0.13 to 10 microns. However, as previously mentioned, the ideal range for most microbolometer cameras is 8-14 microns. If the sight glass only transmits up to
10.0 microns, then the camera misses 71.5% of the energy from the target. This reduction of signal/energy on the detector results in lower image quality and severely compromises temperature values based on the camera algorithms established for the energy from 8-14 microns.
IR sight glasses also have other drawbacks. Some crystal materials are hygroscopic, so they absorb moisture over time, reducing image quality. Additionally, sight glasses allow only a small portion of a cabinet interior to be viewed through a camera. Because a standard lens cannot focus closely enough, it has to be backed off, which limits the useful field of view. The crystal material can also be easily scratched or cracked, which prevents the camera lens barrel from being pushed against it. And any sight glass will attract dirt and oils over time, which creates a dilemma for cleaning the inside surface. Sight glasses are also expensive, costing hundreds of dollars for just a few square inches, and four or more might be required in each cabinet (at a cost of about $200-300 each) to allow the thermographer to image the interior.
The new interchangeable IR camera lenses now allow a thermographer to scan a panel looking through a 0.5-in. opening in the door. This new system combines a viewport, which has no IR sight glass, with a mating fisheye lens. The patented viewports are UL approved and meet NEMA Type 1, 2, 3, 3R, 4, 5, 12, 12K and 13 enclosures.
The fisheye lens/viewport system enables closed-door thermal inspections of connected electrical switchgear, while maintaining the original safety rating of the cabinet. This applies to cabinets energized with 480 to 15,000 volts, as well as motor junction boxes with 4160 volts.
The fisheye lens has a wide field of view (53° horizontal by 40° vertical, or 66° diagonal) that allows easy scanning of the interior of the electrical cabinet through the viewport, while providing a temperature measurement accuracy of ±3°C. The lens focuses as close as 3 in. and provides a significant depth of field, thereby reducing the need to refocus the lens for different electrical cabinet depths. The viewport can be installed on any application where an expansive and unobstructed view behind a panel or door is desired, including mechanical rooms, motor control centers, transformer terminal boxes, air circuit breakers, motor lead boxes and a wide range of mechanical equipment.
The viewport is designed to dock with the cone-shaped tip of the fisheye lens barrel. The tip works with the viewport like a ball-and-socket joint, allowing the camera to be rocked at different angles to look up, down and to the sides, as well as straight ahead. When not in use, the port is covered by a sealed, screw-on or lockable cap to maintain the safety rating of the cabinet.
Recent advances in IR imaging technologies have addressed these issues, enabling thermal imaging to be used as a cost-effective process control and predictive maintenance solution for many of today's glass and ceramic manufacturing facilities.
For more information about thermal imaging equipment, contact Mikron Infrared at 16 Thornton Rd., Oakland, NJ 07436; (800) 631-0176 or (201) 405-0900; fax (201) 405-0090; e-mail firstname.lastname@example.org; or visit http://www.mikroninfrared.com.
Vanadium oxide detectors have traditionally been reserved for medical applications where real-time imaging, with precise temperature sensitivity and repeatability, is required. Many industrial infrared cameras use detectors with pixel counts of 160 x 120, with or without interpolation, as a cheap alternative. However, this is one-fourth the resolution of the 320 x 240 detector (see Table 1). The larger detector produces an image twice as wide and twice as high, with four times the data for a given field of view. From a practical standpoint, the high-resolution camera can be farther from the target without loss of temperature resolution. The 160 x 120 systems trade marginal resolution, lower image quality and overall lack of performance for a lower cost.