From thermocouples to thermal imaging

Paul Boughton

Temperature is one of the seven SI base units and its measurement is critical to many industrial processes for reasons of performance, quality, efficiency, safety or a combination of these. Because of the diversity of the applications for which temperature measurement is required, several alternative temperature measurement techniques have been devised, and the development process is ongoing, especially in relation to systems that measure the infrared radiation emitted by an object.

No single temperature measurement technique is dominant, as the range of applications is so vast. In some cases a simple and low-cost thermocouple is adequate, whereas other applications may call for the better accuracy offered by a platinum resistance thermometer or other type of resistance temperature detector. Infrared technologies may be more applicable if non-contact sensing or a thermal image is required. In all cases, there is a choice of portable handheld instruments or fixed units that measure the temperature on a continuous or periodic basis.

Infrared vision-based systems can also be used for applications where the surface temperature is used to indicate faults that would not normally be visible. For example, by heating along the length of a helicopter rotor blade and using an infrared camera to 'see' how the heat is absorbed and re-radiated, any changes in infrared radiation along the blade can be readily identified. Such changes could be due to pockets of air trapped between the laminations as a result of a defective production process.

Thermal imaging has also proved very useful in condition monitoring for checking electrical and mechanical components; a localised temperature rise can be an early indication of impending component failures.

Progress in packaging

It has already been mentioned that temperature measurement technologies continue to develop, and this is true even for the mature markets such as thermocouples. However, here it tends to be the packaging that is progressing, rather than the core thermocouple technology. For example, Labfacility has recently launched a range of thermocouples for measuring the surface temperature of pipes 13-55 mm in diameter . In each case the type K thermocouple is housed in a stainless steel disc located within a stainless steel pipe clip; the sensitive grounded measuring junction ensures rapid response to any temperature changes. Four alternative pipe clips cover the 13-55 mm range of pipe diameters and screw adjustment allows for precise fitting and good thermal contact. All of the assemblies are supplied with a silicone varnished, glass-fibre insulated extension lead, 7/0.2mm conductors and stainless steel overbraid. The sensors are rated to 350 degrees C.

While thermocouples and resistance thermometers are relatively low-cost devices, there are applications for which they are simply unsuitable - typically due to the temperature range or inaccessibility. Non-contact infrared temperature sensors offer an alternative for such applications, and the price of these devices is falling. Calex Electronics claims that its Pyrocouple series sensors represent a 'first' in the field of infra-red thermometry due to the sensors' combination of high quality, low cost and compact dimensions. The sensors can measure temperatures from -20 to +500 degrees C accurately and consistently, with a response time of just 240ms. Built-in adaptive filtering provides a very stable signal without compromising the short response time.

Pyrocouple sensors are available as either two-wire or four-wire units. The two-wire sensors transmit the target temperature as a 4-20 mA output and are simple to use. Unusually, the four-wire sensors transmit the target temperature as a 0-50 mV or thermocouple output (type J, K or T) plus the internal sensor temperature as a 4-20 mA output. This second output can be used to ensure that the sensor is operating within the correct ambient temperature limits, thereby preventing damage caused by overheating or overcooling.
While the Pyrocouple PC21 has 2:1 optics that make it suitable for most applications where the sensor can be mounted close to the target, the model PC151 is designed for small or distant targets and has an optical resolution of 15:1. A third unit, the PC301, is designed for very small or distant targets and has an optical resolution of 30:1. In each case the sensor itself is just 103mm long by 18mm diameter and is made of polished stainless steel, sealed to IP65. If the sensor is required to operate above 70 degrees C or below 0 degrees C it can be ordered with a water/air-cooled jacket built around it. For dusty environments, such as in paper or textile mills, an air-purge collar can be fitted to keep the lens clean.

For engineers that want to connect an infrared temperature sensor directly to a PC, Calex Electronics offers the PyroUSB that is similar to the Pyrocouple models but connects to a computer's USB port and has a measurement range of -40 to 1000 degrees C.
Another company that is active in the development of non-contact temperature sensors is Impac Infrared. The latest product from Impac is the series 50 digital fibre-optic pyrometers. These are said to be high-performance, cost-effective devices for measuring temperatures from 250-3500 degrees C (Fig. 3). Initially there are four models available in the series 50 family to cover different temperature ranges. For example, the IGA 50-LO plus measures temperatures in the range 250-2500 degrees C, while the ISR 50-LO plus dual waveband pyrometer covers the range 700-3000 degrees C.

Thermal imaging

The aforementioned instruments from Calex and Impac are designed primarily for permanent installation and measurement of 'spot' temperatures, but there are many applications for which an infrared vision-based system is more appropriate. Of course, with appropriate software, such systems can still measure the temperature at one or more fixed points, but the power of such systems often lies in the image that is obtained, which can be analysed automatically or by a human operator. Infrared cameras can either be fixed or handheld.

For those interested in handheld infrared cameras, one of the most exciting product launches recently is the i5 pocket-sized infrared camera that Flir Systems is selling for around €2500, which is 30-40 per cent less than the company's previous lowest-cost infrared camera. Flir Systems claims that the i5 is the first camera of its kind in terms of weight, size, functionality and price (Fig. 4). Weighing less than 340g, it has an 80 x 80 pixel IR resolution and thermal sensitivity of 60 mK that provides a clear infrared image. Temperatures up to 250 degrees C can be measured, and images are displayed on the camera’s bright 2.8inch LCD. JPG files of the infrared images can be stored on a built-in SD card or downloaded to a PC.

As the price of infrared imaging technology falls, more applications are able to benefit from fixed cameras for continuous monitoring of components or processes. Within this field some developments have resulted in reduced response time, others have taken advantage of faster communications between the camera and the process monitoring/control system, and there are also companies that have put their efforts into developing infrared cameras for niche markets including those where explosion-proof systems are required. Alongside these developments there is a drive to simplify the user interface so that, as Flir Systems claims for its recently introduced Thermovision A-Series camera, infrared imaging becomes as easy to use as machine vision systems.

When Flir launched the Thermovision A-Series cameras, it said they were the first thermal cameras to be compatible with Gigevision and Genicam standards and to feature a trigger/synchronisation capability. With a high frame rate of 60 Hz for a full 16-bit image, together with a built-in 25-degree motorised lens with autofocus and other lenses optionally available, the Thermovision A-Series cameras are described as both versatile and affordable.

Another new infrared camera with a choice of communications interfaces is the NIR-300 from VDS Vosskühler. This near-infrared camera is offered with Cameralink or Gigabit Ethernet communications and operates in the 0.9-1.7 μm spectrum thanks to its InGaAs sensor with 320 x 256 pixel resolution. The camera delivers 12-bit data at a rate of 50 frames/second, or there is an NIR-300F model that delivers 100 frames/second. A cooled version is available for applications in high ambient temperatures.

Hazardous zones

One area where thermal imaging is extremely useful but also difficult to implement is applications in potentially explosive atmospheres. Hawk IR International specialises in infrared vision systems for use in hazardous areas. One of the company's most recent products, launched earlier in 2008, is the Ex|CAM AF series FM approved explosion-proof (XP) infrared camera. Based on a robust infrared camera, the Ex|CAM AF series is designed to detect potential equipment failures that could result in plant downtime or even catastrophic explosions. Tony Holliday, director of sales and marketing for Hawk IR International, explains: "Unlike standard infrared cameras aimed at the automation market, the Ex|CAM AF series is designed specifically for explosive areas. In order for equipment to be acceptable for this kind of installation and application, rigorous third-party testing and certification must be completed. In Europe, explosive areas are classified by Zones and covered by the ATEX Directive. In North America, explosive areas can be Zones or alternatively Classes and Divisions, dictated by the relevant National Electric Code standard. Thanks to this latest FM Approval, Ex|CAM AF Series now holds certification for installations into ATEX Zone 1 and NEC Class I Division 1/Class II Division 1 areas."

Hawk IR's Ex|CAM AF series is described as 'maintenance-free' once installed. Constructed from stainless steel and using the innovative Clirvu-Ex optic material, the Ex|CAM AF cameras enable plant managers and process operators to inspect equipment and plant areas within the explosive atmosphere in real time. Coupled with factory-fitted and certified pan/tilt modules, Ex|CAM AF cameras are capable of scanning large areas remotely. The Ex|CAM AF cameras deliver real-time, high-resolution thermal images that can be displayed via existing closed-circuit television systems for fast and simple information dissemination.

Temperature measurement is certainly a diverse segment of the instrumentation market, with widely varying requirements depending on whether the application is, for example, monitoring thermal images of a set of components in a potentially explosive atmosphere or, at the other extreme, using a thermocouple to measure the temperature of a component in a simple low-risk, low-accuracy task. As the foregoing illustrates, there are still ongoing developments surrounding the more mature temperature measurement technologies, though vision-based systems are advancing at a much faster pace and are creating opportunities beyond the measurement of temperature.

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