Sensors for extreme temperatures

Sheila Kennedy, contributing editor, says they go where humans can't.

By Sheila Kennedy, contributing editor

New combinations of materials and designs allow today’s sensors to operate reliably in temperatures not previously possible. From extreme heat to cryogenically cold, new and emerging sensor technologies are up to the challenge.

High-temp accelerometers: Gas turbine vibration monitoring is a prime example of a high-heat application. One maker relies on Inconel heat shields in its extreme high-temperature piezoelectric accelerometers to enable continuous operation in temperatures to 1,200°F (650°C). The Inconel provides a low oxidation rate compared to other types of steel and retains its strength in high temperatures. Meggitt’s Endevco model 6237M70 and 6237M71 accelerometers have sensing elements and integral shield isolated from the case, and contained within the hermetically sealed Inconel housing. The sensors are suited for vibration monitoring of gas turbines, exhaust systems, compressor systems and equipment, environmental chamber testing and similar extreme applications.

To accommodate the extreme heat, the devices operate in charge output mode rather than using IEPE-type circuitry.

– Sheila Kennedy, contributing editor

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Combining customized ceramic or piezo-crystalline materials with specialized manufacturing processes allows PCB Piezotronics to produce a variety of piezoelectric accelerometers that perform at temperatures in excess of 500°F (260°C). To accommodate the extreme heat, the devices operate in charge output mode rather than using IEPE-type circuitry. Target applications include power turbine, steam pipe and engine exhaust system monitoring. Most PCB models in the high-temperature category withstand exposure to radiation, making them suitable for deployment in nuclear environments.

Titanium alloys are lightweight, highly corrosion-resistant and maintain their strength even in extreme temperatures. Endevco’s Isotron (IEPE-type) triaxial accelerometers are constructed of welded titanium and operate to 347°F (175°C). The unit is hermetically sealed and contains micro-electronic circuits designed to operate continuously at these temperatures. High-temperature cable assemblies are a standard accessory. Vibration monitoring of turbines and other rotating equipment, environmental stress screening and product reliability testing are among the applications this technology supports.

High-temp pressure sensor: Pressure sensors that operate normally at excessively high temperatures are necessary in certain applications, as when drilling at great depths. A new pressure sensor system developed by engineers at the Fraunhofer Institute for Microelectronic Circuits and Systems (IMS) withstands temperatures as high as 250ºC (482ºF), as well as shocks and vibrations. The sensor and memory components are stored on a microelectronic chip, or wafer. The modified wafer, made of silicon oxide rather than the usual monocrystalline silicon, allows the sensor to operate at extremely high temperatures. The oxide layer improves electrical insulation and prevents leakage current, and improves insulation of the memory component by three to four orders of magnitude.

The engineers have proven the sensor’s stability at 250ºC and are planning additional studies at higher temperatures, conceivably as high as 350ºC. Petrochemical, geothermal and vehicle engine applications are the target markets.

Cryogenic hydrogen sensor: At the other end of the spectrum are sensors that operate at cryogenic temperatures. NASA needs sensors of this sort to warn against explosion hazards in the space shuttle, for example. New chemochromic hydrogen leak detectors developed through the Kennedy Space Center’s Innovative Partnerships Program operate in temperatures as low as 78°K (-319ºF), thanks to insulating additives.

The color-changing sensor provides a quick, visible indication of low-level hydrogen leaks caused by fatigue or other failures. It incorporates a chemochromic pigment into a base polymer and can be manufactured into polymer parts, such as injection molded plastic parts, fiber-spun textiles or extruded tapes. When exposed to hydrogen, the part reversibly or irreversibly changes color, for instance from light beige to dark gray. The pigment’s sensitivity is tailored to the application.

Fuel cells and other hydrogen-fueled energy-generating systems used in deep-freeze conditions are likely candidates for this technology. Partnering researchers at Kennedy Space Center and Florida Solar Energy Center envision the sensors being fabricated as casings or coatings for high-pressure hydrogen storage tanks and handling system components, or optoelectronically instrumented to provide measured digital indications of the presence of hydrogen.

Wide-range temperature sensor: Researchers at the NASA Glenn Research Center developed a temperature sensor that operates in conditions ranging from hot jet engine compartments to cryogenic space exploration missions. It uses silicon-on-insulator (SOI) technology that reduces leakage current and power consumption. A high-temperature polyimide circuit board, Teflon wire interconnects, high-temperature lead-free solder and a ceramic-packaged chip timer are among the components that help the sensor weather temperature extremes. The sensor performed successfully in tests ranging from -190°C to +210°C.

E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at sheila@addcomm.com.

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