Signal conditioning
Making the most of thermal sensors
From motion sensors that aid safe parking, to pressure sensors that measure air flow in air conditioning units, many applications used in our daily lives are guided by sensing technology. In industrial heating applications, temperature sensors are key components that provide temperature feedback, used to monitor and control the process. However, there is a lot more to thermal sensors than simply recording temperature, as Randy Brown, chief system designer at industrial equipment heating manufacturer Watlow, explains.
W
ithout temperature sensors, a system is essentially blind. While some simple thermal systems can rely on a
thermostat device to maintain a relative temperature within 5°C many processes require a far greater degree of accuracy. This is where temperature sensors come in. However, with various types and purposes of thermal sensors, how do engineers determine which is best for a given application and how to correctly apply them?
Types oF sensoRs There are two categories for temperature sensors: contact and non-contact. Contact sensors include resistance temperature detectors (RTDs), thermocouples and thermistors that must be in contact with the solid, liquid, or gas it is measuring. Non-contact sensors involve thermal imaging and infrared sensors, which determine temperatures from a distance by measuring the thermal radiation emitted by an object or heat source. These sensors are often found in hazardous environments where there are high temperatures, such as high voltages or within moving machinery. Two of the most used sensors in thermal
applications are RTDs and thermocouples. RTDs use resistance to determine temperature of a source. They are often used in industrial applications because they are comprised of metal, instead of ceramic or polymer, and are highly stable and accurate, making them ideal for repeatable precision and a more accurate assessment of a processing system. Thermocouples measure the temperature
delta between two points, using a metal junction of two dissimilar types of wire to measure the micro-voltage. Changes in voltage are read by a process controller, which calculates the relationship between voltage and heat. This allows engineers to adjust the temperature of the heater bundle to increase, decrease or maintain the ideal
28 January 2022 Instrumentation Monthly
temperature for an industrial process. Watlow, which manufactures temperature sensors including thermocouples and RTDs, used its expertise to develop an industrial thermocouple that supported an aircraft manufacturer in heat treating engine components. The manufacturer needed a thermocouple that could easily route through the furnace wall and be safely used in temperatures exceeding 1,000°C. Meeting these design requirements, Watlow delivered a thermocouple that overcame these challenges for easy and safe use.
suITaBle sensoR pRacTIce In addition to the type of sensor chosen for an application, understanding where temperature sensors fit into a production line is also key to making the most out of them. To achieve an accurate, reliable and durable performance from industrial temperature sensors, several considerations must be made. Sensors should be placed in the areas that
represent critical process steps, such as inlets and outlets of heater bundles. Additional sensors should be applied to monitor for process extremes that may cause safety or quality problems. By nature, thermal system temperatures
often change throughout the process with some areas heating or cooling faster than others based on the laws of thermodynamics, making sensor placement key. While every application is different, careful consideration needs to be applied into sensor placement. Besides this, it should also be understood
how to identify when a sensor is damaged and how to replace it, to achieve the best outcome for a thermal process. RTDs and thermocouples can
become damaged and incorrectly read the temperature of heating systems, creating inconsistencies and compromising quality. This can be caused by secondary junctions in thermocouples or element damage to RTDs. Unusual temperature fluctuations or temperature differences between multiple heater bundles signifies that a sensor needs replacing. Eliminating damaged thermocouples or RTD’s can keep industrial processes moving forward with optimal performance. While temperature sensors in
industrial environments differ from those that help us to park our cars without bumping into a bollard, they are crucial components that allow systems to safely run in medical, food processing and energy applications. Design engineers need superior sensors for uncompromising quality and control in thermal applications. Understanding how sensors work, how to identify damage and how to replace them will ensure optimal performance.
Watlow
www.watlow.com
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