How to make the most of thermal sensors…

01 January 2022

Watlow_How to make the most of thermal sensors
Watlow_How to make the most of thermal sensors

From motion sensors that aid safe parking to pressure sensors that measure airflow in air conditioning units, many everyday applications are guided by sensing technologies. In industrial heating applications, temperature sensors are key components that provide feedback, used to monitor & control the process. However, there is much more to them than simply recording temperature, as Randy Brown, Chief System Designer at thermal systems specialist, Watlow explains here...

This tutorial was originally featured in the January 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.

Without 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 demand a far greater degree of accuracy. This is where temperature sensors come in. However, with the 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 which must be in contact with the solid, liquid, or gas being measured.

Non-contact sensors involve thermal imaging and infrared sensors, which determine temperature from a distance by measuring the thermal radiation emitted by an object or heat source. These sensors are often found in hazardous environments, such as where there are high temperatures, 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 to increase, decrease or maintain the ideal temperature for an industrial process.

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 of them. To achieve an accurate, reliable and durable performance from industrial temperature sensors, there are several considerations.

Sensors should be placed in the areas that represent critical process steps, such as inlets and outlets of heater bundles, for instance. 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. 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 signify that a sensor needs replacing. Eliminating damaged thermocouples or RTDs can ensure industrial processes maintain optimal performance.

While temperature sensors in industrial environments differ to 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.

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