System measures heat without temperature sensors

Winona, MN--Monitoring the temperature of an industrial heater isn't always easy. In many applications, size constraints preclude the use of thermocouples. What's more, engineers often must locate temperature sensors a safe distance from the heating load.

Yet, temperature control remains critical to many industrial processes. So how can industrial users measure temperature in difficult applications?

Engineers from Watlow Controls say they've found a way. Using a new Heater Integrated Sensor System (HISS), they can accurately monitor heater temperatures in applications where conventional sensors don't work. To do so, HISS employs a microprocessor, solid-state relay, voltage and current sensors, and a resistance-type heater. During operation, the system uses the sensors to measure current and voltage. Employing those values, the microprocessor calculates the electrical resistance in the heating element. As operation continues, it compares the initial resistance in the heating element to continuous resistance changes. Using those numbers to form a resistance ratio, it then infers the temperature value from data stored in the microprocessor's memory.

Key to the operation is the pre-programming of look-up tables for the particular heater. "By knowing the properties of the material beforehand, the microprocessor can go to the look-up table and determine the temperature," notes James R. Datta, the Watlow engineer who co-developed the system.

Watlow engineers have mapped the values for a variety of heater elements, including those made from pure nickel, nickel-chrome, platinum, Alumel, and Chromel. "Knowledge of heaters and electronics was needed to make this system work," Datta says. "There are a number of different heating materials available today. And it's necessary to understand their characteristics for HISS to work."

To aid in the dynamic heat control of industrial processes, Watlow engineers designed HISS to perform various kinds of failure detection. If, for example, the heater is operating, but the sensors detect no electrical current, the system determines that the heater has failed and sounds an alarm. Or if it detects a ground fault, HISS can decrease the current it sends through the heating element.

Currently, HISS employs a thermocouple to measure the initial ambient resistance in the heater element. But Datta says that the system does not use a thermocouple during regular heater operation.

Watlow engineers believe the system can serve in a wide variety of applications.   The key, in Datta's view, is to use HISS in applications where size constraints or environmental considerations play a role. "HISS is best used in applications where standard sensing elements don't work," he explains.

Additional details…Contact Jim Datta, Watlow Controls, 1241 Bundy Blvd., Winona, MN 55987, (507) 454-5300.