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issue: November 2006 APPLIANCE Magazine

Electronics Report
Combination Sensing

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Combination temperature sensors are heating up the sensors market by offering HVAC engineers an alternative to conventional low-temperature cut-outs.

The Chill-Out combination temperature sensor from Minco (Minneapolis, Minnesota, U.S.) was designed to simplify installation and prevent the problems associated with conventional freeze stats. A solid-state design and rugged tubing are said to eliminate concerns of gas leaks or kinking the capillary during installation.

System integrators have been using conventional low-temperature cut-outs, also referred to as “freeze stats,” in unit ventilators for decades. This fact has brought many sensor suppliers to the design room, hoping to develop a new, more sophisticated temperature sensor that not only provides easier and faster installation, but reduces life-cycle costs and eliminates problems associated with the traditional freeze stat. According to component supplier Minco, its patented Chill-Out™ combination sensor fits the bill.
The combination temperature sensor incorporates multiple solid-state low-temperature cut-out sensors with an averaging resistance temperature sensor in one package. The low-temperature cut-out senses a cold spot every 12 inches (30.5 cm) along its length and contains IC temperature sensors that output 0 V or 5 V. Under normal conditions, the sensor’s output is 5 V. When in an alarm state, the voltage drops to 0 V, and the relay contacts change state. The alarm state can be displayed on the controller.
According to Greg Parker, design engineer at Minco Instruments Group, the combination sensor allows an HVAC engineer to not only improve the performance of their design, but also reduce costs. “The greatest benefit comes from combining two sensors into one package, which significantly reduces the cost to install the device,” he says.
In fact, Parker says the sensor overcomes all of the shortcomings of the traditional freeze stat. The solid-state design of the combination temperature sensor replaces traditional gas-filled capillary tubes, eliminating the risk of kinking during installation. The solid-state design also prevents the occurrence of field failures due to leaking and makes the combination temperature sensor sensitive to low temperatures in any direction up to 12 inches. This is in comparison to 18 inches (45.7 cm) for gas-filled capillary tubes. “By strategically placing our sensors within the probe, we are able to increase the sensitivity of our device to within any 12-inch segment,” Parker says. “Furthermore, the sensitivity of the sensor is not affected by the mounting direction, unlike traditional gas-filled capillary devices, in which the vertical segments of the probe are not functional.”
Another benefit is the device’s failure detection feature, which incorporates a microprocessor with error checking algorithms to ensure accuracy and reliability. “The microprocessor controls a relay in which the coil is energized in its normal, non-alarm state,” Parker explains. “The relay will change state into its alarm condition if a low-temperature cut-out sensor were to fail, or the device were to lose power.”
One key application for the combination sensor is the replacement of a mixed-air temperature sensor and a freeze-stat in air-handling units. “By combining both devices into one convenient package, the expensive installation costs are reduced in half by requiring only one durable device to be installed,” Parker says. “With the flexibility of the installation, the probe length needed may be less than that of previous installations, further reducing the cost of the device.”

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