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issue: November 2007 APPLIANCE Magazine

Electronics Report
Stable Sensing


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An improved ceramic pressure sensor is said to offer engineers of HVAC equipment a wide temperature range and increased reliability.

The ceramic material of the new ME501 is said to create a reliable sensor, providing a linear mV output with little hysteresis. The sensor’s stability makes it impervious to mounting direction, allowing OEMs to create a specific custom sensor housing based on the application.

The new ME501 piezoresistive ceramic pressure sensor from Metallux features manufacturing process improvements that have led to increased stability, consistency, and reliability. Designed by Servoflo Corp. of Lexington, MA, U.S., the sensor comes in a variety of pressure ranges, from 0–0.5 up to 0–600 bar.

With an operating temperature range of –40° to 135°C, the ME501 offers wide temperature compensation directly mounted on the sensor. This is said to offer excellent protection from corrosion, impact and vibration resistance, and easy mounting options.

Applications for the new component include automotive and industrial sensing, but David Ezekiel, vice president, says it can also be used to measure refrigerant circular flow in HVAC systems. “Generally speaking, [it can be used] anywhere you need to measure one or more of the following elements: harsh medium to measure, potentially high pressure ranges, and varying temperature environments.”

In air-conditioning applications, Ezekiel says the sensor’s ceramic material is especially beneficial. “Ceramic is resistant to all cooling agents in air-conditioning systems,” he explains. “Thus, no additional protection is required.” As a result, Ezekiel says, the engineer doesn’t have to worry about the reliability of the sensor material and degradation of the sensor signal.

The sensor’s ability to perform over a wide temperature range is due to a thick-film technology and the stabilization of the sensor during the manufacturing process. The thick-film technology allows the sensor’s measuring bridge to be printed directly on one side of the ceramic diaphragm. The rear part of the diaphragm can be exposed directly to the medium with no additional protection.

“The manufacturing process of packaging the resistors and traces on the ceramic itself is a very specialized and precise process. It is not easy to do,” Ezekiel explains. “All necessary structures—such as conductors, piezoresistive gauge elements, compensation resistors, and coatings (polymer or glass)—are applied by screen printing with a successive drying and firing. The typical thickness of such layers is 8.15 micrometers. It is an iterative process.”

Ezekiel says the design of the sensor’s stable base plate guarantees a very high mechanical stability. “The high resonance frequency reduces the risk of damage caused by vibration to a minimum,” he explains.

In addition, the ceramic sensor’s rugged construction is said to make mounting easy. “A competent mechanical engineer can house the sensor, securing it with clamping or screwing, and create a stable sensor,” Ezekiel says.

Overall, it is the combination of the sensor’s mechanical stability, ease of installation, and ability to resist corrosion that benefits engineers of HVAC equipment, according to Ezekiel. “In varying thermal environments such as HVAC applications, sensor stability is a very important issue,” he notes. “For general-purpose applications, the user does not have to provide additional temperature compensation unless they are looking for extremely precise measurements.”

 

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