TechArchive

Original article date: July 1998

Colin Lussenden product manager with Gems Sensors explains how ASICs and CVD have contributed to a new design of pressure transducer.

Monitoring and process control devices can significantly affect overall performance by helping to ensure that processes remain within the correct operational limits. Accurate pressure monitoring for example forms an important part of many processes across a range of industries from monitoring heating and air conditioning systems to operational tests in hydraulic systems power equipment and environmental control units.

For many years manufacturers requiring pressure measurement have been faced with a choice – either purchase a low cost pressure transducer which falls within typical spending limits but may be subject to considerable long term drift poor stability and low accuracy or install a high specification device which exhibits excellent performance and reliability but for which the cost may be prohibitive in many applications.

Gems Sensors’ new range of transducers Psibar has been designed with a number of operational advantages over traditional pressure sensors. One of the main advances of the new transducers arises from the use of a combination of both application specific integrated circuit (ASIC) and chemical vapour deposition (CVD) technologies both of which provide inherent advantages.

The primary advantage of CVD is that it allows large quantities of devices to be produced quickly at a low unit cost. Each sensor is produced on a stainless steel wafer using semiconductor manufacturing processes. The resulting devices are inherently more stable more robust and less temperature sensitive all of which can provide significant cost savings within an OEM environment. Overall the device offers a design fatigue life of 100 million cycles accuracy to within 0.5% a short lead time and resistance to pressure shocks and mechanical vibration. The stability level is particularly useful in applications such as long term testing and monitoring and on-going leak testing.

In addition high temperature vacuum brazing of stainless steel during sensor production provides a structure that offers both low hysteresis and creep together with high strength and corrosion resistance.

The use of ASIC technology also reduces production time as the need for laser trimming to compensate for minor changes during the manufacturing process is eliminated. Furthermore the output and range for the circuit are factory-programmable and customised for each application.

Internally Psibar incorporates special diaphragms to provide a capability of four times over-pressure. This provides an increased level of ruggedness and resilience while retaining good levels of long term repeatability and performance. The high overpressure ensures that Psibar can also be used in those applications where pressure spikes or shocks can occur which can cause unwanted output signals or in more extreme cases damage to traditional pressure transducers.

The internal design of the transducer also allows it to be connected directly to the process fluid without chemical seals separation diaphragms or fill liquids. Psibar is compatible with many aggressive liquids and gases which may have previously required specialised pressure sensors. The use of stainless steel throughout the capsule and extensive thermal and pressure cycling during manufacture minimises residual stresses and improves stability while at the same time greatly increasing resistance to corrosion. Psibar also features stainless steel wetted parts eliminating the need for O rings or oil screens and thus ensuring freedom from possible leak paths and oil media contamination.

A choice of outputs can be supplied for a wide range of electrical and pressure connections. Compensation data can easily be programmed into the transducer enabling users to achieve fast set up.

Psibar specifications

• Gems Sensors
• Colin Lussenden
• 01256 320244

July 1998