Pressure transducers using silicon-on-sapphire (SOS) technology have been used in industry because of their high temperature operation, high accuracy and repeatability, and long term stability. Now, SOS pressure transducers should find their way into off highway equipment.

What is silicon on sapphire?

Sapphire is derived from the Greek word sappherios, referring to a naturally occurring gemstone of Al2O3. Single crystal sapphire is the purest form of aluminum oxide and is artificially produced by heating Al2O3 powder in a controlled furnace to more than 3600° F, then slowly cooling it. The result is an extremely pure single crystal of sapphire that can weigh more than 100 lb.

Sapphire wafers are cut at a specific angle to allow silicon atoms to bond to them to form SiO2 on their surface. The SiO2 is an atomic extension of the sapphire itself — a process known as epitaxial growth — which makes SOS technology possible.

SOS technology deposits a thin layer of silicon onto a sapphire wafer at high temperature. Doped silicon strain gauges are etched from this layer, and individual strain gauges are electrically isolated from each other by the outstanding insulating characteristics of the sapphire substrate. The ability to operate at high temperatures, chemical inertness, and virtual absence of hysteresis make the strain gauges ideal for use in pressure sensors.

However, pressure transducers used in hydraulic systems of off-highway machines must tolerate punishing conditions, such as severe pressure spikes caused by valves shifting, prolonged exposure to continual pulsation, broad temperature ranges, heavy shock, and vibration. Pressure spikes can be especially harmful or even destroy a pressure transducer. Prolonged exposure to continual pulsation can cause fatigue failure of the pressure transducer and influence long term stability and accuracy.

The benefits of SOS sensor technology would make them ideal for off-highway equipment. However, price sensitivity has k ept them from penetrating the high-volume off-highway equipment industry.

Ellison Sensors International, Wrexham, Wales, has produced SOS pressure transducers for more than 25 yr, primarily for aerospace, defense, oil, gas, and subsea industries. Ellison Sensors is now part of pressure switch and pressure sensor manufacturer SUCO, Bietigheim-Bissingen, Germany, a supplier to many off-highway equipment manufacturers.

The combined capabilities of Ellison and SUCO allowed SUCO to introduce its Type 07XX pressure transducer based on silicon-on-sapphire technology. Key characteristics of the transducer include:

  • 4 nominal overpressure
  • 8 nominal burst pressure
  • maximum rise rate of <72,500 psi/sec (<5000 bar/sec), and
  • mechanical life of 10,000,000 pulses

For more information on SUCO’s SOS-based pressure transducers, visit www.suco-tech.com.