Pressure-transducer options for fluid-power measurements

March 23, 2000
Applying pressure to a coupled port in a Wheatstone bridge increases resistance in two resistors and decreases it in the other two.

Applying pressure to a coupled port in a Wheatstone bridge increases resistance in two resistors and decreases it in the other two. An excitation source drives the bridge, and output is measured as a differential voltage.

In fluid-power systems, users often need to know the operating pressure at any given time. Typically, transducers detect fluid pressure and produce a corresponding electrical signal.

Several electronic-pressure technologies are used in measurement and control of fluid-power systems. Regardless of the technology, the first step is to carefully check prospective transducers for key specifications including pressure range, wetted-path materials, accuracy, overpressure, burst pressure, and temperature effects. Other important specs are response time, long-term stability, EMI effects, NEMA/IP rating, input/output requirements, and safety rating. A transducer with zero and span adjustments may be necessary for optimal measurement accuracy.

Six common types of strain gages are Bourdon tube, semiconductor, capacitance, bonded foil, metal thin film, and piezoelectric. Each type has specific advantages that make them suitable for certain applications.

Bourdon-tube strain gages contain semiflexible tubes normally shaped like the letter "C" and fitted with a pressure port. They are constructed of stainless steel or phosphor bronze and expand under pressure. The resulting change in length drives a local indicator or is converted to an electronic output. The simplest and lowest priced designs are accurate to 1% full scale (FS). Larger tubes or more precise movements improve accuracy.

Semiconductor strain gages have four pressure-sensitive silicon resistors arranged in a Wheatstone-bridge configuration. Silicon strain elements in Wheatstone bridges may be epoxied, glass or ceramic fused, or chemically deposited to stainless-steel diaphragms. Although epoxy-based designs are common, experts suspect that epoxy causes excessive signal drift. If this is a concern, piezoresistive dies offer an alternative to epoxy.

Piezoresistive dies have four ion-implanted resistors on a micromachined silicon substrate with a glass backing to isolate stress. The dies are usually contained in an oil capsule with a thin, convoluted stainless diaphragm exposed to the fluid media. This maintains low hysteresis and enhanced repeatability.

Silicon resistors are relatively temperature dependent, with typical gradients exceeding 1,500 ppm/°C. Thus, silicon resistors often require internal temperature compensation. In fact, response time usually depends on piping to the transducer rather than the transducer itself. Some amplified units purposely damp response time, so millivolt output might achieve the quickest response times. Elements made of silicon-on-insulator or silicon-on-sapphire are often needed for temperatures above 125°C. Accuracy of silicon sensors is often 0.25% FS or better.

Capacitance strain gages are useful for electronically measuring vacuum or subvacuum pressures. When pressure varies, changes in the distance between two electrodes produce a slight capacitance change, which alters an oscillator frequency.

Because full-scale capacitance changes are typically only several picofarads, temperature compensation is difficult with capacitance gages. Large diaphragms, sophisticated electronics, or even heaters are often used to keep units operating consistently with temperature changes. Accuracies of capacitance transducers are often better than 0.25% FS.

Bonded-foil strain gages use Wheatstone bridges similarly to silicon sensors. The difference is that stable metal elements are deposited with an insulator and epoxy on a metal diaphragm. Although temperature dependence is low, the epoxy is often considered a source of long-term drift. The sensor's output is typically 115 that of silicon, so bonded-foil strain gages generally need amplification to display pressure signals. The devices are accurate to 0.25% FS.

Metal-thin-film strain gages were developed to improve bonded-foil designs. Instead of using epoxy, the strain elements are produced by sputtering glass and a nichrome conductor on a metal diaphragm under vacuum. The resulting molecular bond is very stable over time. The temperature coefficient of the formed resistors drops to 1100 that of silicon resistors. The temperature coefficients of metal-thin-film strain gages also match those of the diaphragms, making them less sensitive to temperature than bonded-foil strain gages. This allows operation at temperatures above 125°C.

Operational amplifiers can convert the low-level output from the strain gages to a useful signal. Recent developments have produced metal thin-film strain gages with long-term stability of 0.125% over six months. The strain gages have accuracies of 0.25% FS or better.

Piezoelectric transducers measure stress changes in quartz to determine pressure changes. Unlike other technologies presented here, which are useful in static or semidynamic measurements, piezoelectric transducers are good for dynamic measurements when quick response times are needed. This comes in handy for viewing explosions or other rapid pressure changes.

Benefits and limitations of fluid-power pressure technologies

Pressure Technology



Bourdon-tube gage

Low cost, local indication possible

Lower accuracy

Silicon gage

Mass produced, general purpose

Large raw temperature errors, not recommended for epoxy-based designs

Capacitance gage

Very low pressure ranges

Large package size, large temperature errors

Bonded-foil gage

Low-temperature errors

Uses epoxy mounting of elements

Metal thin film gage

Ultrastable, low-temperature errors

High gain electronics needed

Piezoelectric sensor

Ultrafast response time

Not for static measurements

Information for this article was provided by Richard Rosenblum, Principal Electrical Engineer, Ametek U.S. Gauge Div., PMT Products, Feasterville, Pa.

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