Pressure transducer technology

All pressure transducers operate on the principle of converting a pressure change into a mechanical displacement, or deformation. Deformation of the sensing element is then converted into an electrical signal that is processed by the measuring system. Types of pressure transducers available in the field, either individually or in combination, are mechanical, capacitance, strain gauge and quartz gauge. This article discusses how each of these types of pressure transducers operate.

Mechanical pressure transducers

The first pressure transducers had mechanical-force-summing elements that converted energy into mechanical displacement, or deformation, and then coupled the generated force to a recording device. In the Amerada gauge, a popular mechanical pressure transducer, the pressure-sensing element is a helical Bourdon tube. The tube is of sufficient length to rotate a clock-driven stylus a full circumference inside the cylindrical chart holder. The chart, usually made of coated metal, is recovered at the end of the test, unfolded until flat, and read on a high-precision optical machine. The transducer also incorporates a vapor-type recording thermometer to make temperature corrections on the pressure measurements.

Mechanical transducers have largely been abandoned because of their obsolete metrological characteristics and lack of surface readout (SRO). They are still used occasionally for basic applications at the lower end of the economic spectrum, for some very high-temperature applications, or as backup for an electronic pressure gauge.

Capacitance pressure transducers

Capacitance transducers have a variable-gap capacitor in which the sensing element is formed by two metallic or quartz plates. As the external pressure increases, the deflection of the sensing plate creates a change in the capacitance that can be mathematically related to the applied pressure.

Advantages of capacitance transducers

Capacitance transducers have the advantages of:

• Good frequency response
• Low hysteresis
• Good linearity
• Excellent stability and repeatability

Disadvantages of capacitance transducers

The disadvantages are:

• High sensitivity to temperature
• Acceleration
• Orientation
• Mechanical noise

Fused quartz has excellent elastic behavior (low hysteresis) and is chemically inert. These properties make it an almost ideal material for manufacturing small capacitor modules with a high temperature rating.

Strain pressure transducers

Many types of strain-gauge transducers are in use. Strain gauges have become very popular because of their ruggedness, low cost, and good dynamic behavior. Their metrological characteristics have greatly improved in recent years; gauges with an accuracy of a few psi and resolution as low as 0.05 psi are available. The primary limitation of strain gauges is their tendency to drift, although that aspect of the measurement has improved.

A strain gauge has a strain-sensitive resistor directly attached to a measuring sensor; when the sensor is subjected to pressure, it deforms. The resulting displacement changes the resistor length, hence its resistance. The applied pressure is calculated from a calibrated relationship to the change in resistance at a given temperature.

Bonded wire transducers

In this design, introduced by the Paine Corporation in the 1970s, two sets of wire, called the "active" windings, are wrapped around a cylindrically shaped tube-sensing member. As pressure increases, the tube bore is stretched, causing a change in the wire resistance. Another two sets of wire—the reference, or "passive," windings—are wrapped on the upper part of the tube, which is not exposed to pressure. These four sets of wire form a Wheatstone bridge that allows the electrical output to be reduced to a pressure reading.

Thin-film transducers

The thin-film sensor consists of a resistor pattern that is vaporized or sputter-deposited onto the force-summing element (the measuring diaphragm). In some transducers the resistors are not directly mounted on the diaphragm but are on a beam linked to the diaphragm by a push rod.

Sapphire transducer

In the Schlumberger improvement of the diaphragm-type thin-film transducer, the sensing resistors are mounted on a miniature substrate of industrial sapphire. The Sapphire* pressure-gauge system is vacuum-filled and the resistor pattern forms a Wheatstone bridge. This system benefits from the elastic performance of the sapphire and its stable deformation properties. The result is a sensor with good repeatability, good stability, low hysteresis, and low drift. A high-gauge factor improves the resolution over traditional designs. The main disadvantages are low output level and high cost.

Quartz pressure transducers

Quartz-crystal pressure transducers vibrate at their resonating frequency when excited by a suitable external energy source. The resonating frequency is affected by both the pressure and temperature to which the crystal is exposed. Because of the excellent gauge factor yielded by this physical process, quartz-crystal pressure transducers have exceptional accuracy, resolution, and long-term stability. The disadvantages are high cost and high sensitivity to temperature, although the most recent designs are much less temperature-sensitive.

Hewlett-packard design

The Hewlett-Packard (HP) design has been in use since the early 1970s. It features a two-crystal arrangement of a measure and a reference crystal. The measure crystal is exposed to both pressure and temperature. The reference crystal is exposed only to temperature and is used to compensate for temperature effects on the measure crystal. Both crystals are factory-matched so that their frequency characteristics in temperature are approximately the same. The measure crystal senses the pressure directly rather than through a mechanical linkage or other force-summing device. This has the effect of optimizing the metrology of the measurement. The output from the crystal pair and associated electronics is calibrated to yield the measured pressure by means of a 2D cubic polynomial including 16 coefficients. The values of these coefficients are determined at least annually during the gauge master calibration.

Quartzdyne design

The Quartzdyne design features three resonating crystals: the measure crystal, which is exposed to both pressure and temperature, and the temperature and reference crystals, which are exposed only to temperature. The measure crystal is a thick-walled, hollow quartz cylinder closed at both ends. The resonating element is a disk placed in the center, which divides the cylinder into halves. Separate conductive plates are located on the front and back of the resonator disk. Fluid pressure on the exterior walls hydrostatically compresses the quartz cylinder, producing internal compressive stresses in the resonator. The oscillating frequency of the resonator changes in response to these internal stresses.

The reference crystal oscillates at a fixed high frequency, which is subtracted from both the measure crystal and temperature crystal resonating frequencies. The temperature compensation is performed based on these low-frequency signals. The calibration procedure involves a fourth-order polynomial. Because of its small size, the Quartzdyne design provides good thermal performance and low cost, although somewhat at the expense of accuracy.

Schlumberger design

In the Schlumberger Crystal Quartz Gauge design, the transducer features a single quartz crystal structure in which a resonator is coupled with a dual-mode oscillator. The resonant frequency of the first mode is highly sensitive to pressure, and that of the second mode is more sensitive to temperature. The sensor consists of a cylindrical quartz body fitted with two end caps. The end caps maintain a vacuum inside the sensor. The resonator is a plate etched out of the quartz cylinder that features shaped surfaces acting as vibrating lenses. The resonating frequency of the plate varies with changes in pressure and temperature.

From a static point of view, the main advantage of this design is that pressure and temperature are measured at the same location, which minimizes time and space delays for thermal corrections. From a dynamic point of view, this design leads to very small peak transient errors in the thermal response that can be further minimized by using real-time dynamic compensation. The calibration involves a fourth-order polynomial.

The main disadvantages of the crystal quartz gauge design are fragility and high cost.

Paroscientific design

The Paroscientific design uses a quartz crystal operating in flexure mode to measure force. To derive a pressure output, a force-summing device such as a Bourdon tube or bellows must be used. Thus, the transducer senses pressure through the force-summing device and is not in direct contact with the wellbore fluid. This design tends to improve temperature characteristics but dampens the response and downgrades the measurement metrology. A temperature sensor comprising a quartz torsional tuning fork provides temperature compensation.

Quartztronics design

The Quartztronics design is a modified HP design, with a specially cut resonator and a noncylindrical cell geometry. The result is a smaller, lower cost pressure transducer with a higher pressure range.

The transducer features a temperature-sensing crystal and a reference crystal, both located close to the measure crystal. This configuration provides improved pressure- and temperature-transient responses in comparison with the HP design. The two crystals are thermally matched to the measure sensor and within a pressure-proof package bonded to one of the end caps of the measure sensor.

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See also

Reservoir pressure and temperature

Bottomhole pressure and temperature gauges

Acquiring bottomhole pressure and temperature data

PEH:Reservoir_Pressure_and_Temperature