Process measurement / transducers--part 3: Pressure transmitters

.Pressure is probably the second most commonly used and important measurement in process control. The most familiar pressure measuring devices are manometers and dial gauges, but these require a manual operator. For use in process control, a pressure measuring device needs a pressure transmitter that will produce an output signal for transmission, e.g. an electric current proportional to the pressure being measured. A transmitter typically that produces an output of a 4-20 mA signal is rugged and can be used in flammable or hazardous service.

Terms of pressure reference

Pressure is defined as force per unit area and may be expressed in units of newtons per square meter, millimeters of mercury, atmospheres, bars or torrs. There are three common references against which it can be measured:

1. If measured against a vacuum, the measured pressure is called absolute pressure

2. Against local ambient pressure it’s gauge pressure

3. If the reference pressure is user supplied, differential pressure is measured.

There are seven principle methods of electronically measuring pressure for use in process control and each of these is listed and described under its numeric heading, in principle detail below:

1. Strain gauge (bonded or unbonded wire or foil, bonded or diffused semi conductor)

2. Capacitive

3. Potentiometric

4. Resonant wire

5. Piezoelectric

6. Magnetic (inductive and reluctive)

7. Optical.

Strain gauge

In process control applications, one of the most common ways to measure pressure is using a strain gauge sensor. There are two basic types of strain gauge, bonded and unbonded, each utilizing wire or foil, but both working in the same electrical manner. A thin wire (or foil strip), usually made from chrome-nickel alloys and sometimes platinum, is subjected to stretching, and hence its resistance increases as its length increases.

Strain gauges are commonly made using a thin metal wire or foil that is only a few micrometers thick, so they can also be known as thin film-based bonded pressure sensors.

The unbonded strain gauge consists of open wire wound round two parallel mounted posts which are flexed or pulled apart, hence imparting a stretching dynamic to the wire, reducing its resistance, these being physically much larger units.

Composition of strain gauges:

Bonded strain gauges are the most common type in use. They comprise an insulated bonded sandwich usually made of two sheets of paper, with the gauge wire laid in a specific pattern between them. Strain gauge wires of less than 0.001 in. (0.025 mm) diameter are used as they have a surface area several thousand times greater than the cross-sectional area.

++++Composition of bonded and unbonded strain gauges-- Strain gauge windings (R1 and R2) Strain gauge windings (R3 and R4) Posts Spring element Reference pressure Process pressure Diaphragm Foil grid pattern Terminal wire Insulating layer and bonding cement Structure under bending Neutral axis (a) Unbonded foil straw (b) bonded foil gauge

Foil gauges have been commercially made where the foil thickness can be as low as 0.0001 in. (0.0025 mm). Semiconductor types are available which have sensitivities close to one hundred times greater than the wire types.

Strain gauge sensitivity and gauge factor:

The ratio of the percentage change in resistance to the percentage change in length is a measure of the sensitivity (S ) of the gauge:

Strain gauge sensitivity and gauge factor:

The ratio of the percentage change in resistance to the percentage change in length is a measure of the sensitivity (S ) of the gauge:

Where:

L is the initial length of the wire or foil.

R is the specific resistivity in the unstrained position.

Many things affect the axial strain and gauge resistance, such as the geometry of the wire or foil in the gauge, direction of strain. This is expressed by the constant called gauge factor or GF:

Typically, four strain gauges are bonded to a metal or plastic flexible diaphragm and connected into a Wheatstone bridge circuit to yield an electrical signal proportional to the strain caused by the displacement of the diaphragm by the pressure applied to it.

The changes of resistance in a strain gauge are very small and as such precise and accurate instrumentation is required in order to obtain useable and accurate readings. The most common form of measuring is in a Wheatstone bridge circuit. --- a typical arrangement using this type of instrument.

++++ Wheatstone circuit for strain gauge measurement -- +- -, +, + = 8 Excitation power supply; Output E + delta_E; Calibration resistor; Terminal resistance adjustment; Temperature compensator resistor; R1 R2 R3 R4 = 0 ohm

Effects of temperature on the gauge's resistance is minimal due to their influence on the output being subtractive.

The active gauges are located on opposite arms of the bridge, making effects additive; the other two gauges are for compensation and are either 'dummy' gauges or resistors with equal resistance to the active gauges.

This circuit is suited to both static and dynamic strain measurements, the output is, however, a differential output and care must be exercised in 'grounding' any part of this circuit.

An alternative arrangement, called a ballast or potentiometric circuit is arrived at by removing R2 and making the value of R3 equal to 0.

Unfortunately this circuit is best used for dynamic sensing only; however one of the signal leads can now be grounded or electrically referenced to 0 V.

Measurement errors for strain gauges:

A number of errors exist when strain is measured using the Wheatstone bridge arrangement. Typical of these are:

• Gauge factor uncertainty (typically 1%)

• Bridge non-linearity (typically 1%). This is a result of the assumption that the change in strain gauge resistance is very small compared to the nominal gauge resistance

• Matching of compensation resistors to the strain gauge (typically 0.5%)

• Measurement errors caused by the accuracy and resolution of the measuring device and lead resistances

• Temperature effects: Resistance varies with changes from the temperature at which a bridge is calibrated

• Self-heating of gauges.

Strain gauge pressure transducer specifications

Strain gauge elements can detect absolute gauge, and differential spans from 30 in. H2O to upwards of 200 000 psig (7.5 kPa-1400 MPa). Their inaccuracy is around 0.2% and 0.5% of span. Units are available to work in the temperature range of -50 up to +120 °C with special units going to 320 °C. 2.8.3 Vibrating wire or resonant wire transducers

This type of sensor consists of an electronic oscillator circuit, which causes a wire to vibrate at its natural frequency when under tension. The principle is similar to that of a guitar string. A thin wire inside the sensor is kept in tension, with one end fixed and the other attached to a diaphragm. As pressure moves the diaphragm, the tension on the wire changes thereby changing its resonant vibration frequency. These frequency changes are a direct consequence of pressure changes and as such is detected and shown as pressure.

The frequency can be sensed as digital pulses from an electromagnetic pickup or sensing coil. An electronic transmitter would then convert this into an electrical signal suitable for transmission. This type of pressure measurement can be used for differential, absolute or gauge installations. Absolute pressure measurement is achieved by evacuating the low pressure diaphragm. A typical vacuum pressure for such a case would be about 0.5 Pa.

++++ Resonant wire transducer -- Wire grip; Vibrating wire; Wire grip Pressure sensitive diaphragm; Electromagnetic plucking and sensing coil—Magnet—Magnet--Resonant wire--To oscillator circuit, High-side backup plate, Metal tube, High pressure diaphragm, Fluid transport port, Low pressure diaphragm, Electrical insulator, Pre-load spring, Low-side backup plate

Transducer advantages:

• Good accuracy and repeatability

• Stable

• Low hysteresis

• High resolution

• Absolute, gauge or differential measurement.

Transducer limitations, or disabilities

• Sensitivity to vibrations

• Temperature variations require temperature compensation within the sensor, this problem limits the sensitivity of the device

• The output generated is non-linear which can cause continuous control problems

• This technology is seldom used any more. Being an older technology, it’s typically found with analog control circuitry.

Capacitance

Capacitive pressure measurement involves sensing the change in capacitance that results from the movement of a diaphragm. The sensor is energized electrically with a high frequency oscillator. As the diaphragm is deflected due to pressure changes, the relative capacitance is measured by a bridge circuit.

++++ Capacitance pressure detector -- High frequency oscillator Output Reference pressure Process pressure; Pressure bellows; Insulated standoffs; Diaphragm; Capacitor plates; Pressure port

Two designs are quite common. The first is the two-plate design and is con figured to operate in the balanced or unbalanced mode. The other is a single capacitor design.

The balanced mode is where the reference capacitor is varied to give zero voltage on the output. The unbalanced mode requires measuring the ratio of output to excitation voltage to determine pressure.

This type of pressure measurement is quite accurate and has a wide operating range.

Capacitive pressure measurement is also quite common for determining the level in a tank or vessel.

Transducer advantages:

• Inaccuracy 0.01-0.2%

• Range of 80 Pa-35 MPa

• Linearity

• Fast response.

Transducer limitations:

• Temperature sensitive

• Stray capacitance problems

• Vibration

• Limited over pressure capability

• Cost.

Many of the disadvantages above have been addressed and their problems reduced in newer designs.

Temperature-controlled sensors are available for applications requiring a high accuracy.

With strain gauges being the most popular form of pressure measurement, capacitance sensors are the next most common solution.

Linear variable differential transformer

This type of pressure measurement relies on the movement of a high permeability core within transformer coils. The movement is transferred from the process medium to the core by use of a diaphragm, bellows or bourdon tube.

The LVDT operates on the inductance ratio between the coils. Three coils are wound onto the same insulating tube containing the high permeability iron core. The primary coil is located between the two secondary coils and is energized with an alternating current.

Equal voltages are induced in the secondary coils if the core is in the center. The voltages are induced by the magnetic flux. When the core is moved from the center position, the result of the voltages in the secondary windings will be different. The secondary coils are usually wired in series.

To measuring circuit; Transducer limitations or disabilities

• Mechanical wear

• Vibration.

Summary:

• This is an older technology, used before strain gauges were developed.

• Typically found on old weigh-frames or may be used for position control applications.

• Very seldom used anymore; strain gauge types have superseded these transducers in most applications.

Optical

Optical sensors can be used to measure the movement of a diaphragm due to pressure. An opaque vane is mounted onto a diaphragm and moves in front of an infrared light beam.

As the light is disturbed, the received light on the measuring diode indicates the position of the diaphragm.

A reference diode is used to compensate for the aging of the light source. Also, by using a reference diode, the temperature effects are canceled as they affect the sensing and reference diodes in the same way.

LED Opaque vane; Measured pressure; Reference diode; Measuring diode

++++ Optical pressure transducer

Transducer advantages:

• Temperature corrected

• Good repeatability Pressure

++++ Schematic representation of a linear motion variable inductance prior transducer element (LMVIPTE)

• Negligible hysteresis as optical sensors require very little movement for accurate sensing

• Optical pressure measurement provides very good repeatability with negligible hysteresis.

Transducer limitations or disabilities:

• Expensive.

Pressure measurement applications

There are a number of requirements that need to be considered with applications in pressure measurement. Some of the more important of these are listed below:

• Location of process connections

• Isolation valves

• Use of impulse tubing

• Test and drain valves

• Sensor construction

• Temperature effects

• Remote diaphragm seals

• Corrosion may cause a problem to the transmitter and pressure sensing element

• The sensing fluid contains suspended solids or is sufficiently viscous to clog the piping

• The process temperature is outside of the normal operating range of the transmitter

• The process fluid may freeze or solidify in the transmitter or impulse piping

• The process medium needs to be flushed out of the process connections when changing batches

• Maintaining sanitary or aseptic conditions

• Eliminating the maintenance required with wet leg applications

• Making density or other measurements.

NEXT: part 4: Flow meters

PREV: part 2

All related articles   Top of Page   Home