Basics of pressure transducers

A transducer is a device that converts a physical phenomenon into an electrical signal. In the industrial world, that task includes strain gauges, linear variable differential transformers, pressure transducers, and load cells.

06/01/1999


A transducer is a device that converts a physical phenomenon into an electrical signal. In the industrial world, that task includes strain gauges, linear variable differential transformers, pressure transducers, and load cells. The term "pressure transducer" or "transducer" can refer to either a pressure transmitter or pressure transducer. There is a distinction.

Pressure transmitters are current loop instruments that incorporate an electronic amplifier and pressure sensor into one package. This arrangement allows measurement signals to be transmitted over long distances without the loss of accuracy or the introduction of electrical noise.

Current loop systems are designed for applications where the signal is processed remotely and high noise immunity is required. Since they transmit a 4-20 mA output signal, these systems operate with lines over 1000 ft without signal degradation. Current loop systems are used for direct interface with most computers, data acquisition systems, and industrial process controllers.

Voltage transducers and current loop transmitters normally have amplifiers and signal conditioning built in. They can accept an unregulated or regulated power supply, which means the excitation voltage can vary between limits set by the manufacturer and, for a given pressure, the output signal will not vary. Therefore, an expensive, regulated power supply and signal conditioner are not required.

Pressure transducers use a millivolt or voltage system to convert input pressure into low-level electrical/electronic signals. These signals are not suitable for transmission over long distances.

Millivolt systems are ratiometric units with a relatively low cost, because the power supply, voltage regulator, and signal conditioning functions are remote from the transducer. Separating these components allows the transducer to be small in size. Because the signal conditioner is separate, these transducers are compatible with most instrumentation. They provide 0-30 and 0-100 mV signals.

Amplified voltage systems have the same basic sensor technology as millivolt systems, but are enhanced by instrumentation-grade amplifiers. An amplified voltage system is appropriate for many industrial applications. These systems are directly compatible with most older process control and computer interfaces and provide a 5-10 V output signal.

Selecting a transducer

Consider accuracy, system pressure, temperature, fluid compatibility, excitation, processor compatibility, noise, sensor type, and the system's loop resistance for transmitters when making the equipment choice.

Accuracy

Perform a complete analysis of how tightly the pressure should be controlled. The accuracy of each sensor and control element within the loop needs to be determined and the statistical accuracy of the control loop calculated. Most applications don't require this degree of sophistication, but accuracy will affect both the system performance and the product quality.

A rule-of-thumb is to select a transducer with a 0.25% full scale accuracy (including hysteresis, linearity, and repeatability) to provide proper control.

Pressure

Determine the maximum pressure in the system, including spikes and surges. The transducer should have a proof pressure greater than this value. If there are safety relief valves in the system, select a transducer based on the valve's setting. If the system is subjected to a vacuum, as in a cleaning cycle, consult the manufacturer because this condition affects selection.

As the overall range increases, transducer sensitivity decreases. It is not advisable to add too many safety factors. A prudent precaution is to add a pressure snubber to minimize the impact of surges and spikes. If a snubber is used, it alters the frequency response and rise time characteristics of pressure spikes and surges. The response time could change from 1 to 10 ms. For many systems this degree of dampening has no effect.

Temperature

The temperature rating of the transducer should match the normal temperature range of the process and the environment. Pressure transducers have three temperature ranges: operating, compensated, and storage.

Operating range defines the maximum temperature range for using the transducer. Exceeding the maximum operating temperature affects accuracy. At extremely low temperatures the unit may not energize.

Compensated range establishes the extent to which thermal shift is accurately predicted. Outside this range thermal shift can cause unpredictable readings. If the storage temperature is exceeded, instrument electronics can be permanently damaged.

If the process fluid exceeds the upper/lower temperature limits of the transducer, the transducer should be isolated from the temperature extreme. One method is to install a length of pipe or tubing between the piping and transducer. If there is any question regarding temperature, the temperature of the transducer should be confirmed with a thermocouple or other temperature device.

Fluid compatibility

Determine that diaphragm and process fittings are compatible with the process fluid. Most manufacturers can make material substitutions for unique applications.

Isolating the transducer can be accomplished by adding a chemical seal and vacuum filling with oil or water to eliminate air, which can affect frequency response and accuracy.

Excitation

Excitation power can be provided by an independent power supply or from process instrumentation. Power supplies can be regulated or unregulated.

All millivolt output transducers require a regulated power supply. Voltage and current loop pressure transducers can generally use either type of power supply.

Processor compatibility

Transducer output must match the input requirements of the processor. To determine if the transducer is operational, even when there is no process pressure, most voltage output units provide a zero offset. If true zero is used, it cannot be determined whether the system pressure is zero, the transducer is inoperable, or there is a break in the line.

Many processors and recorders specify a 0-5 or 0-10 V input signal. Their span can be adjusted to accept a standard offset of 0.5-5.5 or 1-11 V input signal.

Noise

EMI and RFI compatibility with electrical and electronic equipment is an international concern. IEC and ISO standards were implemented in January 1996 by the European Community (EC).

The EC requires all electrical and electronic products delivered after this date to conform to these standards.

Sensors

A pressure sensor is a subcomponent of the transducer which must be normalized, temperature compensated, and output conditioned. A signal conditioning circuit is generally required to provide temperature compensation, excitation, offset, and span adjustment. A typical transducer should be designed to provide EMI/RFI protection, shock and vibration resistance, and have a NEMA 4 enclosure.

Open cell designs are normally used to measure gauge pressure up to 150 psi. Changes in atmospheric pressure have no effect on accuracy. Closed cell sensors are used to measure absolute pressure, gauge pressure over 150 psi, and vacuum to 30-in. Hg.

Loop resistance

To specify a control system, determine the total loop resistance and maximum supply voltage. Then operate within a specific zone defined by the transmitter's loop resistance and maximum/minimum excitation voltage.

Example:

Maximum total loop resistance = V(subsmax) - V(subsmin) / I(submax)

Where:

V(subsmax) =rated maximum supply voltage, Vdc

V(subsmin) =minimum supply voltage, Vdc

I(submax) =full scale current, mA (usually 20)

The maximum supply voltage, V(subs) , must be determined to plot the minimum and maximum slopes of the operating zone, using the maximum total loop resistance value.

( V(subs) - V(subsmin) ) / I(submax) = ( V(subs) - V(subsmax) ) / I(submin)

Where:

V(subs) =maximum supply voltage, Vdc

I(submin) =minimum system current, mA (usually 4)

To use the loop resistance chart, determine the rated power supply excitation voltage. Next, determine total system resistance, including external wiring resistance and internal resistance of the receiver.

Compare the intersection of the total resistance and excitation voltage to the operating zone given in the specifications. If they intersect in the zone nothing more needs to be done. If the intersect falls outside the zone, add resistance to the loop until it falls within the zone.-- Edited by Joseph L. Foszcz, Senior Editor , 847-390-2699, j.foszcz@cahners.com

Key concepts

Pressure transducers are compatible with most instrumentation.

Sensitivity decreases as the range increases.

Relief valve setting detemines the upper pressure range.

Components of pressure measurement system

- Pressure transducer

- Excitation power supply

- Signal processor

- Digital or analog readout meter

- Oscilloscope

- Digital voltmeter

- Tape recorder

- Computer

- Chart recorder

- X-Y plotter

- Controller

More info

For additional information on pressure transducers, contact Dennis Echternach at 800-835-1060.





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