Specifying a Pressure Transducer for a Process Measurement

Numerous things must be considered when specifying a pressure transducer for a process measurement. Some of the more crucial ones are discussed in terms of the overall measuring system and the transducer itself. This information is user-oriented and is used as a practical guide in the choice and application of strain-gage pressure transducers.

Introduction

Bonded strain-gage pressure transducers have supplied an accurate, convenient and reliable means of measuring fluid pressure over the years. The present day foil strain-gage is much more sophisticated than it's wire-gage predecessor.

In the early 1950s, moving from wire to foil brought about huge enhancements in overall performance of a transducer. The improvement has been ongoing and today's strain-gage transducers are worlds apart from the models built in the past.

Key advances have been made in many areas, such as superior adhesives, improved heat dissipation, improved temperature compensation, better bonding technique and reduced creep and hysteresis effects. This leads to a transducer that is more reliable and accurate than other types.

Transducer

To understand the do's and don'ts of pressure transducers better, it is worth having a brief look at the way they work. The transducer is installed in the system by means of its pressure fitting. This can be either an external or internal threaded connection.

The thread itself can either be a tapered pipe thread or a straight thread with a plug seal or ‘O’ ring. After installation, the pressure is applied to the transducer, which results in a force-summing element that converts the pressure into a physical displacement.

The force-summing element develops surface strains proportional to the pressure; this element is a diaphragm in most instances. By design, strain-gage pressure transducers are rugged, stable and accurate.

They can perform in severe vibration and shock environments. In addition, a wide range of pressure and electrical connections can be installed directly at the point of measurement.

Transducer Signal

Transducers are user friendly as they are able to convert fluid pressure into an electrical signal. The signal can be transmitted from the pressure sit to remote locations and utilized for specific control, measurement and monitoring purposes. They interface with data acquisition systems, data loggers, readout instruments and computers.

Strain-gage transducers are available in both high and low level outputs. High level outputs are 5 VDC and 10 VDC at an input voltage of 24 to 32 VDC and 4-20 mA at a 12 to 36 VDC input. The low level output for foil-gage transducers is a 3 mV/V or 30 millivolts at 10 VDC input.

The analog-to-digital (A/D) converter is employed in the digital world to translate the analog signal into digital bits. Although this is a rugged device that requires minimal maintenance, transducers are subject to misapplication and damage. Long transducer life and high integrity performance can be assured with proper application and knowledge of certain limitations.

Foil Strain-Gage

Foil strain-gages are bonded to the diaphragm and become the primary sensors in order to use the pressure to strain relationship. They alter resistance as a function of the diaphragm strain, which is, in turn, related to pressure.

Typically, four strain-gages are wired in series to create a Wheatstone Bridge. The bridge concept is employed as it is the simplest and most accurate technique conceived for measuring small resistance modifications.

An electrical output signal is developed proportional to the applied pressure with voltage applied to two opposite corners of the bridge. The output signal level is in millivolts per applied volt and is gathered at the remaining two corners of the bridge.

Considerations

The factors which must be considered in the feeding and care of transducers can be placed into the categories below:

1. Pressure/Electrical Connections

All transducers need two connections, an electrical connection and a mechanical pressure connection. As there are no industry standards established, the transducer manufacturer's data must be checked to establish the configuration offered with the product.

Pressure fittings are usually created from stainless steel and are designed to be leak-free within the operating parameters of the transducer. Typically, the electrical termination on the transducer is a multi-pin connector or a cable.

The connector normally has six pins for compatibility with six conductor cable assemblies that go from the transducer to the instrumentation. It must be established the transducer chosen meets not only performance specifications but that it properly mates with the mechanical and electrical connections.

It is frustrating to purchase a transducer and discover that it does not fit in the standard pressure port. The same is true of your electrical connection, which has added dimension.

The connection must be mechanically compatible with your system and it must also have the same wiring code for electrical pin-to-pin compatibility. Both the pressure and electrical interface must be considered.

2. Pressure Range

Transducers are designed to supply a certain electrical output for a given pressure range. The manufacturer's product specifications always supply this relationship. Typically, transducers are available in various discreet pressure ranges from 0-5 psi to 0-100,000 psi rated pressure.

The majority of suppliers can supply some or all of the ranges in low or high level electrical outputs. When selecting a transducer, a pressure range should be chosen such that operating pressure is around 80% of full-scale.

An electrical output level that is compatible with your system requirements should also be chosen. A high level unit should be considered if the environment is intolerant of low-level signals and if transducer cable runs exceed 100 feet.

3. Media Compatibility

Transducers must have some built-in protection against hostile fluids, as they are called upon to perform in a variety of pressure media. This is done by making the pressure-sensing end out of hardened stainless steel. In most instances, the material is 17-4 PH or 15-5 PH.

The outer shell or cover is usually made from 303 or 304 stainless steel in order to enhance protection further. Corrosion of the transducer's diaphragm can alter the output sensitivity and influence its structural strength.

In the majority of system applications, the media is never a problem and should not be of concern but consult the manufacturer if there is any doubt as to which material is best for a certain situation.

4. Installation and Handling

Built to withstand the rigors of the industrial world, a pressure transducer is a rugged device. It requires very little attention once installed and operating and carries out its task extremely well. It is a precision measuring device and does need a certain amount of care to keep its integrity.

Take care not to damage the electrical connector during installation, as it is probably the most vulnerable part of the transducer. Bending a pin or denting the connector shell could put the unit out of commission. A number of high level transducers have zero and span adjustment access holes located on the transducer's connector plate.

Make sure that these units are installed so any adjustments can be reached with a screwdriver. The pressure fitting on the transducer is designed to secure the unit in place properly. Make sure to tighten the transducer well to avoid the chance of leaks. Correct tightening will not harm the transducer's performance and will assure leak-proof performance.

5. System

Attention has to be paid to the whole system to ensure accuracy and reliability. The transducer is one component in a system that usually includes signal conditioning, cabling, amplification and readout devices. The input wiring to the transducer is the largest source of system error.

This error is a result of the noise produced by electrostatic coupling and inductive pickup (EMI/RFI). The former produces noise because of the coupling of the electric fields surrounding signal wires.

This is due to the capacitance between the conductors and ground and between individual conductors. Capacitive coupling is a bigger problem in longer cables as capacitance between conductors heightened with cable length.

Utilize shielded cable and ground only on one end of the shield in order to eliminate these unwanted signals. It will become a signal conductor, capacitively coupled to the measured signal if both ends are grounded. Power lines, motors, transformers and similar sources produce electro-magnetic fields, which can be picked up inductively.

These strong magnetic fields are a source of noise, which can induce signal errors. Utilizing twisted pairs will cancel the noise from inductive pickup effectively. In instances where a low signal level cable is near to high voltage cables, the low level cable may be run in a metal conduit.

Probably the least understood problems are those caused by grounds and they may well be the most troublesome of all noise sources. A signal circuit should be grounded in one place only to help decrease grounding problems.

The difference in potential between numerous ground points can lead to circulating currents and generate noise because of this. If all else fails, a high level of system isolation will keep ground effects to a minimum.

Finally, all electrical interconnects between system components should be mated properly, cleaned and have the highest integrity. More problems are solved in the field by merely tightening a loose transducer connector.

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.

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