How to Select  Pressure Transducers

Pressure Range

• What will be the pressure range?
• What will be the units of measurement?
• Converting Measurements Into PSI
• What is the pressure reference? Local, absolute, sealed or differential?

Accuracy

• What are the components of transducer accuracy?

Application Considerations

• What output signal is needed?
• What will be the output at zero pressure?
• What degree of transducer interchangeability is required?
• What power is available?
• What type of electrical connections are there for the transducers?
• Transducer Circuit-Type: two-, three- or four-wire?
• What are the pressure media to be measured?
• How will the transducer be connected to the pressure source?
• What environment will the transducer be in?
• Special Transducer Requirements

Pressure Range

What will be the pressure range?
The pressure range of the application will greatly influence the transducer selection process. The specifier should select a transducer with a pressure range that is more than the anticipated pressure including any spikes, transients, pulsations or other atypical behavior. A pressure range much larger than the application really needs will decrease accuracy. A range lower than the application needs may result in damage to the transducer, including the possibility of bursting the transducer if severe overloads occur.

A good rule of thumb: figure out what your anticipated pressure spikes will be and then pick a transducer rated 25% higher than your highest spike. An additional margin is suggested where "high cycling" may occur. Consult with Trans-Metrics if you are unsure how to select the most accurate model with appropriate proof-pressure and burst-pressure protection.

What will be the units of measurement?
The units of measurement are determined by the user's custom. Trans Metrics offers pressure ranges as low as 2 psi and as high as 60,000 psi. This chart is provided for reference only since Trans Metrics can calibrate transducers in any unit of measurement.

For example:
A pressure of 75 bar would convert as follows:
75 bar X 14.5 = 1087.5 psi

What is the pressure reference? Local, absolute, sealed or differential?
Trans Metrics offers gage, vacuum, compound, sealed gage, absolute, and differential pressure transducers.

All transducers measure pressure relative to a second or reference pressure. The vast majority of pressure measurements are made relative to the local atmospheric pressure. In the USA, these are called gage type transducers (psig).

When a transducer measures pressures relative to the local atmospheric pressure and less than the ambient pressure, it is called a vacuum type transducer (psiv). A transducer that measures both positive and negative pressures is called a compound transducer (psig).

A sealed gage transducer (psis) measures pressure relative to the standard atmospheric pressure at sea level (14.69 psia). In moist or humid environments, sealed gage transducers are frequently used to protect the sensors internal components.

An absolute transducer (psia) measures pressure relative to a vacuum (zero pressure).

A differential transducer (psid) has an output that is the difference between the pressures applied to its two pressure ports.

Accuracy

What are the components of transducer accuracy?
Accuracy is the percentage of the error to the full-scale output (or "FSO," the algebraic difference between end points). It is usually denoted as a percentage ± FSO. In some cases (as indicated by a given transducers specifications), accuracy is the percentage of the error to the output.

While the terms "accuracy" and "static error band" are frequently interchanged, they are not the same. Many factors can contribute to a transducers accuracy. Actually, the term "error" is preferred for specifications and other specific descriptions of transducer performance. The static error band is, however, a good measure of the accuracy that can be expected at constant temperature. The static error band consists of the three following components:

Non-linearity is the deviation of a calibration curve from a specified straight line. The best straight line (BSL) is a line between two parallel lines that enclose all output vs. pressure values on the calibration curve.

Hysteresis is the maximum difference in output at any pressure within the specified range, when the value is first approached with increasing and then with decreasing pressure.

Non-repeatability is the inability of a transducer to reproduce output readings when the same pressure is applied to the transducer repeatedly, under the same conditions and in the same direction.

In addition to the three components of static error band, you may need to consider the effects of temperature.

Thermal effects typically create the largest errors in pressure measurement. Thermal errors are commonly expressed in percent per degree, although this can be deceptive. An apparently very small percentage error per degree may actually be a significant problem when checked over the entire expected range.

To combat thermal effects, transducers should be 100% tested and compensated over a specified temperature range (as are all of Trans-Metrics transducers). The compensated temperature range is usually smaller than the operating temperature range. Note that the thermal sensitivity shift is usually expressed as a percent of output.

Application Considerations

What output signal is needed?
Transducers can be categorized by their output characteristics as either low-level types or high-level types.

Low-level transducers usually have only passive devices (strain gages and resistors) in their electrical circuits. Thus they are typically more rugged and are able to operate over a wider temperature range. Signal conditioning is external. The highest accuracy over the broadest environmental conditions can be provided with low-level transducers. A typical low-level output is 3 mV/V.

High-level transducers have internal electronic circuits to condition the excitation or supply voltages and the signal. They are easy to install because most of the signal conditioning is already provided. Some typical high-level output are 0-5 VDC, 0-10 VDC, 1-5VDC, and 4-20 mA.

NOTE: Most high-level Trans-Metrics transducers have zero and FSO (span) adjustment pots; therefore, they are field-adjustable.

What will be the output at zero pressure?
Zero balance is the transducer's output signal at zero pressure. For most applications, the zero balance will be zero volts. However, some transducer circuits will intentionally have an offset output at zero, particularly if zero or negative outputs (the result of vacuums) would be outside the parameters of the monitoring computer or recording device. Typical offset voltages include outputs of 1-5 VDC or 2.5-7.5 VDC. The 4-20 mA current loop type is another common type of offset output.

Trans Metrics offers all of the above and more.

What degree of transducer interchangeability is required?
Interchangeability means that two or more transducers will provide the same indication when measuring identical pressures. The degree of interchangeability required must be determined by the user. However, the user should consider the static error band, thermal errors and tolerances on zero balance and full scale output if the units do not have field adjustable end-points.

What power is available?
Usually power requirements are not a primary consideration in transducer selection. However, there are some notable exceptions.

In remote locations where power is supplied by battery or solar power, a low-consumption transducer is needed. Circuits would typically consist of a low dropout regulator, a voltage doubler, a negative voltage converter and a signal amplifier. Trans-Metrics P100 Series with L circuit type is an example of a low-power consumption transducer series. With excitation voltages of 4-26 VDC, it can provide a regulated 0-5 VDC output. If the application's power supply is unregulated, the transducer package needs internal voltage regulation. All high-level Trans-Metrics transducers have internal voltage regulation if the power supply is unregulated.

What type of electrical connections are there for the transducers?
Transducers are supplied with either an electrical connector already mounted or some provision of the input and output conductors to exit directly from the transducer.

There are many makes and models of electrical connectors suitable for transducer use.

When the conductors exit directly from the transducer they may be called a cable output, pigtails or flying leads. Conduit connections may also be included with this approach. It is important that the strain relief and seal are adequate for the application regardless of the method.

Note that applications with high temperatures, high humidity (including submersion), or high shock and/or vibration may require special treatment. Discuss your application with your Trans-Metrics Application Engineer.

Transducer Circuit-Type: two-, three- or four-wire?
The reason for selecting one type of transducer circuit over another is usually determined by the availability of power supplies and interface modules, as well as the user's familiarity with specific types.

Transducers with high-level outputs may have two-, three- or four-wire connections.

The 4-20 mA current loop transducer is a two-wire type where the transducer, power supply and readout are all connected in series.

The three-wire circuit has a common return for both the excitation and signal. This type of circuit may have the zero balance elevated so that the signal will vary from between 1-5 VDC or 2.5-7.5 VDC (such as the Trans-Metrics P100, D circuit type). Some Trans-Metrics three-wire circuits (such as our P100 C and L circuit type) have an internal negative voltage source so that the transducer will have zero volts out at zero pressure.

The four-wire circuit has a separate return connection for the excitation and signal. The signal is differential with a common mode voltage different from the excitation return. Note that on this type of transducer, the power supply and signal lines cannot be connected to the same point.

What are the pressure media to be measured?
Some media may be corrosive. This is not usually a problem with Trans-Metrics pressure transducers because their sensing diaphragms and pressure ports are machined from vacuum-melted 15-5 PH stainless steel, with no welds or epoxy joints at the diaphragm edges. 316L stainless steel is also available as an alternate metal on most models. Other metals have also been used to provide corrosion resistance against particular fluids. The final determination of whether a material is suitable for an application is the responsibility of the end-user.

How will the transducer be connected to the pressure source?
The standard pressure port fitting for most applications is the 7/16-20 UNF female, SAE-type, with the military specification of MS16142. This is a "straight thread" fitting, using O-ring compression to form a seal against leakage. An alternative pressure port is the 1/4-18 NPT (National Pipe Thread). NPT ports form a seal via a mechanical fit-metal against metal, with TeflonĘ tape usually applied to help prevent leakage. Care must be taken not to over-tighten the metal-to-metal seal on a female pressure port or else the zero balance can be offset. VCR fittings seal using a metal gasket. VCR fittings are also known as zero clearance fittings.

Anther type of pressure port is the flush mount. Flush-mounting transducers, like Trans-Metric's P37 and P38 models, do not have any cavities, making them easier to clean. Please consult instruction manuals for mounting recommendations.

By using adapters, virtually any transducer can be connected to almost any pressure source.

What environment will the transducer be in?
Since gage pressure transducers measure pressure relative to the local atmospheric pressure, they can expose their internal components to humid or corrosive environments. Sealed gage or absolute types should be considered for humid or corrosive environments. If a gage pressure transducer is required, a no holes housing with vented cable may be a good alternative.

Additional environmental considerations include temperature, vibration and EMI/RFI. Trans-Metrics can offer recommendations for all of these problems.

Important: be sure to note any underwater applications.

Special Transducer Requirements
Certain special requirements will affect all other considerations when selecting a pressure transducer. For example, you may need a pressure transducer with unusual temperature extremes, such as -300 F or +450 F, when 0 F to 160 F is a common operating range. Yet another example is extreme pressure ranges, e.g. 0 to 25,000 psi.

If you don't know what requirements may be considered "special," just briefly state the application on the Quote Form.  For example the statement, "I have a down hole application," will prompt the transducer vendor to ask: "How big are the well holes?" or, "What size transducer diameter will fit in the well holes?"

Special characteristics do not necessarily require custom transducers. If customizing is required, please note that some manufacturers welcome custom orders while others do not. Trans-Metrics, Inc. specializes in custom transducers.

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