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Practical Considerations when Choosing a Pressure Transducer

When selecting a transducer performance specifications like linearity and hysteresis are important but consideration of the nature of the process, the media being measured, the ambient environment that the transducer will be located in, and the output signal is also needed.

Accuracy specs are often the most challenging criteria to understand. Accuracy is the difference between the true value and the indicated value expressed as a percent of the full scale output of the transducer. For any application repeatability is the most important specification. Repeatability is the maximum difference between output readings for repeated pressures under identical environmental conditions. When a process will vary over a wide band of pressure the errors from hysteresis (the maximum difference between output readings for the same applied pressure approached from opposite directions) and linearity (the maximum deviation of transducer output from a Best Straight Line) come into play. Often times these parameters may be combined by the root sum square of these three error results and is called static accuracy. Below is an example of how errors are calculated:

Typical 5-point Calibration. Click for larger image.

If Balance and Span Setting Errors need to be minimized it is good to select a transducer with trim pots. Installation variables, such as transducer orientation or process fitting torque, will have an effect on the balance output for lower range transducers. The trim pots allow the user to adjust the output to compensate for any of these effects.

For the given process it is important to understand what pressure range and pressure type is required. There are a variety of pressure types available, but what it comes down to is what the process parameter is in reference to. A gauge pressure transducer will be referenced to the ambient air pressure; changes in ambient pressure due to weather systems or altitude will not affect the measurement. Absolute pressure is the pressure of a perfect vacuum. Measurements taken in absolute pressure use this absolute zero as their reference point, an example is the measurement of barometric pressure.

Pressure Measurment Types. Click for larger image

Probably the single most important decision in selecting a pressure transducer is the range. One must keep in mind two conflicting considerations: the instrument's accuracy and its protection from overpressure. From an accuracy point of view, the range of a transmitter should be low (normal operating pressure at around the middle of the range), so that error, usually a percentage of full scale, is minimized. On the other hand, one must always consider the consequences of overpressure damage due to operating errors, faulty design (waterhammer), or failure to isolate the instrument during pressure-testing and start-up. Therefore, it is important to specify not only the required range, but also the amount of overpressure protection needed.

Where higher overpressures are expected and their nature is temporary (pressure spikes of short duration…seconds or less), snubbers can be installed. These filter out spikes, but cause the measurement to be less responsive. If excessive overpressure is expected to be of longer duration, one can protect the transducer by installing a pressure relief valve. However, this will result in a loss of measurement when the relief valve is open. The process fluid that is going to be measured should guide your decision as well. Often referred to as the “wetted parts” these materials should be selected for their compatibility with fluid being measured. For environments with clean dry air just about any material is permissible but for conditions using sea-water high nickel content alloys such as INCONEL® alloy 718 (UNS N07718) should be considered.

Extremes in temperature or vibration will limit what transmitters will function properly. For temperature extremes thin-film technology is superior. The extreme temperatures also create errors in the output of the transducer. The error is often expressed in percent full scale over 1°C (%FS/°C). High vibration environments favor smaller un-amplified traducers. The transducer housing should be selected to meet both the electrical area classification and the corrosion requirements of the particular installation. Corrosion protection must take into account; both splashing of corrosive liquids or exposure to corrosive gases on the outside of the housing. If the installation is in an area where explosive vapors may be present, the transducer or transmitter and its power supply must be suitable for these environments. This is usually achieved either by placing them inside purged or explosion-proof housings, or by using intrinsically safe designs. If compact size is required an unamplified transducer is best.

The pressure port and electrical connection are important as well. If a transducer must be removed for calibration annually it is best to have a removable connection as well, as either a face or cone seal fitting. Thread seal fittings like tapered pipe threads are not preferred.

The output signal is a critical parameter to understand. There are several output types to consider: ratiometric, mA, voltage, and digital outputs.

Digital outputs are gaining popularity in the market; they provide versatility that cannot be matched by analog signals. There are many communication protocols available and care must be taken when choosing a digital output so the protocol it uses will be compatible with whatever system you are using. Depending on the protocol the transmission distances for a digital signal can be more than a mile.

Voltage output is a simple output method to use; many meters or I/O ports can utilize a voltage signal. Voltage signals are commonly found as 0-5V or 0-10V. For differential measurements plus or minus output are available, care should be taken to insure that the device receiving the signal is able to read negative voltages. Voltage signals are susceptible to electrical interference. Motors, relays, and “noisy” power supplies can induce voltages onto signal lines corrupting the transducer signal. Also, voltage signals are susceptible to voltage drops caused by wire resistance, especially over long cable runs.

mA output are by far the most common found in use today. The signal is designed to vary from 0 or 4 mA to 20 mA. It is a two wire installation where the power supply lines provide voltage to the transducer and the transducer controls the current in the circuit to generate the signal. This configuration makes the signal more immune to electrical interference and allows very long cable runs exceeding 1000 ft.

Ratiometric (or millivolt) output are common with unamplified sensors. Often they are passively compensated and are simple devices. The output signal is proportional to an input (or excitation voltage). For many pressure sensors the ratiometric output is 1 or 2 mV/V. This means that when the sensor is measuring its full scale output that the signal will be 2 mV for every volt of excitation provided to the sensor. If the sensor was hooked up to a 10V power supply the output would be 20 mV. If the excitation fluctuates, the output will change also. Because of this dependence on the excitation level, regulated power supplies are suggested for use with millivolt transducers. Because the output signal is so low, the transducer should not be located in an electrically noisy environment. Because this type of output lacks a signal conditioning stage the sensor tend to be more compact, and these devices can more easily handle harsher environments then the other output types.

Unique Considerations May Require a High Line Transducer

An example, the new PX509HL sensor offers superior differential pressure sensors measure the difference in static pressure between two reference points. When selecting a differential pressure sensor one must always consider the expected line pressure for the design conditions.

We use the term High Line pressure, or HL, to describe a situation where the line pressure is many times greater than the desired differential pressure to be measured. The PX509HL sensor offers superior performance in terms of low differential pressure at high line pressures: 2000 psi Standard / 5000 psi Safe Overpressure / 10,000 psi Ruggedized Containment Pressure. It also provides high overload protection: A sensor with the sensitivity to accurately measure 5 psid (pounds per square inch differential) will be destroyed if exposed to a differential pressure equal to the Line Pressure. A high overload sensor has protection built in that isolates the sensor should an overload condition occur.

Ideal applications for the PX509HL sensor include:

  • Filter monitoring: Provides data for maintenance. As the filter clogs the differential pressure across it builds.
  • Pump pressure monitoring: Insures that pump is working properly.
  • Flow measurement: Flow through a restrictive orifice generates a differential pressure proportional to the square of the flow. Learn More

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