What is a Pressure Transducer?

A pressure transducer measures pressure and translates it into an electrical signal. Employed in diverse applications like hydraulic pumps, air compressors, and transportation systems, it finds utility wherever pressure readings are essential. Compatible with a variety of media, such as water, hydraulic oil, air, and chemicals, the pressure transducer can be used in a variety of applications. This exposition delves into the key components of pressure transducers and delves into their accuracy.

Components

Pressure transducers, come in a compact package that include the pressure port, sensing elements, and electrical components. Manufacturers offer a spectrum of designs, ranging from compact packages to robust constructions tailored for extreme conditions.

Pressure Port

The pressure port is what connects the pressure transducer to the media, some of the most common pressure port sizes for pressure transducers are ¼" NPT, 7/16-20 UNF or G1/4 threads. It is important to choose the appropriate pressure ports based on applications, for example, high pressure and certain media may not be supported by all threads. For pressure greater than 10,000 psi autoclave pressure connections are commonly used.

Sensor

The sensing element, the core of pressure transducers, receives pressure-induced energy and translates it into an analog signal via an electronic component known as a PC board. Different manufacturers leverage diverse sensing technologies. Barksdale offers three sensing elements—ceramic, piezo resistive, and "monolithic" sensors—each possessing different accuracy, construction, and thermal factors. This adaptability allows Barksdale to tailor solutions to specific customer needs.

Electrical Components

The output from the PC board is transmitted to the equipment control system via an electrical connector. There are various common electrical connectors with M12, Deutsch, DIN 43650 and Delphi Metripack being some examples. These signals are usually transmitted to a datalogger or control systems, where it provides continuous monitoring of the system pressure.

Accuracy

How precise must a pressure transducer be, and how is this accuracy quantified? The accuracy of a pressure transducer is typically evaluated through five combined metrics: Zero Offset, Span Offset, Linearity, Hysteresis, and Repeatability.

Zero Offset is calculating how much the measured zero output is from the ideal zero. For example, if a pressure transducer is supposed to read 0-100 psi and give out 4-20mA, this means that the ideal output of the transducer will be 4mA when measured at 0 psi. The zero error is calculated as percentage of the difference of measured output at zero to the ideal output over the full scale reading.

Span Offset gauges the error by analyzing the endpoints of the measurement range. In the example of a pressure transducer designed for 0-100 psi and an output range of 4-20mA, the span is 16mA (20-4mA) across 100 psi.

The span error is similarly measured as a percentage of the difference of measured span to the ideal span over the full scale output.

Linearity assessment can be calculated by multiple methods. The common ones are the end-point method, the absolute method and the best-fit straight-line (BFSL) method. In the absolute method, the accuracy is computed between the measured value and the value on ideal straight line between theoretical zero and full scale. The end point method involves calculating a straight line between the measured end-points and the accuracy is computed between the measured value vs the corresponding computed “ideal” value on the end-point line. The BFSL entails calculating the linear line that best fits the sensor's response curve across its entire range. The accuracy is computed between the measured value vs the corresponding computed “ideal” value on the best fit straight line. Barksdale uses the BFSL method for specifying accuracy.

Hysteresis is the measure of how differently the reading is measured on one the same pressure when increasing vs decreasing. When we are calculating a certain pressure within the range, we have to also calculate if the pressure transducer can hold the same readings as the pressure increases or decreases to the value.

Repeatability is simply a calculation of how similar the pressure reading is when the same pressure is provided to the transducer repeatedly.

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