Introduction to pressure measurement

Pressure transmitter working principles: a brief overview

 4 October 2017

The purpose of a pressure measurement

The use of pressure measurements in the industry is becoming more and more widely spread. Pressure is the second most measured process variable after temperature. Millions of pressure measurements are produced every year to satisfy the needs of the customer.

But what is the purpose of all these measurements?

Well, pressure measurements are used primarily for three reasons:

Composition of a pressure transmitter

A pressure transmitter consists mainly of the pressure sensor, the secondary electronics, and the enclosure. Each of these components has a specific function.

The pressure sensor converts the measured pressure into a measurable electrical signal.

The secondary electronics is the electronic part of the transmitter that is reading the sensor signal, conditions that signal (linearization, compensation of the temperature effect, amplification …), and finally transforms it into an industry-standard output signal, for example, 4 – 20 mA.

The enclosure eventually holds all the parts together, includes the components for connecting the signal cable, provides the necessary protection against water and dust, and occasionally provides protection against explosion (Ex d version) when used in hazardous areas.

Composition of a pressure transmitter
Composition of a pressure transmitter

What is a transducer?

A transducer is a device that converts a signal from one physical form to a corresponding signal having a different physical form. There are six different types of signals – mechanical, thermal, magnetic, electrical, chemical, and radiation. A transducer is converting one of these six types of signals to one of the other five types of signals.

If we take the example of a pressure transducer, it will convert the mechanical pressure signal, such as the deflection of a diaphragm, to an electrical voltage signal.

What is the difference between a transducer and a transmitter?

The biggest difference between these 2 is that a transmitter has secondary electronics for conditioning and amplifying the signal. A transducer is, in fact, a part of a transmitter. If a pressure sensor has an output signal in the magnitude of millivolts, it is called a transducer. If this output signal is amplified and converted into a milliamp signal, then it is called a transmitter. The transmitter converts the transducer’s millivolt signal into an industry-standard signal such as a 4-20mA or a 0-20mA.

There is a lot of confusion about the use of these terms in the industry. A transmitter is often referred to as a transducer and vice versa, or a sensor is considered a transducer.

How to measure pressure?

If you look at industrial pressure sensors, you will see that almost every sensor is using a diaphragm to measure the pressure. The impact of the pressure is deflecting the diaphragm and in one way or another, this deflection leads to a change in resistance, capacitance, reluctance, or generated voltage. Below is a brief description of the various working principles of a pressure transmitter.

Recent developments

Since the beginning of the new millennium, there have been many new developments for pressure transmitters in industrial applications. The progress of electronics and particularly its miniaturization has made that a lot of problems were solved.

New technologies have resulted in lower power consumption, faster response, and reduced dimensions and weight. This could seem of less importance at first sight but is of great importance for certain industrial applications. Due to the low-power consumption of pressure sensors and transducers, pressure measurements can be performed in shorter intervals of about 0,5s. This will then also lead to a faster response time in dynamic pressure applications.

Smaller dimensions and weight can be important for applications in narrow spaces, like under the hood of your car for measuring the content of the fuel tank in natural gas vehicles.

In addition to these improvements, progress has also been made in terms of accuracy, repeatability, and reliability.

Today’s most common pressure measurement principles include strain gauges, piezoresistive, piezoelectric, and variable capacitance.

Each of these techniques and a few others will be explained in the next articles.

References

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