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:
Indication of pressure at a distance
Pressure control in control loops
Monitoring of pressure thresholds in automated processes
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
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.
Resistive pressure measurement
Resistive pressure measurements can be subdivided into potentiometric, strain gauges and piezoresistive pressure measurements, depending on whether you use a potentiometer, metal or semiconductor strain gauges. In each of these cases, the idea is that the measured pressure causes a resistance change within the sensor. This can be done by turning a potentiometer or expanding and contracting metal or semiconductor strain gauges. The resistance change is a measure of the measured pressure. Read more about potentiometric pressure sensors.
Piezoelectric pressure measurement
The piezoelectric pressure measurement uses a monocrystalline quartz crystal. When pressure is exerted on this crystal in a certain direction, a positive and a negative charge results on opposing surfaces of the crystal. This charge causes an electrical voltage across the crystal proportional to the measured pressure. This voltage generates an electrical current that can be amplified and converted to an output signal of the transducer.
The piezoelectric pressure measurement is only suitable for measuring dynamic to quasi-static pressures. Truly static pressures cannot be measured because of charge leakage causing rapid decay of the crystal’s electrical signal.
Capacitive pressure measurement
The principle of the capacitive pressure measurement is based on the change in capacitance between a metal diaphragm and a solid metal plate. When the diaphragm bends under the influence of the process pressure, the distance between the 2 plates decreases, which increases the capacitance of the capacitor. Secondary electronics measure this capacitance and convert it into a proportional pressure signal. Capacitive pressure measurements have a high sensitivity, allowing very low pressures to be measured.
Resonant wire pressure measurement
An oscillator circuit driven by a magnetic field causes a wire to oscillate at its resonant frequency. One side of the wire is firmly fixed to a static part while the other side is attached to the diaphragm. A spring causes the wire to be pretensioned. Due to the effect of the process pressure on the diaphragm, the wire will stretch at increased pressure and shrink at reduced pressure. The resonance frequency will increase or decrease accordingly. A digital counter measures the change in resonance frequency and passes its signal to a microchip that also compensates for linearization and temperature compensation.
Inductive pressure measurement
The principle of inductive pressure measurement is based on the change in impedance of a coil due to the displacement of a ferromagnetic core which is firmly connected to the pressure sensing element. A Bourdon tube as well as a diaphragm, bellows or a capsule can be used as pressure sensing element. The effect of the pressure on the pressure sensing element causes the displacement of the ferromagnetic core. The change in impedance of the coil resulting from this is proportional to the measured pressure.
Examples of this measurement principle are the LVDT (Linear Variable Differential Transformer) transducer and the variable reluctance transducer.
Optical pressure measurement
Optical sensors use glass fibers through which a light beam is sent. This beam of light is characterized by parameters such as intensity, frequency, phase and polarization. When the glass fiber is exposed to the process pressure, one or more of these parameters will change. The change of these parameters is a measure of the measured pressure. There are several types of sensors designed to measure either one or the other parameter.
Hall effect pressure measurement
The Hall effect sensor consists of a thin plate of conductive material through which a constant electrical current is passed. The connections for the output signal on the plate are perpendicular to the direction of current flow. When the Hall effect sensor is placed in a magnetic field perpendicular to the plate and to the direction of the current, an electric potential difference occurs at the output terminals. For pressure measurement applications, the magnetic field is generated by a permanent magnet which is fixedly connected to a pressure sensing element. At increasing process pressure, the magnet will move towards the Hall effect sensor. This results in a stronger magnetic field in the Hall effect sensor and its output voltage will increase proportionally to the process pressure.
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
Béla G. Lipták – Instrument Engineers’ Handbook, Vol. 1 Process Measurement and Analysis, 4th Edition-CRC Press (2003)
ABB – Pressure Measurement Theory and Application Guide (2012)
4 October 2017
