What are the different pressure sensing elements?

Sensing pressure usually begins with converting the force exerted by the pressure media – gas or liquid - into a physical displacement. This can be used to move a pointer relative to a calibrated scale, or to cause an electrically measurable response such as resistance or capacitance change proportional to the pressure.

The pressure sensing diaphragm, capsule, Bourdon tube, and expanding bellows are proven mechanisms for converting pressure to displacement.

Pressure sensing diaphragms

How they work

The pressure-sensing diaphragm is a circular plate, fixed around the edge, and exposed to the pressure media on one side (see diagram below). On the opposite side may be a sealed chamber, in the case of an absolute pressure sensor, or it may be vented in the case of a gauge or differential sensor.

Cross-section of a pressure sensing diaphragm deflection under applied pressure

When pressure is applied, through the media, the diaphragm deflects to an extent proportional to the magnitude of the pressure. This deflection can be used to create a change in capacitance or resistance.

In a capacitive sensor, the diaphragm represents one electrode of a capacitor that has a fixed plate as the second electrode. Pressure-related deflection of the diaphragm reduces the separation of the electrodes, causing a capacitance change proportional to the applied pressure.

Alternatively, a network of resistive elements is attached to the surface of the diaphragm. These may be foil strain gauges bonded to the surface, or metal resistors deposited using a thin-film sputtering or thick-film process depending on the diaphragm material. Deflection of the diaphragm, under pressure, causes these elements to stretch and changes their resistance.

The resistors are placed in locations subject to both compressive and tensile force (see diagram to the right) to maximise the resistance change and so enhance resolution. A Wheatstone bridge connection eliminates drifts and offsets from the measurements.

The diaphragm may be metal or ceramic. A metal diaphragm is often made from stainless steel or titanium, which allows compatibility with a variety of pressure media. These types of diaphragms can withstand a wide range of applied pressures, and high proof-pressure and burst-pressure ratings.

Ceramic diaphragms offer broad compatibility with various types of pressure media, and good corrosion immunity at a relatively low cost. On the other hand, the measurement range, proof-pressure and burst-pressure ratings are usually lower.

Another type of so-called slack-diaphragm sensor can be used for measuring very small pressures. The diaphragm material is typically a synthetic non-elastic material, such as polythene, or a natural material like silk. The non-elastic nature of the material requires external springs to oppose the diaphragm, to enable calibration and ensure precise operation.

Diaphragm-type sensors are used throughout various industries. Care must be taken in applications such as food preparation or pharmaceuticals manufacturing, to allow proper cleaning of equipment and prevent contamination by germs or bacteria.

Oil-filled sensors may be used, which feature an oil-filled cavity between the sensor diaphragm and an outer diaphragm that is installed flush with the wall of the vessel containing the pressure media to permit thorough cleaning.

This diaphragm must be extremely compliant, to fully transfer the applied pressure to the internal sensing diaphragm. The temperature characteristics of the oil may affect the sensing accuracy, and the potential for leakage risks contaminating the pressure media. Alternatively, the sensor may be designed with a fully flush sensing diaphragm, which is designed to come into direct contact with the pressure media.

Piezoresistive and MEMS pressure sensors typically feature a silicon diaphragm and resistors fabricated as part of the same structure. The diaphragm for a standard piezoresistive sensor is machined from silicon. For a MEMS sensor, the diaphragm/resistor structure is produced by selective doping and etching as part of the standard MEMS fabrication process.

Advantages and disadvantages

Pressure-sensing diaphragms have a simple construction and are easy to miniaturise. Precision resistors require only small deflection, minimising diaphragm fatigue. Media-isolated sensors maintain high accuracy. And diaphragm-based sensors can measure lower pressures than a Bourdon tube.

The choice of materials for the construction of the diaphragm enables broad media compatibility. Metal diaphragms can measure high pressures. And piezoelectric sensors allow a wide range of measurement.

One potential downside is that conventional, i.e non-MEMS, diaphragms have limited low-pressure measurement capability.

Pressure sensing capsules

How they work

The pressure-sensing capsule adapts the diaphragm sensing principle to allow measurement of low pressures that would otherwise require an impractically large and thin diaphragm.

The capsule comprises two diaphragms, welded at the edge, to allow the pressure media to act on both simultaneously. The resulting structure displays twice the displacement, relative to the pressure applied, compared to a single-diaphragm.

Pressure sensing can be done using a single capsule, as shown in the first diagram below, or using a stack of capsules as shown in the second diagram. Some capsules also feature profiling (such as the corrugations shown below, right) to optimise linearity and mechanical strength.

Advantages and disadvantages

Stability, simplicity and its small-size are the main advantages of the pressure-sensing capsule - as well as its ability to measure lower pressures, compared with a diaphragm sensor of a similar size.

However, the capsule does not self-drain so it is not suitable for measuring pressure in liquid media.

Bourdon tubes

How they work

A Bourdon tube can be either c-shaped or helical, with an oval cross section. When the pressure media enters the tube, the pressure acts to change the oval towards a circular cross section. The effect of this distortion causes the tube to move – opening the c-shape, or extending the helix. The closed end of the tube is attached to a movement, so that displacement causes an indicator needle to deflect. The deflection can be measured on a scale, calibrated to represent the pressure exerted by the media.

The diagrams below illustrate the operating principle of the c-shaped and helical Bourdon tube, respectively. Alternatively, the movement mechanism can be attached to a potentiometer to provide an electrical representation of the pressure.

Depending on application requirements, such as corrosion resistance, cost, size, measurement range, proof pressure, and burst pressure, the tube may be made from a metal such as copper, brass, aluminium, or a nickel alloy such as monel.

A cross-section of a C-shaped Bourdon tube

A helical Bourdon tube

Advantages and disadvantages

The operating principle of the Bourdon tube is well understood, and tube-production techniques are mature.

However, the minimum measurable pressure is about 600mbar. In addition, miniaturisation can be difficult, and liquid pressure media cannot drain fully from the tube. Drainage may not be a problem if the media is inert. However, other types of media may decompose or solidify, impairing function or accuracy, and possibly contaminating fresh media.

The Bellows operating principle

Bellows sensing elements

How they work

The bellows sensing element is a container that expands in response to the force applied by the pressure medium within. The bellows is typically made from a metal such as phosphor bronze, brass, beryllium copper, or stainless steel. It can be machined from solid stock, rolled from tube, or fabricated with a series of welded annular rings.

An internally mounted - or external - spring enhances the bellows’ response to positive- and negative-going pressure changes. As a result, the deflection characteristics are a combination of the mechanical properties of the bellows, and those of the spring.

An attached mechanical movement converts the expansion and contraction of the bellows due to changing media pressure into a proportional deflection of the pointer to indicate the pressure on a calibrated scale (see diagram to the right). In this sense the bellows is quite similar to the Bourdon tube. Alternatively, the movement may be attached to a potentiometer to provide an electrical analogue of the applied pressure.

Advantages and disadvantages

Advantages of the bellows sensor include simplicity, low cost, and the ability to connect directly to a pointer. The movement and pointer can be designed to give a large change in indication relative to the change in unit pressure, resulting in high resolution.

The bellows must operate within the elastic limit defined by the material and construction. And the mechanism can fatigue over time. As with capsules, drainage can be a challenge that may complicate use with liquid media. However, the bellows can be filled with an inert liquid, such as oil, and the open end sealed with a diaphragm to create an element suitable for monitoring liquid pressure.

Modern pressure sensors for automotive, medical, industrial, consumer and building applications will typically use diaphragm or capsule elements due to factors including space constraints, measurable pressure requirements and durability. Understanding how they work should allow you to make an informed decision on the type of element your sensor will require.

In chapter 2 we'll take a look at how pressure sensors are being used in some of today's most innovative applications. If you want to learn more about how pressure sensing elements are used in the different types of sensor technology, check out the later chapters of the guide below. Alternatively, if you're pressed for time, the full guide is available as a downloadable PDF here.