Since every fluid power system (either gas-based or hydraulic-based) depends on the accurate monitoring and control of pressure, pressure transducers play a key role in every aspect of the industrial and commercial world.
Pressure transducers tend to be poorly defined in comparison to other types of industrial products. For the sake of increased clarity, it is helpful to contrast pressure transducers with extremely similar – but not identical – devices.
The term pressure sensor is often used interchangeably with pressure transducer. However, differences exist between these two terms that should be noted. A sensor is technically a device that detects and reacts to the presence of physical phenomena or changes. A transducer, on the other hand, not only senses physical phenomena but also translates (or transduces) the information it senses into a different, more easily interpretable format. Although sensors may also translate the physical information they sense, they do not necessarily have to do so. Pressure gauges are arguably the best example of a device that functions as a sensor without necessarily doubling as a transducer. Analog pressure gauges, in particular, sense and react to mechanical force without necessarily reconfiguring their input in non-mechanical formats. Transducers, by definition, must always perform translation in addition to sensing. Practically speaking, equating these two terms usually works since the vast majority of pressure sensors are also pressure transducers. It is still helpful to keep the technical distinction between them in mind.
A distinction should also be drawn here between pressure transducers and load cells. The boundaries between pressure transducers and load cells are largely blurred. Load cells are usually defined as pressure transducers which translate mechanical force into meaningful electrical output. Since the majority of pressure transducers emit electrical output, it follows that a majority of pressure transducers could also be considered load cells. The key point to remember is that not every single type of pressure transducer is necessarily a load cell. For example, there are some pressure transducers which produce optical (rather than electrical) output. Furthermore, pressure transducers often specifically measure pressure, which is defined as force acting over an applied area or surface (measured in Pascals, bars, psi, etc.). Load cells often simply and directly measure force (measured in Newtons).
Today, high accuracy pressure transducers are made using a variety of materials. Main parts of the transducers are made from stainless steel, copper, ceramic, titanium, carbon, and many others. Wetted materials (parts of the transducer which come into contact with pressurized media/fluid) may be made from a slightly different range of materials, which not only includes previously mentioned metals (e.g. stainless steel) but also bronze, plastics, glass, and even silicon. Pressure transducers can vary widely in terms of size as well as in material construction. The average pressure sensor measures about one cubic inch. However, they may be fabricated to be less than 1/100th of a cubic inch.
How They Work
Even though many types of pressure transducers exist, they all operate according to the same basic principles. In particular, pressure transducers translate mechanical pressure into meaningful electrical signals by the deflection of a sensing, measuring element within them. This sensing element may assume the form of a Bourdon tube, bellows, or a diaphragm. (These pressure sensing elements are explained in further detail in the article on Pressure Gauges.) The deflection and corresponding strains in these sensing elements produce proportional electric changes – e.g. changes in electrical resistivity – that can then be meaningfully interpreted and correlated in order to deduce the existence of and changes in mechanical pressure.
Strain gauge pressure transducers are by far the most common type of pressure transducer. Strain gauges are sensing elements (made of materials such as silicon, polysilicon film, metal foil, etc.) which change in electric resistivity as mechanical pressure deforms it. Typically, the gauge deforms in the secondary step of a process as pressure initially deforms a primary sensing element (e.g. a diaphragm) that the gage is connected to. Furthermore, strain gauges are typically wired in a well-known configuration known as a Wheatstone bridge circuit that amplifies and maximizes the transducer’s output. Pressure transducers which utilize this type of technology are typically can-shaped and ideal for high pressure applications.
Piezoelectric pressure transducers function very similarly to strain gauge devices. Piezoelectric products measure pressure by directly measuring electric charges that accumulate and develop across sensing elements in proportion to applied force. Some piezoelectric transducers integrate strain gauges into their configuration to form a hybrid type of transducer.
Other pressure transducers infer pressure changes from decreases in electric capacitance (i.e. the ability to store electric charge) when a diaphragm is deformed. Accordingly, such transducers are known as capacitive pressure transducers. Generally speaking, these are used for low pressure applications (e.g. around 40 bar).
Pressure transducers that do not depend on sensor deflection to measure and convert pressure do exist. Resonant pressure transducers derive their name from correlating pressure changes to changes in a sensing mechanism’s resonance frequency. Sometimes, the resonating element is directly exposed to media being measured, making resonance frequency dependent on media density. Another example of a non-deflection pressure transducer is a thermal pressure transducer (such as a Pirani gauge), which calculates pressure change based on changes in the thermal conductivity of a gas.
Pressure sensors may be categorized in a number of ways, including by the kind of pressure they measure, the range of pressure they measure, and their range of operating temperatures. A survey of some different approaches to categorization follow below.
Categorizing by Types of Output
Pressure transducers typically require excitation, or input from an electric power source, in order to function (unlike analog pressure gauges). As mentioned previously, they are mostly defined by their ability to generate electric output signals based on pressure detection. Such electric output can assume the form of voltage (e.g. mV or V), current (e.g. mA), or frequency.
Transducers that generate millivolts often do not exceed 30 mV and are very sensitive to changes in excitation. On the other hand, transducers that generate volts are able to produce higher electric outputs due to the presence of integral signal conditioning. Unlike millivolt transducers, their output is not directly proportional to changes in excitation. Transducers that generate current are also known as pressure transmitters. Although pressure transducers in general are sometimes identified as pressure transmitters, such nomenclature is incorrect. Technically, the term only applies to transducers which generate current. More specifically, these types of transducers are known as 4-20 mA output pressure transducers due to their normal output range.
As mentioned before, a few pressure transducers do not generate electric output signals at all. The most common alternative to electric signal output is optical signal output. Many pressure transducers based on optical output depend on physical changes in an optical fiber to sense mechanical strain and corresponding changes in pressure. Similarly, other pressure transducers use layered elastic film to deduce pressure changes through changes in reflected optical wavelengths.
Categorizing by Different Measuring Standards
One approach to pressure transducer categorization revolves around the standard they use to operate. From this perspective, all pressure sensors can be divided into five main categories: sealed pressure sensors, gauge pressure sensors, vacuum pressure sensors, absolute pressure sensors, and differential pressure sensors.
Gauge pressure sensors measure pressure relative to atmospheric (or barometric) pressure. Atmospheric pressure is generated from the weight of the air in Earth’s atmosphere.
Sealed pressure sensors measure pressure against a predetermined fixed pressure (which may or may not correlate to surrounding atmospheric pressure).
Vacuum pressure sensors are used to measure pressures that are lower than atmospheric pressure. They are ideal for showing the difference between said low pressure and atmospheric pressure.
Absolute pressure sensors, which calculate pressure relative to a perfect vacuum (a state in which no particles exist). Unlike gauge pressure sensors, absolute pressure sensors include atmospheric pressure in their total pressure calculations.
Differential pressure sensors measure the difference between two or more pressures found at various spots on the sensor. They are used to evaluate the rate of flow and pressure drops inside pressurized or enclosed vessels.
Categorizing by Measured Phenomena
Sometimes, pressure transducers are simply categorized by the specific phenomena they are used to measure (e.g. a blood pressure transducer). One major grouping of pressure transducers consists of air pressure sensors. Air pressure sensors are commonly used with pneumatic tools or air compressors to determine the pressure of airflow and provide a digestible reading of this measurement to overseers. One important type of air pressure sensor is an atmospheric (or barometric) pressure sensors, which is used to measure and provide readings of atmospheric pressure for meteorological applications. Overall, air pressure sensors are typically absolute pressure sensors or differential pressure sensors.
Often, pressure transducers are simply categorized by the many types of applications they are used for. Examples of such pressure transducers include pc board mountable transducers, heavy duty/industrial pressure transducers, and high temperature pressure transducers. Overall, pressure transducers can be adapted to or customized for a host of varying applications, including altitude sensing (useful for aerospace and navigation applications), flow sensing, and leak testing and pressure sensing (essential to weather instrumentation, automobile functioning, aircraft operation, and chemical processing).
Categorizing by Size
A type of pressure transducer that is primarily defined by its size is the miniature pressure transducer. These very small sensors are designed for application in critical proceedings, like biological and medical procedures where instruments must be as non-intrusive as possible. Most miniature sensors exhibit a margin of error of less than one percent. They are kept this accurate via proper calibration and backup systems.
To further hone results, operators frequently use additional devices and mechanisms in combination with pressure transducers, such as pressure regulators, pressure calibrators, level transmitters, torque transducers, temperature transducers, and integrated circuits.
Pressure regulators offer greater observation and control over the amount of pressure running through a system and are generally programmable; if pressure exceeds the threshold of safety, indicated by a programmed point, a regulator will alert operators. Pressure regulators are particularly useful in conjunction with millivolt transducers.
Pressure calibrators calculate and report back the pressure, flow, and level of certain system instruments, with the goal of maintaining safe and efficient operations. Connected to an established system, they receive input from it, that they can then compare to input from the system’s gauge. This comparison helps operators quickly determine whether or not their gauge is working properly.
Level transmitters measure the levels of variables like solids, slurries, or liquids in a given space. Generally, level transmitters will sound an alarm or trigger a shut-off switch if the buildup is too great for safe operations.
Both torque transducers and temperature transducers broaden the usefulness of pressure transducers by measuring rotational movement and heat content (respectively). Torque transducers (also called torque sensors or torque transmitters) get their readings by measuring both the static and dynamic twist in a rotating system. Temperature transducers can either draw conclusions remotely or directly (by basing their readings on thermal radiation in the former scenario or by literally going into a substance).
The integrated circuit (IC) is also known as a silicon chip or microchip. It is, in essence, a miniature electronic circuit. Integrated chips are increasingly being used by pressure transducers to communicate with other equipment and maintain accuracy levels.
When selecting a pressure transducer, it is prudent to determine priorities in your specific application and choose a device accordingly. For example, millivolt transducers are popular because they are very economical. On the flipside, they are very vulnerable to electrical noise or interference. If you will be using a pressure transducer in an industrial environment, selecting a transducer with a higher voltage output may be the wiser choice. If a pressure transducer needs to operate over a long distance, a transducer that generates volts in any form may not be an ideal choice at all. Rather, choosing a pressure transmitter (which is most resistant to electrical noise and can have lead wires of over a thousand feet) may be the best way forward.
One important aspect of pressure transducer selection is factoring in physical threats to the transducer. Like most industrial products, pressure transducers are sophisticated but vulnerable pieces of equipment that should be handled carefully in order to maximize their usage. While pressure transducers are designed to operate in a range of different conditions, using them under conditions they are specifically not designed for is an easy way to render them inoperable.
Over-pressurization is a particularly dangerous threat to pressure transducers. (One of the most common indicators that a pressure transducer has been over-pressurized is an upward shift from a zero reading.) One of the best ways to combat over-pressurization is adopting a preemptive strategy during the initial selection process. Many pressure transducer manufacturers or suppliers recommend choosing pressure transducers with maximum pressure ranges that are significantly higher than the expected operating ranges, in order to obtain a safety margin against unexpected pressure spikes and over-pressurization. Generally speaking, expected operating ranges should constitute 50-60% of a pressure transducer’s maximum operating pressure. (Applying a 5000 psi pressure transducer to a hydraulic system with expected operating pressures of 2500-3000 is an example of this strategy.) Other safety measures can also be implemented to extend the life and usefulness of pressure transducers from over-pressurization. One common strategy is the use of snubbers, orifices which are installed in piping to shield a transducer from pressure spikes. Extreme temperature variation is another major threat to pressure transducers’ well-being. Even though transducers are designed to operate independently of non-pressure variables, extreme temperature still tends to affect them since transducers often consist of several different materials. Once again, adopting a preemptive strategy during transducer selection helps to mitigate this threat. If you know you will be using a transducer in a high temperature scenario, only high temperature pressure transducers should be used.
Considerations for Pressure Transducer Supplier Selection
When selecting a pressure transducer supplier, some standard considerations apply. Look for a supplier that has a wide range of experience, a wide range of products, etc. More so than other industrial scenarios, working with a transducer supplier who has extensive experience in your particular application or sub-field may benefit you by eliminating the need for extensive product testing on your part. When searching for a supplier, you should also make sure to educate yourself on industry specific terms in order to know the right questions to ask. As with the term pressure transducer itself, much jargon and many publishing standards in the transducer field are not strictly defined. To give a single example, publishing standards for resolution (the minimal pressure change that a given transducer can detect) tend to be very inconsistent among manufacturers; some will publish maximum resolution values while others publish average resolution values. Asking targeted questions about topics such as this will reveal a supplier’s level of expertise as well as their commitment to customer service, enabling you to make a well-informed decision.