What is a pressure transducer? A pressure transducer is a sensor used to measure pressure. Depending on where you put it and how you connect it, a pressure transducer can tell you the pressure in a pipe or tank, the weight of an object, or even the depth of fluid above it. Most industrial pressure transducers are made of two technical parts, a transducer and a transmitter, inside a third, equally important part, the housing.
What is a transducer? A transducer converts a physical action--often a vibration--into an electrical signal. Microphones and the pickups on an electric guitar, which both convert sound waves into electrical signals, are transducers. Pressure transducers, then, convert pressure--air pressure, fluid pressure, oil pressure, etc.--into electrical signals.
Pressure Transducer Types
What is the difference between a piezoelectric and a piezoresistive transducer? Piezoelectric and piezoresistive sound very similar, and for good reason: transducers made of each material type change the characteristics of an electrical circuit as they flex. The difference is that piezoelectric transducers produce a voltage as they flex, while piezoresistive transducers change the resistance of the circuit as they flex. So, in theory (which means "to a fairly decent degree in practice, too"), a flexing piezoelectric transducer could be used to power the "deflection detection" circuit (i.e., the circuit the transducer is part of). However, it also means that a piezoelectric transducer under constant pressure will not provide any voltage to the deflection detection circuit. (Really, just say that three times: deflection detection, deflection detection, deflection detection. It just sounds so cool.) Piezoresistive transducers, on the other hand, always need power supplied to them, so long-term, constant pressure will not cause problems with measurements.
It's easy to mix the two up. A quick internet search will turn up plenty of conflicting uses and descriptions, sometimes even within a single site. Throw in more general terms, like "strain gauge," or specific configurations, such as "Wheatstone bridge," and it looks like chaos reigns. But remember, that's the internet. (Also, "Hello," from the internet!) Some careful consideration helps solve most confusion. (I did say _most_.) Strain gauge really is a more general term than either piezoelectric or piezoresistive, but is usually used as shorthand only for piezoresistive transducers. For example, some people use piezoelectric for piezoelectric transducers, and strain gauge for all others, including piezoresistive ones. A Wheatstone bridge is a specific configuration of the bridge electric circuit that is usually used on pressure transducers.
How Does A Pressure Transducer Work?
We touched on part of how a pressure transducer works above: an electrical circuit (usually a bridge) is attached to the backside of a diaphragm, a small piece of flexible material. As the pressure that the diaphragm is exposed to changes, the diaphragm flexes, changing the characteristics of the circuit. That's the transducing part of a pressure transducer. However, in order for the measurement to be of much use, it has to be transmitted somewhere: to a display, or a control system, etc. So a transmitter converts the measurement into a standardized signal--4-20 mA, Modbus, mV/V, etc.--and communicates it to whatever control or display system it is connected to.
There are some instances when it is advantageous to denote a pressure transducer as separate from a pressure transmitter, i.e., differentiating between the physical measurement and the electronic signal. In reality, neither can be used without the other. A pressure transducer without a pressure transmitter generates a signal far too weak to be communicated to a controller, and a transmitter without a transducer has no message to communicate.
An integral part of how a pressure transducer works is how it is built to measure pressure. There are five main types of pressure measurement:
- Gauge Pressure: Reads “0” when measured pressure is equal to local atmospheric pressure (i.e., same pressure inside and outside the tank/vessel/pipe, etc.). Registers pressures higher than atmospheric pressure. The body of the transducer is vented to accommodate changing atmospheric pressure.
- Vacuum Pressure: Also reads “0” when measured pressure is equal to atmospheric pressure, but only registers when measured pressure is less than atmospheric (negative pressure). Vented.
- Compound Pressure (or Compound Gauge Pressure): Combines gauge and vacuum in a single sensor: Reads "0" when monitored pressure is equal to atmospheric pressure, registers both lower and higher pressures than atmospheric, and is vented.
- Sealed Pressure (or Sealed Gauge Pressure): Transducer body is sealed to protect the electronics from moisture, dust, or other environmental hazards. Because the body is sealed, the pressure measurement will reflect the changes in atmospheric pressure (i.e., a storm blowing in will cause readings to increase because atmospheric pressure is decreasing), so sealed pressure is generally reserved for high pressure measurements that are impervious to atmospheric pressure changes.
- Absolute Pressure: Transducer body is sealed under vacuum conditions (absolute 0 pressure). Used for relatively low pressure ranges so that atmospheric changes can be observed.
Some pressure transducers are used for single-point-pressure detection, i.e. as pressure switches. But most are used for continuous pressure readings, with the analog or digital output signal always indicating the current pressure reading. In fact, it's often easy to configure a control system for individual, pressure-based outputs, thus allowing a continuous output pressure transducer to generate single-point actions.
How to Choose a Pressure Transducer
What a pressure transducer is needed to measure, and where, and the required output will do most of the determining when choosing between pressure transducers. Is a local readout for the pressure reading all that's necessary? Then a pressure gauge--a transducer and display combined into a single unit--will do the job nicely. On the other hand, line pressure on fracking rigs can only be measured safely by pressure transducers with hammer union process connections. So, as we say quite frequently around here, the application will largely determine what kind of pressure transducer one chooses. Some of the questions that need to be answered to ensure a good technology fit are:
- What pressure range will the transducer measure under "normal" circumstances?
- Is this range best served by a gauge, vacuum, compound, sealed, or absolute pressure measurement?
- What pressures might the transducer face under "abnormal" circumstances?
- How will the transducer be connecting to the pipe, tank, line, vessel in which it is measuring pressure?
- Will the transducer be measuring a substance that is corrosive or chemically aggressive?
- Will the transducer body be exposed to the elements, somewhat sheltered, or submersed in liquid?
- Will the transducer be exposed to strong vibrations, shocks, or otherwise rough treatment?
- What kind of electrical output is needed or desired from the transducer?
- How far away is the nearest/intended power supply?
- How far does the output signal need to travel (for control, display, or both)?
Pressure Transducer Applications
There are so many ways and places a pressure transducer can be used. Any kind of commercial or industrial system that uses pumps needs to be able to monitor line pressure before and after those pumps; pressure transducers can do that. Pressure transducers can be used to monitor the weight applied by a hydraulic ram, or even a fighter jet! The output of a pressure transducer at the bottom of a liquid-containing-vessel can be used to calculate the level of the liquid above it. Thus, submersible pressure transducers can measure the level of water in a deep well, sewage at a lift station, or even refined diesel fuel in a storage tank. Need to make sure that a pressurized system has the appropriate pressure in multiple places? Pressure transducers are the way to go. From fracking rigs to industrial process systems to water system pressure relief valves, pressure transducers provide important pressure readings to keep people and equipment safe.
Best Practices for Pressure Transducers
Best practices for pressure transducers are going to vary a bit, depending on what kind of pressure transducer is being used in what sort of environment. Pressure transducers built for light-duty applications (i.e., indoors, low vibration, low moisture and dust) are going to need different care than hammer union transducers in an oil field. That said, installing any threaded pressure transducer is fairly easy if you follow the right procedure. And while this video is specifically for cleaning submersible pressure transducers, we hope it's obvious that touching the diaphragm on any pressure transducer is just asking for trouble. In general, the Measurement Experts here at APG suggest the following:
- Only install a pressure transducer in an environment it is designed to operate in.
- For threaded process connections, use Teflon tape as necessary, and appropriately.
- Be sure to match straight threaded sensors with straight threaded process connections, and tapered threaded sensors with tapered-thread connections.
- Do not over tighten process connections.
- Ensure desiccant cartridges for submersible pressure transducers are situated in a dry location that is easily accessed and observable.
- Check sensor networks for ground loops.
- Check units at least yearly for recalibration.
If you have further questions about pressure transducers, including how a pressure transducer can help your system operate more safely, let us know. Our Measurement Experts can help you find the right technology to fit your application like a glove. Give them a call or drop them an email today.