What is a Pressure Transducer and a Pressure Transmitter?

Every engineer has been there. You start a new job and you need to figure out how the equipment works and what it’s used to measure.

Maybe you need to measure the pressure of some system but don’t know exactly where to get started.

With a few of our tips and resources, you can learn all about pressure measurements.
Here we are going to go over the difference between pressure transducers and pressure transmitters.

What is Pressure?

Pressure is a measure of how much force an object is pushing perpendicularly on a given area. Although pressure can be exerted by solids, it is most often associated with liquids and gases.

There are many different units that measure pressure, but the international standard unit is the Pascal (Pa). Other common units include the pound per square inch (psi), the atmosphere (atm), the millibar (mb), and the millimeter of mercury (mm Hg).

The pressure of an ideal gas is given by p=nRT/V, where n is the amount gas measured in moles, R is a constant, T is the temperature of the gas measured in Kelvin, and V is the volume of the gas.

Although no real gas behaves exactly like an ideal gas, many can be approximated by this equation. Even the ones that can’t usually have a similar equation that relates pressure to the other physical quantities.

The ideal gas law, however, doesn’t apply to liquids. The equation for the pressure in a liquid is p=ρgh. Here, ρ is the density of the liquid, g is the acceleration due to gravity, and h is the height or depth of the liquid.

For fluids in general, which includes both liquids and gases, the pressure represents the compressive stress at some point within the fluid. Bernoulli’s principle governs the pressure in all fluids.

There are two types of environments regarding fluid pressure: open and closed. An open system is something like the ocean where the fluid is free to move wherever it wants. A closed system is like a sealed pipe where the fluid remains in a set place.

Pressure Sensor

A pressure sensor is exactly what it sounds like, any tool that is used to measure pressure. There are many types of pressure sensors depending on exactly what kind of pressure you want to measure and what environment you’re measuring in.

An absolute pressure sensor is one that measures absolute pressure, that is, a pressure value relative to the pressure in a vacuum. It has several applications including use in vacuum pumps, measuring pressure in liquids, and packaging.

A gauge pressure sensor measures the pressure relative to the ambient temperature in the environment. This is useful for measuring the pressure of things like vehicle tires.

A vacuum pressure sensor is sometimes used interchangeably with an absolute pressure sensor but can also mean a sensor that measures pressures lower than atmospheric pressure.

A differential pressure sensor is one that can measure the difference between two pressure values. For example, when you are trying to measure the flow rate of a fluid through some kind of obstruction, you can measure the pressure before and after.

You can also use them to find out how pressure changes in filters or to measure the level of a fluid in a container. Most pressure sensors are differential sensors, but they have some set value of pressure as one input.

A sealed pressure sensor is a sensor that measures the pressure relative to a given reference pressure.

Pressure Transducer and Pressure Transmitter

A pressure transducer is similar to a pressure sensor. The terms are often used interchangeably, but there is a difference. A transducer is a pressure sensor that measures the pressure and converts it to an electrical signal, usually as a voltage.

There are many different operating principles that transducers use to measure pressure which depends on the type of pressure and the environment the pressure will be measured in.

Force Measuring

Force measuring transducers use parts that measure force to deduce or calculate the pressure of a fluid.

A piezoresistive strain gauge uses a special material that changes resistance when it is deformed. When the pressure being measured increases, the material deforms more, and the resistance increases. This change is then converted to a voltage.

A capacitive transducer uses a capacitor that can change capacitance when it is deformed by a change in pressure. This change in capacitance is then converted to a voltage.

An electromagnetic transducer is similar to a capacitive one except that is changes inductance. Some also use different principles of electromagnetism such as the Hall effect.

A piezoelectric transducer is similar to a piezoresistive one except that the material builds up an electric charge in response to changes in pressure which causes a current to flow rather than a change in resistance. This current is then converted to a voltage.

An optical transducer uses optical fiber and light to measure pressure. When the fiber is deformed due to a change in pressure, the light shifts slightly which can be measured and converted to a voltage.

A force balancing transducer measures force using a beam of light and then corrects the force by applying a current. The current applied depends on how much force was generated which depends on how much the pressure changed.

Other Types

A resonant transducer uses changes in the resonant frequency of a material-to-measure pressure.

A thermal transducer uses changes in the thermal conductivity of a material due to changes in density caused by a change in the pressure applied.

An ionization transducer measures the flow of charged ions which also depends on the density of a material.

Pressure Transmitters

A pressure transmitter is similar to a pressure transducer, but it converts the voltage output to current or some other useful quantity. It can also transmit the results of the measurement to another location.

In addition to a direct measurement of pressure, pressure transmitters find many other uses. They can measure the altitudes of aircraft, the flow rate of fluids, or the depth of liquids.

Pressure transmitters also have a use in fabrication services and even in detecting leaks in systems that use fluids.

Next Steps

Now that you know the difference between pressure transducers and pressure transmitters, feel free to contact us for more information.

 

 

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