Pressure Sensor, Transducer, and Transmitter ExplainedAll sensors are transducers, but not all transducers are sensors.
Over the years, the terms sensor, transducer, and transmitter have become confusing because they are often used interchangeably and indiscriminately by different vendors. The terms sensor and transducer have been around for a very long time, but in the world of process control, the term transmitter is not very old.
In particular, the term transducer is tossed around as a name for many devices and instruments. If you conduct a web search of the terms sensor and transducer, or transducer vs transmitter, you’ll get lots of different definitions. Many organizations have their own definitions.
Just to confuse the issue, sensor and transducer have often been used as synonyms.
Ok… Let’s start with some suitable definitions that we’ll use in this article.
1) What is a sensor?
A sensor is a device or element that performs the initial measurement as it senses the changes in a physical variable.
A sensor is a device that makes physical contact with the variable being measured and detects changes and provides a corresponding output signal that can be measured.
2) What is a transducer?
A transducer is a device that converts one form of energy to another. A Transducer provides an output quantity having a specified relation to an input quantity.
In the field of instrumentation, the output quantity is an electrical or electronic signal. So, we say that a transducer measures pressure, load, force, or other states, and converts the reading into an electrical or electronic signal.
For example, a strain gauge is a transducer whose resistance value changes proportionally to the strain it experiences.
Sensor vs transducer
Here’s an interesting statement… All sensors are transducers, but not all transducers are sensors.
What do we mean by that? Well, consider a loudspeaker that converts an electrical signal into motion. The loudspeaker is a transducer, but not a sensor.
3) What is a transmitter?
Alright… let’s talk about the term transmitter because it has more than one definition.
The process control term was likely borrowed from the Telecommunications world where a Transmitter is a device that produces and transmits radio waves radiating from an antenna. These radio waves are eventually detected and demodulated by a Radio Receiver. The Receiver produces sound waves for the human ear.
In the world of process control, a Transmitter is a device that transmits a standard instrumentation signal representing a physical variable being measured.
The standard electrical signals are 1-5 V or 4-20 mA which represents 0-100% of the physical variables being measured. This transmitted signal is detected by a Receiver such as an Analog Input on a PLC or a DCS.
Pressure sensors vs transducers
So,… We just said that the transmitter signal represents the physical variable. How does the transmitter know what the physical variable is?
That’s where a sensor or a transducer comes into the picture. A sensor performs the initial measurement. If necessary, a transducer converts this initial measurement into an electrical signal that is then amplified and standardized by the transmitter.
Here’s an example… We are measuring a process pressure with a range of 0-100 psi. Our Sensor/transducer performs the initial measurement and converts it to an electrical signal of 0-3 mV. The transmitter then converts that signal to a standard signal of 1-5 V, which is received by a PLC analog input module.
Many vendors use the terms pressure transmitter and pressure transducer interchangeably. But we now know that the 2 devices are not the same. So… it’s buyer beware…
If you are curious, conduct a web search of the two terms and you may be surprised at the results!
4) What is a pressure switch?
Let’s talk about a Pressure Switch. A Pressure Switch is a two-part device consisting of a sensing transducer and an electrical switch.
Pressure switch vs pressure transmitter
Is a pressure switch different than a pressure transmitter?
There are some common components, but they are functionally very different. A pressure switch has a transducer just like a pressure transmitter. That’s where the similarity ends.
As we’ve already discussed, a Pressure Transmitter produces an analog electrical voltage or a current signal representing 0-100% of physical process pressure. A pressure switch has electrical contacts that open and close at a specific pressure.
Can a Pressure Transmitter be used to function as a Pressure Switch? The answer is absolutely yes, but is it really worth it?
First of all, a pressure transmitter is larger, more expensive, and often more difficult to install. There are other deterrents as well.
A practical example
Let’s illustrate with an example… Here we have a process control system where we need to open a valve when the pressure in a pipe reaches 50 psi. We can use a pressure transmitter calibrated to produce a 4-20 mA output for a pressure range of 0-100 psi.
We need an analog input module on our PLC, and we need to scale the analog module and insert code into the PLC program so that the 50 psi value triggers an event to open the valve.
It’s much easier to use a pressure switch connected to a simple PLC digital input module. The PLC valve control logic is much simpler as well.
Bottom line, if you don’t need to know the exact pressure and only need to know when a specific pressure point has dropped below or risen above a setpoint value, then stick with a pressure switch.
Ok… let’s summarize what we’ve discussed…
– A transducer is a device that converts one form of energy to another
– A sensor is a device or element that performs the initial measurement of a physical variable
– All sensors are transducers, but not all transducers are sensors
– All Pressure Sensors, pressure switches, and pressure transmitters must have a sensing transducer
– A pressure switch consists of a transducer and an electrical switch that opens or closes a contact at a specific pressure
– A pressure transmitter is a device that converts the electrical signal from the transducer into a much larger electrical signal.
Please let us know if you have any questions about Sensors, Transducers, and Transmitters, add them in the comments below and we will get back to you in less than 24 hours.
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