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⌚Timestamps:
00:00 Intro
01:00 1) What is a sensor?
01:18 2) What is a transducer?
01:57 Sensors vs transducers
02:17 3) What is a transmitter?
03:12 Pressure sensors vs transducers
04:22 4) What is a Pressure Switch?
04:31 Pressure switch vs pressure transmitter
05:23 Pressure switch vs pressure transmitter in practice

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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.

Just to confuse the issue, sensor and transducer have often been used as synonyms.

1) 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) 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.
A strain gauge is a transducer whose resistance value changes proportionally to the strain it experiences.

All sensors are transducers, but not all transducers are sensors.
Consider a loudspeaker that converts an electrical signal into motion. The loudspeaker is a transducer, but not a sensor.

3) The term transmitter 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.

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 15 V or 420 mA which represents 0100% 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.

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 0100 psi. Our Sensor/transducer performs the initial measurement and converts it to an electrical signal of 03 mV. The transmitter then converts that signal to a standard signal of 05 V, which is received by a PLC analog input module.

4) A Pressure Switch is a twopart device consisting of a sensing transducer and an electrical switch.
A pressure switch has a transducer just like a pressure transmitter. That’s where the similarity ends.

A Pressure Transmitter produces an analog electrical voltage or a current signal representing 0100% of physical process pressure. A pressure switch has electrical contacts that open and close at a specific pressure.

Let’s illustrate with an example… 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 420 mA output for a pressure range of 0100 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.

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