Sensors, Transducers, & Transmitters
Objectives
- Define sensors, transducers, and transmitters.
- Discuss the relative strengths and weaknesses of each.
- Explain the purpose of using a live zero scale when transmitting sensor information.
- Walk through a simple transmitter zero and span calibration.
- Construct a standard instrumentation current loop.
Sensors vs. Transducers vs. Transmitters
The words sensor, transducer and transmitter are often used interchangeably. But, strictly speaking, they are not exactly the same.
All transducers and transmitters contain sensors.
Some sensors, but not all, are transducers or transmitters. For example, a traditional U-tube manometer is certainly a pressure sensor, but it is not a transducer or a transmitter. A piezoelectric pressure sensor, on the other hand, naturally produces an electrical signal, so it is a sensor and transducer.
Sensors
A sensor is any device that reacts to a physical stimulus and produces a signal that can be measured or recorded. Here are some examples:
The mercury in a thermometer rises and falls in reaction to temperature.
Litmus paper changes colour in the presence of an acid or base.
A piezoelectric sensor produces a small voltage when squeezed.
Transducers
A transducer converts one form of energy into another. In the industrial world, a transducer translates the output from a sensor into voltage—usually in the millivolt range. Which is great if you have a use for a millivolts signal that can't travel very far without degradation.
This small signal is also vulnerable to electromagnetic interference.
Transmitters
What do you do when you need to transmit the signal over a long line, or when electromagnetic interference is a risk? Add a transmitter!
A transmitter is capable of reading the tiny millivolts signal from the transducer, and converting it into something much more useful.
Most transmitters are adjustable, so you can calibrate them to produce one of a variety of standard instrumentation signals. Back "in the day", the most common instrumentation signals were
- 4-20 mA
- 1-5 V
- 3-15 psi (used in pneumatic systems)
Today, 4-20 mA remains as a frequently used standard, but Pulse Width Modulation (PWM) signals are also increasingly common.
Milliamps vs. Millivolts
The 4-20 mA signal outperforms a millivolt signal in 2 ways. It is less vulnerable to signal degradation over long lines, and it is also less vulnerable to electromagnetic interference.
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