In industrial applications, a “switch” typically means “a snap-acting electrical component that is combined with a process sensor to provide electrical switch output and take action to alert or control a process”.
While a transmitter is defined as, “an instrument that continuously monitors and provides information about a process with no switch point”.
Switches take action and transmitters inform
Users can also think about a switch as a point monitor and a transmitter as a continuous monitor.
Switches are old technology and transmitters are the next-generation product that everybody wants. The real question is “Should you upgrade or stay with a switch?”
functional differences
Let's look at the functional differences between the two technologies to understand when each would be ideal.
Switches
Switches are simple to install and understand, and easy to adjust when controlling a process
A “switch” typically is an electromechanical instrument. It has an electrical switching component that can open or close contact and a sensor that monitors the process.
Switches, invented over 80 years ago, are still widely used in most plants. They are simple to install and understand, and easy to adjust when controlling a process. Set points are decision points that the process takes action.
Transmitter
Similarly, a transmitter is an electronic instrument that has an electronic sensor and electrical output. The difference between the two is in the way their outputs operate.
A transmitter, unlike a switch, does not have an on/off output. Its electrical output is a continuous proportional signal directly related to the process variable. It has no capability to take any action based on process conditions. Therefore, comparing a switch to a transmitter it is like comparing apples to oranges.
Application
A water pump operating in an industrial plant needs three switches to protect it from a variety of damages. A low-pressure switch, to prevent it from running dry and overheating, a high-pressure switch, to prevent it from running when pipes are clogged, and finally a high-pressure switch, to prevent system-wide high-pressure damage.
For these three set points, three separate switches are used. With switches, users have specific set points and specific actions to take when reached.
Transmitter signal comparison
The transmitter would supply a continuous feed of live process pressure information
Comparing that to a transmitter signal, users can see that the transmitter would supply with a continuous feed of live process pressure information, but would not be able to provide any switching points.
Based on the comparison, if users need switching points to control the process, they are unable to replace a switch with a transmitter.
Upgrading with PLC
How is a transmitter then considered an upgrade compared to a switch? The answer is not that simple. When using a transmitter to control a process need to go through an intermediary, called PLC.
To take action, pair a transmitter with a Programmable Logic Controller (PLC) and it will provide the switching points for actions. The transmitter provides the input and the PLC takes the action.
Strengths and weaknesses
To convert the input to action users program the switch points into the PLC. It is a very powerful concept that gives you a lot of flexibility as an upgrade but adds complexity! Therefore, an upgrade is a balancing act between the simplicity of a switch and the flexibility and complexity of the transmitter/PLC combination.
When considering a change from switch to transmitter/PLC combo users should always consider all the strengths and weaknesses of both solutions to avoid ugly and unexpected surprises later on. It is not always a yes or no decision; there are many factors to consider.
