Keep Using a Switch or Upgrade to a Transmitter?

The answer might surprise you!

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”.

Simply stated; switches take action, transmitters inform!

You 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?”

Let us look at the functional differences between the two technologies to understand when each would be ideal.

A “switch” typically is an electromechanical instrument. It has an electrical switching component that can open or close a 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, easy to adjust when controlling a process. Set points are decision points that the process takes action.

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, when you compare a switch to a transmitter it is like comparing Apples to Oranges.

Let us look at a simple 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-high pressure switch, to prevent system wide high pressure damage. For these three set points, three separate switches are used. With switches, you have specific set points and specific actions to take when reached.

Comparing that to a transmitter signal, you can see that the transmitter would supply you with a continuous feed of live process pressure information, but would not be able to provide any switching points.

Based on above comparison, if you need switching points to control your process, you are unable to replace a switch with a transmitter,

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 you need to go through an intermediary, called PLC.

To take action, pair a transmitter with a Programmable Logic Controller (PLC) and it will provide you with the switching points for your actions. The transmitter provides the input and the PLC takes the action. To convert input to action you 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 flexibility and complexity of the transmitter/PLC combination.

When considering a change from switch to transmitter/PLC combo you 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. To learn more about these considerations, follow the UE Blog.

Future blog discussions related to switch upgrades will include the following topics and if you have additional suggestions or comments please contact me:

• System Simplicity
• Overall Cost
• Electrical requirements
• Programming
• Performance & Maintenance
• Regulatory Compliance