Sensors are a part of everyday life at home and work. There’s probably not a day that goes by where you aren’t impacted in some way by a sensor.

In this article , we’re going to talk about what a sensor is, what it can do, and how it can
be used in process control. A sensor is a device that “senses” something. Today we have sensors that can see, feel, hear, smell, and even taste. Without sensors, our home and work lives would be quite difficult.

For example, as you drive to work, the traffic lights at an intersection are controlled by sensors embedded in the road. These sensors detect your arrival at the intersection. As you approach the grocery store, the door automatically opens because of a sensor.

In your plant, the batch process temperature and pressure are displayed and controlled as a result of output from Sensors. In the world of instrumentation and process control, we define a Sensor as a device
that detects changes in physical, electrical, or chemical properties and produces an electrical output in response to that change.

What are the typical physical properties that sensors are detecting? Let’s name a few… Level, Temperature, Flow, Pressure, Speed, and Position. From a process control perspective, we can classify sensors as either Passive or Active.

A Passive Sensor requires an external source of power to operate while an Active Sensor does not. A Thermocouple is an Active Sensor as it does not require any external power supply to operate. As a thermocouple is exposed to an increase in temperature, it will develop an increasing voltage across it. Another example of an Active sensor is a piezoelectric sensor. A Resistance Temperature Detector or “RTD” is a Passive Sensor. It is a device that’s resistance will change with a change in temperature. To take advantage of this change in resistance, an external supply, or an excitation circuit is required to produce a change in voltage. Another example of a Passive sensor is a Strain Gauge.

Now that we’ve talked about sensors and the physical properties that they can sense, let’s discuss how they are used in the industry. Almost every sensor used in process control will be connected to a Transmitter because a sensor’s output needs to be conditioned or amplified. Here’s an example… We’ve already talked about a thermocouple and the voltage output created when it is heated. Unfortunately, the voltage output of a thermocouple is minuscule! In our example, the thermocouple will produce a voltage output from 8 mV to 18 mV over a 450 degree Fahrenheit change in temperature! In process control, we condition that 8mV to 18mV thermocouple voltage and convert it to a 4 mA to 20 mA industry standard signal that represents our controlled temperature range.

Let’s review what we’ve covered today… Sensors are a part of everyday life at home and work A sensor is a device that can See, Feel, Hear, Smell, and even Taste. In process control, sensors are classified as Passive, requiring an external excitation to produce an electrical output, or Active, producing a voltage
output without any external excitation In process control, sensors are used to measure physical variables such as Level, Temperature, Flow, Pressure, Speed, and Position. Sensor output voltages are very small and therefore require a Transmitter to amplify or condition the output to make it useable in process control applications.

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