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 video, we’re going to talk about what a sensor is,
what it can do, and how it can be used in process control.
OK…off we go.
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.
Alright…, 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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