Lesson 4 - Process Instrumentation

There are many ways to classify instrumentation; no single system can really cater for all variables. Let us explore some of the key aspects that differentiate instrumentation:

Manual vs electronic instruments

The first classification is whether they are manual or electronic. Most manual instruments provide visual feedback in the field and do not interact with the control systems. However, some manual instruments can directly control equipment. This is handy, as they can provide a method of control that is independent of the control system.

Digital vs analogue

One important classification is whether sensors are digital or analogue. Digital can be a confusing term. If you need a reminder, we covered digital and analogue signals in the lesson on Principles of Process Control. In process control, digital means that the instrument can only have two possible outputs. In fact, ‘binary’ might be a better word to use to describe it. As you may recall, analogue sensors output a value across a range. This can be via a 4-20 mA signal, or via other network connections.

Positional vs physical vs analytical

This classification centres around what a sensor is measuring.

Many sensors are used to detect the position of an item. They detect whether valves are open or closed, or whether pipe connections are in place.

Physical conditions such as level, pressure, temperature, flow and mass, conductivity, and density, can all be measured.

Analytical instruments measure conditions such as dissolved oxygen, pH, alcohol, gravity, turbidity, yeast cell viability, and yeast cell counts.