Selecting Wrong Sensor As a Mistake #1 In Instrumentation

Instrumentation issues

Despite ongoing advancements in measurement technology, instrumenting a process for feedback control remains a technical challenge. Today’s sensors are certainly more sophisticated than ever before, and fieldbus technology has simplified many installation issues considerably.
Nonetheless, much can still go wrong with an instrumentation project.

Selecting the wrong sensor

Technology mismatch //

Although it’s generally obvious what quantity needs to be measured in a flow, temperature, or pressure control application, it’s not always obvious what kind of flow meter, temperature sensor, or pressure gauge is best suited to the job.

A mismatch between the sensing technology and the material to be sensed can lead to skewed measurements and severely degraded control.

This is especially true when measuring flow rates. All flow meters are designed to measure the rate at which a gas or liquid has been passing through a particular section of pipe, but not all flow meters can measure all flows. A magnetic flow meter or magmeter, for example, can only detect the flow of electrically conductive materials by means of magnetic induction.

Non-conductive fluids like pure water will pass through a magmeter undetected. Magmeters also have trouble distinguishing air bubbles from the fluid in the pipe.
As a result, a magmeter will always yield an artificially high reading when bubbles pass through because it cannot sense the decrease in fluid volume caused by the presence of the bubbles. In a feedback loop, this occurrrence would cause the controller to throttle back the flow rate more than necessary, preventing the required volume of fluid from reaching the downstream process.

The problem gets even worse if the pipe is so full of air that it is only partially filled with liquid, a condition known as open channel.

Although recent technological innovations allow certain magmeters to work in such a challenging environment, mechanical sensors such as turbines yield artificially high readings, since a trickle of fluid will move the meter’s mechanism just as much as a full-pipe flow traveling at the same speed.

On the other hand, mechanical sensors are not affected by the conductivity of the fluid, so they will sometimes work where magmeters fail.

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