Impact of Operating Environment
All of the sensors described in this module are widely used in control and
instrumentation systems throughout the power station.
Their existence will not normally be evident because the physical
construction will be enclosed inside manufacturersí packaging. However,
each is highly accurate when used to measure the right quantity and within
the rating of the device. The constraints are not limited to operating
pressure. Other factors include temperature, vapour content and vibration.
Vibration
The effect of vibration is obvious in the inconsistency of measurements,
but the more dangerous result is the stress on the sensitive membranes,
diaphragms and linkages that can cause the sensor to fail. Vibration can
come from many sources.
Some of the most common are the low level constant vibration of an
unbalanced pump impeller and the larger effects of steam hammer.
External vibration (loose support brackets and insecure mounting) can
have the same effect.
Temperature
The temperature effects on pressure sensing will occur in two main areas:
The volumetric expansion of vapour is of course temperature dependent.
Depending on the system, the increased pressure exerted is usually already
factored in.
The second effect of temperature is not so apparent. An operating
temperature outside the rating of the sensor will create significant error in
the readings. The bourdon tube will indicate a higher reading when
exposed to higher temperatures and lower readings when abnormally cold
- due to the strength and elasticity of the metal tube. This same effect
applies to the other forms of sensors listed.
Vapour Content
The content of the gas or fluid is usually controlled and known. However,
it is mentioned at this point because the purity of the substance whose
pressure is being monitored is of importance - whether gaseous or fluid ñ
especially, if the device is used as a differential pressure device in
measuring flow of a gas or fluid.
Higher than normal density can force a higher dynamic reading depending
on where the sensors are located and how they are used. Also, the vapour
density or ambient air density can affect the static pressure sensor readings