A pressure switch trips at a certain predefined pressure. Its output is either on or off. They are used as warning devices for valve leakage, as integrated control sensors and as notification mechanisms for exceeding thresholds.
There are many different forms of pressure switches. The oldest is a simple spring resisted bellows that activates a physical switch when the spring tension is overcome.
A pressure bellows is used to measure a range of pressures with a certain level of granularity. They are used in meteorological equipment, to measure liquid levels in tanks and gas or liquid pressures in pipelines and storage facilities.
A pressure sensor has a bellows which compresses or expands in response to an outside pressure. The movement of the bellows typically moves a transformer core and alters its electrical coupling. This signal is converted from an analog current or voltage measurement into a digital signal and fed to the PLC or RTU. In some cases the PLC/RTU is integrated into the sensor itself in others it simply receives the output in a known format.
Thermocouples measure temperature and convert it into a voltage variance. They are used to monitor temperature remotely.
Thermocouples work by taking advantage of the differing electrical conductivity values of two dissimilar metals in contact with each other. Because different metals have a different electrical response at different temperatures they will form a potential difference at their junction point. This potential difference is measured. The voltage variance indicates the difference in temperature between the voltage measuring point and the temperature at the point that the dissimilar metals connect. Knowing these points of information allow the calculation of the temperature at the measuring point.
Valve Position Indicator
The simplest valve position indicator is a switch that is activated when the valve stem reaches a specific position. One switch is used for fully open and one for fully closed.
Repositioning of a transformer core is used in some implementations and activation of a traveling linked rheostat is used in others. These implementations can provide readings indicating a percentage of how open or closed a valve is. This is essential where the valve is used to throttle flow rate.
Fluid Flow Rate
Fluid Flow rate is usually measured using a Bernoulli gauge. For a given volume of flow with a smaller cross-sectional diameter path pressure of a fluid will increase. By measuring the pressure at both the smaller cross section and larger cross section and knowing the temperature and density of the fluid it is possible to calculate the flow rate.
I did this off the top of my head then realized
A lot of good stuff on Wiki