Remote indication is necessary to provide transmittal of vital level information to a central location, such as the control room, where all level information can be coordinated and evaluated. There are three major reasons for utilizing remote level indication: Level measurements may be taken at locations far from the main facility The level to be controlled may be a long distance from the point of control The level being measured may be in an unsafe/radioactive area.
Figure 15 illustrates a block diagram of a typical differential pressure detector. It consists of a differential pressure (D/P) transmitter (transducer), an amplifier, and level indication.
The D/P transmitter consists of a diaphragm with the high pressure (H/P) and low pressure (L/P) inputs on opposite sides. As the differential pressure changes, the diaphragm will move. The transducer changes this mechanical motion into an electrical signal. The electrical signal generated by the transduceris then amplified and passed on to the level indicator for level indication at a remote location. Using relays, this system provides alarms on high and low level. It may also provide control functions such as repositioning a valve and protective features such as tripping a pump.
Density of the fluid whose level is to be measured can have a large effect on level detection instrumentation. It primarily affects level sensing instruments which utilize a wet reference leg.In these instruments, it is possible for the reference leg temperature to be different from the temperature of the fluid whose level is to be measured. An example of this is the level detection instrumentation for a boiler steam drum. The water in the reference leg is at a lower temperature than the water in the steam drum. Therefore, it is more dense, and must be compensated for to ensure the indicated steam drum level is accurately indicated.
Ambient temperature variations will affect the accuracy and reliability of level detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in the instrumentation circuitry, and, therefore, affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the level detection instrumentation in the proper environment. The presence of humidity will also affect most electrical equipment, especially electronic equipment. High humidity causes moisture to collect on the equipment. This moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components. The effects due to humidity are controlled by maintaining the equipment in the proper environment.