MEASUREMENT AND INSTRUMENTATION
storage tanks. Care must be taken to make sure the motor-driven wheel has the strength to pull up the plumb bob if it is buried or coated with product.
ELECTRONIC/ELECTROMAGNETIC LEVEL DETECTION AND MEASUREMENT SYSTEMS Level measurement and detection systems for which electronic and electromagnetic principles are used have grown considerably. Advances in microprocessor/software, mechanical design, and measurement techniques have made it possible to develop measurement systems that can now do economically in a process plant what was possible only in the laboratory and at great expense. Level Detection of Liquids by Use of Conductivity This method is suitable only for level measurement in conductive liquids. The difference in conductivity of a partially insulated electrode is measured when the probe is covered and not covered with the conductive product. The metal wall of the tank can be used as the other electrode, or a second electrode can be inserted into the tank. Additional electrodes of different lengths can be added to detect other levels. The advantages of this method are simple, inexpensive and suitable for dual or multiple point control. The disadvantages are probe cannot become contaminated with grease or other deposits and has limited suitability for products of varying conductivity (Fig. 4). The principle for this form of level measurement is that the presence of a product will cause a change in the resistance between two conductors. A practical application of this principle is as follows. An indication of the level of electrically conductive products in a metal tank, or other container, can be obtained very simply by means of a probe insulated from the vessel and a conductivity amplifier. When the product is not in contact with the probe, the electrical resistance between the probe and the tank wall will be very high or even infinite. When the level of the product rises to complete the circuit with the probe and the tank wall, the resistance will be relatively low. The calibration of conductivity switches is relatively simple. The production process should have been in operation, so there would be some natural contamination, or parallel resistance, caused by the buildup. The switch point is then chosen for a probe with an immersion of approximately 5 cm, then adjusted so the relay is energized. When the level subsequently drops and exposes the probe, the relay will deenergize.
L0 L1 L2 L3 L4
L0 L1 L2
Stop (Start) Start (Stop)
High Alarm Latching for control Low Alarm
FIGURE 4 Conductivity switches.
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Mechanical BE (Measurement and Instrumentation)