TABLE OF CONTENTS Instrument suppliers offer more than 20 different level measurement technologies. All work, when properly applied. However, each has its strengths and its weaknesses, and some are not suitable for certain applications.
For a given acceleration of gravity, the liquid head in a tank or vessel generates a force per unit area or pressure (P) that is directly proportional to the liquid level (L) above the measurement point times the average density ( ?) of the liquid in the column. Solving for L:
L = P/ ?
While this formula is simple, its usage can be complicated. Virtually all applications using pressure transmitters for liquid level include one or more of the following issues:
Transmitter is not located at the zero level point
Transmitter is remote from the tank, above or below the primary pressure connection
Transmitter is isolated from process fluid with a flange or seal system
Tank is closed and, hence, subject to pressure or vacuum above the liquid
The fluid above the liquid may be the vapor of the liquid itself or an outside sourced fluid, such as a nitrogen blanket
Tank pressure reference connection is filled with a vapor (dry leg)
Tank pressure reference connection is filled with liquid (wet leg)
External wet legs can exist on both high and low pressure sides of the transmitter
Environmental conditions can be different for each of these external legs
Environmental conditions are usually different than tank conditions, e.g., a wet leg temperature might...
