Key features of differential pressure level measurement include:
In cases, where the density of the process fluid is not stable, an additional differential pressure transmitter is employed to evaluate density. This will then compensate for the variations introduced in the process.
“To accommodate the measurement of light slurries, differential pressure transmitters are available with extended diaphragms that fit flush to the side of the vessel. However, if the d/p transmitter diaphragm becomes coated, it may require recalibration, which can be impractical and will add to the "cost of ownership".”
Differential pressure transmitters are generally found to be more cost effective as compared to other level sensors. However, they often require substantial extra hardware like legs and valve manifolds and labour for successful and stable installation purpose.
Differential pressure level sensors are particularly useful for preventing the problem of GIGO i.e. Garbage In, Garbage Out, experienced in various level applications.
In actuality, differential pressure level measurement technology does not measure level. It just infers level of the process liquid.
Level measurement method with the help of differential pressure is also known as hydrostatic tank gaging method i.e. HTG.
Differential pressure transmitters basically operate upon “the principle that the difference between the two pressures (d/p) is equal to the height of the liquid (h, in inches) multiplied by the specific gravity (SG) of the fluid. By definition, specific gravity is the liquid's density divided by the density of pure water at 68° F at atmospheric pressure.”
A differential pressure transmitter offers good accuracy over broad level measurement ranges, provided the density of the process remains stable.
Various differential pressure cell designs include motion balance, force balance and electronic etc. The same cell designs are used for level detection in pressurized tanks as well as open tanks.