Differential pressure (DP) for liquid level measurement works by measuring the hydrostatic head pressure at the bottom of the vessel and the specific gravity of the liquid. The height of the liquid in inches is the pressure of the liquid column in inches of water divided by the specific gravity of the liquid.
I share this as background for a Processing magazine article, Better ways to use differential pressure for liquid level measurement, by Emerson’s Nicole Meidl. In it, she shares some practical tips to improve the performance of the measurements and reduce the efforts in maintenance.
Nicole opens describing good applications for DP level measurements.
If the liquid and its characteristics are well understood, including the effects of temperature, the level measurement will have the same degree of precision as the pressure instrument.
Another consideration is if the tank is at atmospheric pressure or not.
If the instrument measuring level is reading against atmosphere, the interior pressure of the system adds to the pressure of the liquid and can change the calculated level reading drastically because it is reading the pressure of the liquid plus the system pressure.
For tanks with non-atmospheric pressures, DP measurement is needed. Nicole notes several ways to implement DP measurement. The first is to:
…put a direct impulse line to the high side from the liquid tap, and then run a line from a tap at the top of the tank to the low side. The first connection is wet and the second is dry.
This method is well suited for non-hazardous liquids and non-excessive pressures and temperatures since this DP measurement is a part of the process. The second method uses:
…a flanged spud and the transmitter connection uses a diaphragm… which keeps the process medium inside because the pressure is transmitted via an incompressible fill fluid inside the impulse lines on both sides.
She explains that impulse lines can introduce inaccuracies from the fill fluid weight to the temperature differences in these lines versus the liquid in the vessel. Reducing the length of the impulse lines can help.
A third approach is to replace the impulse line and associated heat tracing for colder climates with a second pressure transmitter at the top of the vessel as:
…an electronic remote sensor (ERS) to measure headspace pressure and send the data electronically to the main DP transmitter at the bottom.
The Rosemount Electronic Remote Sensor (ERS) systems provide DP level measurements by calculating differential pressure from individual pressure readings. Nicole explains that both:
…readings must be within a tight tolerance band to provide the level measurement, and this is only possible with matched transmitters providing high repeatability.
Read the article for more on learning from difficult applications such as distillation towers where other level measurement technologies, such as guided wave radar, cannot be implemented due to internal tower obstructions.
Visit the Rosemount 3051S Differential Pressure (DP) Level section on Emerson.com for more of the technologies and options for accurate and reliable level measurement. You can also connect and interact with other level measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.