What is the purpose of your level measurement? If you know the answer to that I highly recommend an Intech article, How to measure liquid level in vessels, by Emerson’s Lydia Miller. This article is but one of a number of excellent learning articles in the International Society of Automation (ISA)’s Automation Basics series of articles.
Lydia opens highlighting different purposes for level measurements:
- Level of liquid-How close is the vessel to being full or empty?
- Volume of liquid-How many liters or gallons is in the vessel?
- Level has reached a high limit-Will the vessel overflow?
- Level has reached a low limit-Will the vessel run dry?
The lower two can be measured with level switches or continuous level measurements while the top two require continuous measurement.
Older switch designs used floats. Newer designs might use a vibrating fork (figure 1) that vibrates at a different frequency when immersed in liquid than when exposed to air.
The Rosemount 2120 level switch is an example of one using vibrating fork technology.
For continuous level measurement, she highlights pressure measurement and radar technologies as two options. One pressure measurement is required for a vessel vented to atmosphere and differential pressure (DP) level measurement for vessels not at atmospheric pressure. Lydia notes:
Using DP for level is an excellent approach, because it is unaffected by equipment or structures inside the vessel, or by turbulence and foam, with minimal effects related to liquid characteristics outside of density.
Radar level measurement measures from the top down. It works by:
…bouncing a microwave radar signal off the liquid surface and measuring the time necessary for it to go down and come back to a sensor. This can be accomplished by measuring time of flight for a microwave pulse, or the degree of frequency shift with a frequency-modulated continuous wave (FMCW) signal.
…a metal probe extends down through the air or vapor space and into the process medium. This helps concentrate the pulse, so the reflection is less affected by reflections from vessel walls, internal structures, or agitators.
…provide continuous level measurements, but without touching the process medium. Some models use a microwave pulse, while others send an FMCW signal to perform the measurement.
Visit the Level Measurement section on Emerson.com for more on these and other technologies to address your level measurement requirements. You can also connect and interact with other level measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.