In a recent Control magazine article, First the application, then the product, Editor in Chief Walt Boyes wrote about the importance of thinking about the application before selecting a level measurement technology. He wrote:
Before you do anything else, you have to have the application parameters. Most of us get so practiced with instrumentation design that we seem to start with the last ISA S20 instrument specification form we worked with and just plug it in. But the S20 forms were not designed to be application selection forms. You start with the sensor or transmitter. That’s backwards.
Walt shows a level measurement continuum chart from very easy applications to nearly impossible ones and the types of measurement technologies which may be suitable.
I passed this article by Sarah Parker, an application manager in Emerson’s Rosemount measurement division, for her thoughts. She agrees wholeheartedly that level measurement can be more complicated than it appears. For anything beyond the “very easy” as Walt puts it, there is no simple answer to the question, “What level technology should I use on my XYZ level application.”
Sarah stressed that there are 3 parameters you must consider together: the technology, the installation, and the application conditions.
On the level measurement technology it’s important to understand its capabilities. Questions you should answer include:
- What are its pressure and temperature limits?
- What is its range capability?
- What is its primary mode of level detection?
- Is it using mass, capacitance, or a distance measurement to make determine level?
- What are the restrictions on its use?
- What conditions will impact its performance?
On the specific installation:
- What connections are available on the tank?
- What size and style are they?
- Where are they located in reference to the material you want to measure and any internal structures?
- How big is the vessel/structure?
- Are there any valves?
And on the application conditions:
- What is the expected pressure and temperature ranges?
- What are the properties of the material being measured–liquid or solid, corrosive, viscous, sticky, or crystallizing?
- Do any of its properties such as density, dielectric, or conductivity change?
- Is there agitation?
- Is there foam?
- Is the material vaporous?
- Is there steam?
Sarah summed it up well that ideally you want to find a technology that is able to handle all the application conditions, fits on the existing connections on the vessel and fits within your budget. If you have some thoughts on this, join the conversation and add a comment.