Why Measuring Flow is a Difficult Task: Physical Properties

E-Zine August 2014

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Physical properties can render almost any measurement instrument inoperable. Problems can occur when the physical properties of the fluid are outside the limits of the flowmeter. Applicable physical properties include pressure, temperature, viscosity, specific gravity, lubricity, thermal conductivity, electrical conductivity and the like. Only one constraint need be exceeded to cause excessive measurement error and/or damage the flowmeter. For example, even momentarily operating a plastic flowmeter at 500°C will likely damage or destroy the flowmeter.

Process issues can have a major effect on flowmeter performance. Fluid composition can affect the physical properties of the fluid and adversely affect the measurement. Flow rate can affect the hydraulic characteristics of the fluid in the pipe, and cause the velocity profile to be different from that in which the flowmeter was designed to operate. In addition, the state of the fluid can affect the measurement. For example, consider a flowmeter installed in water service where the sensing lines freeze. Filling the flowmeter with ice, and not the pressure, temperature or composition, renders the flowmeter inoperable.

Remember, performance can be degraded and/or the flowmeter can be damaged if any one of the many limits is exceeded.

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From Flow Control (August 2002)

ISSN 1538-5280