Density Compensation of Differential Pressure Flow

By Wayne Salo

The most elegant programming or control configuration is wasted if the fundamental
physics, chemistry or mechanics is not understood. If you cannot measure it properly,
you cannot control it properly. (Garbage In, Garbage Out – GIGO) The control engineer
or technician must understand the fundamentals of these most used flow metering
devices to obtain the correct flow measurement.
This paper discusses those differential pressure meters with a fixed size bore diameter
or metering area in a pipe or duct. This does not discuss other types of meters with
variable bores such as the Spirax-Sarco Gilflo flow meter. Compensation for these
devices is different from the fixed bore devices and is properly reflected in the
Manufacturer’s Operation/Installation Manuals provided with these instruments.

The areas this paper will discuss are:
Flowmeter Demographics
Basic Differential Pressure Flow Equation
Water/Liquid Density Compensation
Steam Density Compensation
Gas/Air Density Compensation

1 Basics

1.1 Flowmeter Demographics

The majority of industrial plants utilize orifices, nozzles, Venturi or Pitot tubes for flow
measurement (fixed bore/area DP type measurement). This meter type is generally
considered the easiest and least expensive to procure and install. The instrument
technicians and engineers are generally familiar with the operation of these instruments.
However, there is a tendency of these individuals to overestimate the accuracy of these
instruments. Both density compensation and installation location can cause problems
with the accuracy of these reported process values.
Other meter technologies are becoming more common and more cost effective in
specific applications. These other meter types may have better accuracy in these
specific locations effectively right out of the box without the additional need for
compensation. Only the DP type instruments will be addressed in this document.

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