Density Compensation of Differential Pressure Flow Meters
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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