Brooks Instrument Blog

Yesterday I covered flow rate and reference conditions and talked about how important (or not) these are to specifying a variable area (VA) flow meter. Today, I’ll finish up my tips by reviewing fluid density and viscosity as well as accuracy.

Fluid: Density and viscosity
We always have questions about the fluid such as gas or liquid.  What are the density and viscosity?  Is it corrosive or opaque?  If it a know fluid such as air, nitrogen, water, etc.  The questions get much easier because the world has defined how these known fluids behave so we can easily determine density and viscosity for common fluids such as air, water, nitrogen, etc.  Which leads to the questions as to why do we need to know fluid density and viscosity?  Fluid density and viscosity are important because these two values allow us to select the right flow meter (meter size).  We call this sizing.  What is behind sizing?  Briefly, performance data has been collected on all of the different meters we offer.  We query the performance data and look for flow meters that fit the supplied process conditions (density and viscosity).  Usually there are many flow meters that fit your conditions.  From there it becomes a matter of preference, available options, price or accuracy.  This leads me to my last topic on VA meters, which is accuracy.

Accuracy                                                                          
There are other flow measurement technologies that are more accurate than variable area but variable area is still widely used.  VA flow meters are made in all shapes, sizes and types of materials. It has so many positives such as no power needed, easy to install and use and very repeatable which brings me back to accuracy versus repeatability.  Variable area  accuracy is computed using the full scale accuracy method rather than accuracy of reading.  Simply said the variable area flow meter is much more accurate in the upper end of the flow range.  But more VA meters are used for repeatability of flow measurement.  This means given the same process conditions the float will “repeat” and be at the same scale reading.

In conclusion, correctly specifying a variable area flow meter does require several questions to be answered but as always the intent is to get the best product fit for the application.

Learn more about the variable area flow meters that Brooks has to offer here.