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Specifying the Right Variable Area Flow Meter: Part 2 of 3

In yesterday's blog post, I took you through why we need to know about your normal and maximum operating temperatures and pressures in your application to specify the right variable area (VA) flow meter for your process. Today, we'll dive into flow rates and reference conditions. 3. Flow rate – minimum, maximum, normal Of course flow rate is an obvious requirement but it is more complicated than it appears.  The goal is to specify a VA meter where the normal operating flow is in the 60% to 80% of the meter's range.  Why you ask because a variable area meter is more accurate in the upper part of its range.  I will talk more about accuracy in my next blog post.  Of course a flow meter needs to be chosen that handles the minimum and maximum flows too.  The other component of flow rate is the units.  If the unit is too small or too large for the meter selected the flow number can be miniscule (many zeros to the right of the decimal point) or too large (many zeros to the left of the decimal point).  An example would be l/d (liters per day) for a 4" meter.  The maximum flow rate of the meter would be almost 2,400,000 l/d wow that is a lot of something but not very practical on a VA meter scale.  This occurs on very low flows an example would be 0.0006 cf/s (cubic feet per second).  In both of these cases considering a more appropriate unit is necessary. 4. Reference conditions and volumetric flow type, if a gas A component of flow rate is the reference condition and flow type.  These only apply to gases.  My views on these two things are" don't worry about it".  Why you ask? because you can count on one hand how many people want actual volumetric gas flows.  This is extremely unusual.  The reference condition can be a variable but customers know what they need and they usually tell us.  The two "biggies" are standard in the US and normal which is very common in the rest of the world.  US standard reference condition is 70°F and 1 atmosphere.  The normal reference condition is 0°C and 1 atmosphere.  Simply this is a method to correct the flow rate to a standard.  It is important but don't lose sleep over it. I'll wrap up my tips for specifying a VA flow meter in tomorrow's post where I'll focus on fluid density and viscosity as well as accuracy.

About The Author

Jim Dillon
Global Product Manager

Jim is the global product manager for variable area products (rotameters) at Brooks Instrument.

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