Variable Area Flow Corrections

Variable Area Flowmeters

Dwyer Instruments, Inc. offers many flow measuring products including our very popular variable area flowmeters, often called rotameters. Dwyer manufactures variable area flowmeters in polycarbonate, the Series RM and LFM; in acrylic, the Series VF; and in glass, the Series VA and DR.

Visi-Float® Flowmeter Series VFCII

Variable area flowmeters have scales calibrated for specific media at specific conditions. Most commonly, the flowmeters are calibrated for air for gas use, and water for liquid use, at standard atmospheric conditions of 70 degrees Fahrenheit and 14.7 PSIA, which is 0 PSIG.  When using the flowmeter in conditions other than these standards, the flow reading will not be accurate and corrections will need to be made.

Gases can have large differences in density based on pressure and temperature. The observed reading of the flowmeter is the actual flow rate.  The flow rate corrected for pressure and temperature is the standard flow rate.  Often for air use the pressure in the flowmeter is not atmospheric and a correction must be made to get the standard flow rate.

For correcting the flow rate to standard use the equation shown, where actual pressure P1, actual temperature T1, and actual flow rate Q1 are used.

When using the flowmeter for gas other than air, the specific gravity must be corrected for.

For this correction use the equation shown, where actual flow rate is Q1 and gas used in the flowmeter specific gravity is SG.

Scales for Other Media

If manual corrections are not desired, Dwyer offers scales calibrated for other specific media; for example, nitrogen versus air. Dwyer can even create custom scales calibrated specifically for your application media, operating pressure, and temperature.  Please contact our technical sales department for already existing scales or to have your own custom scale created.


Thermal Dispersion Flow Switches

Thermal Dispersion Flow Switch Series TDFS

Dwyer Instruments offers several flow measuring products that use a thermal sensing element.

Thermal dispersion flow sensors use a temperature element that is heated up beyond the media temperature. Based on calorimetry, the flow moving past the sensor will have a cooling effect that is directly proportional to the flow velocity. The heat is convected from the heated probe surface to the boundary layer of the fluid flowing over the heated surface.  Continue reading “Thermal Dispersion Flow Switches”

Flow Measurement with Orifice Plates

Dwyer Instruments, Inc. offers many types of flow measuring products including orifice plates, which are used as a flow sensing element with a differential pressure monitor.

Orifice Plates

Orifice plates are a primary flow element, detecting the flow of a fluid passing through the plate by sensing the pressure drop across the plate. When a fluid flows through a restriction in a pipe, it creates a pressure difference between upstream and downstream of the restriction. This pressure difference is proportional to flow rate according to Bernoulli’s principal, similar to a Pitot tube.  Orifice plates are commonly used as they are simple to use, low cost, work with gases or liquids, and require low maintenance.  Adversely, they do have large pressure losses with about 50% of the pressure drop not recoverable.  Continue reading “Flow Measurement with Orifice Plates”