The Comprehensive Flowmeters Resource

Positive Displacement Flowmeters

About Circular and Oval Gears vs Non-direct Measurements

A circular gear flow meter (also called cylindrical gear) belongs to the family of positive displacement meters just like the oval gear meter.
One of the well-known attributes of a positive displacement meter (also known as a PD meter) is its ability to measure volumetric flow directly as opposed to non-direct methods (inferential) which take a velocity measurement and using mathematical equations calculate volume flow from the velocity. Inferential type meters work very differently but since the velocity is known – it is possible to compute the volumetric flow rate in this way. An example is shown in the video of how a velocity in meters/second (or m/s) can be converted into a volumetric flow in litres per minute for a known pipe size.
The following example shows how a 1 m/s velocity in a pipe with an internal diameter of 1 inch is calculated at 30.39 L/M.
The equation used here is Q = VA where Q is the volumetric flow rate, V = velocity and A = surface area.

However, this calculator calculated the volumetric flow rate with assumption that the flow velocity is uniform across all sections of the pipe.
In reality, this is not happening because the flow velocities nearer the pipe wall will tend to be less than the velocity in the middle of the pipe.
In order to correct for these errors, a Reynolds Number compensation must be configured.


If you take a look at the working principle of this Oval Gear meter as an example, you will see that the gears do not turn unless a void (volume) is filled.
Oftentimes, engineers have likened this principle of measurement akin to filling cups to the full. Imagine each void is a cup and the rotation of the internals will not move until one cup is filled. These meters can be installed in a liquid line, measure accurate flow and continue to keep reporting accurate flow.


This type of flow meter is being used across a wide range of industries dealing with oil, chemicals, solvents where continuous measurement is needed.


Mostly different liquids which have a high or low pH (either acidic or alkaline) saline solutions and toxic chemicals.
Low Flow – Yes! This product (because of its inherent design) is able to accurately pick up low flow rates.


The rotations are measured by using non-contact methods. A popular design is the Hall Effect (but not limited to) which creates a pulse signal proportional to the flow rate.
One of the advantages of pulse outputs is that it is easy to totalise i.e. to measure total flow in volumes or batches (Batch Flow SG Flow Meters supplies circular gear flow meters manufactured by automated precision machining which improves overall accuracy and repeatability in both low and higher flow scenarios, even at high temperatures.

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