## Oval Gear Flow Meters Versus Standard Gear Flow Meters

These two types of gear flow meter are superficially the same but operate in very different ways.

### Standard Gear Flow Meters (Spur Gear Flow Meters)

A standard gear flow meter usually has a few very large gear teeth which are meshed in a chamber with close clearances on all surfaces. The teeth themselves form a seal along their length so the only possible leakage path is around the outside of the meshed cogs to the chamber walls. The movement of the liquid causes a differential pressure across the meter and more importantly across the gears themselves. The pressure imbalance on the gear faces produces a small force, the difference in the area caused over the gear height by the pink and green lines in the drawings below. This force imbalance between the gears causes the gears to rotate displacing a volume of fluid approximately equal to one gear tooth volume, shown in red in the illustration below. The direction of rotation is counter intuitive as the gears rotate towards the incoming stream. Usually a sensor is used to count the passing of each tooth generating a high resolution pulse train.

Standard, or spur gear flow meter

### Oval Gear Flow Meters

Oval gear flow meters rely on an entirely different theory. The teeth are still used to drive the gear but the differential force is developed by the shape of the ovals not the gear teeth on the lobe. The teeth form a fluid seal and drive the lobes. Oval Gear flowmeters from different manufacturers include gears of varying oval shapes depending on the resolution and flow requirement of the target application. By using an oval shape a much greater driving pressure can be generated resulting in a wider flow range and lower pressure drop compared to a standard gear meter. The displaced volume is a product of the oval shape not the gear profile shown in blue below. The sensor is usually magnetic with a detector at the face of the gear. The resolution is lower than the standard gear meter.

Oval gear flow meter

## Standard or Oval Gear Flow Meter? The conclusion

In conclusion – for high resolution applications the standard gear flow meter is the flow meter of choice as they have a very small displaced volume and give a large number of pulses per revolution at high accuracy. However the downside of standard gear meters are their pressure drop and flow range. A lot of force is required to drive and this results in a reduced dynamic range.

By comparison the oval gear flow meter has a far greater dynamic flow range and a much lower pressure drop making these devices suitable for low pressure systems. For the vast majority of applications the lower number of output pulses per unit volume is not an issue. Further the choice of gear materials is greater as the forces they have to deal with are considerably lower, which can even permit all polymer oval gear flow meters to be made.