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April, 2024

Mastering Flow Measurement

Unveiling the Power of Oval Gear Meters

The Oval Gear flow meter is a type of Positive Displacement Flow meter used to measure the volumetric flow of liquids. Positive Displacement flow meters directly measure the volume that passes through the meter rather than the velocity or some other physical characteristic of the liquid to calculate the volumetric flow rate, making them a volumetric meter. Volumetric meters directly measure the total volume passing through the meter rather than measuring the velocity of the flow or inferring the volumetric flow rate from some physical characteristic such as the conductivity or sound velocity. The advantage of volumetric meters over inferential or velocity meters is that the viscosity effects are largely removed. Fluids like oils and greases can be measured at very low flow rates where other technologies require much higher minimum flow rates and so are ineffective for real world application rates.

The Oval Gear principle is a simple concept: two oval shaped rotors, with integral timing gear teeth around their perimeters, rotate within a precision measuring chamber. The rotors are installed at exactly 90 degrees from each other, and this relationship is maintained as they rotate. The Oval Gear Meter requires no external timing mechanism, making it a simple device with only 2 moving parts that are durable and easily serviceable. Each rotation of the Oval Gear rotors will transmit 4 identical volumes of liquid from the meter inlet to outlet providing either a constant number of electronic pulses to a digital instrument, or 360 degrees of rotational motion to a mechanical counter. They are well suited for a broad range of applications and excel in many operating conditions where other measurement principles cannot be used effectively, or at all.

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Strength of the
Oval Gear Meter


  • Accurate with clean, viscous fluids, unaffected by temperature changes
  • No flow conditioning required, they can be placed directly after pumps or valves with no effect on accuracy
  • Robust
  • High turndown rates
  • Low pressure drop
  • Excellent for small volume additions

All of the advantages of the oval gear meter make it excellent for all types of lubricants. From motor engine oils through to heavy hydraulic oils and greases oval gear meters are the most accurate solution for measuring them economically and reliably.



The simplicity of installation of oval gear meters is one of their strengths. Little consideration needs to be given to where they are installed in a piped system. Volumetric meters don’t require flow conditioning to get accurate and reliable readings. They can be placed directly after pumps, before or after valves, bends or flow obstructions unlike velocity or inferential meter types. This makes them very suitable for chemical injection or dosing rigs where compact piping design can be a paramount design consideration. Coupled with the high precision of oval gear meters particularly with measuring small volumes it makes them an ideal solution for batching.

The meters can be installed in horizontal or vertical pipe legs. The constraints on the meter are that the meter needs to run full to be accurate so it should not be placed at the highest point in a line or on a downleg where bubbles may pass through or the pipe may not run full.

A common error with installation is orienting the meter the wrong way. The shafts of the meter must be parallel to the ground so that the weight of the rotors is supported by the shaft and not on the end of the rotor. By installing the meter with the rotor vertical it is like trying to drive a car on its side, the tire will be spinning on the hub cap rather than the tread.

The meters also need to have the air bled out of the system whenever starting up with air in the line. If air is not removed the liquid will compress upon pumping and overspeed the meter. Friction will create heat and bind or crack the bearings, depending on the bearing material, and the meter may fail.

During installation, it is good practice to install the meter with a strainer before the meter to prevent foreign material entering the meter and damaging the internals or effecting the bearings. A bypass around the meter allows the pipe system to be bled and also ensures the meter can be isolated and removed for easy maintenance checks.

Oval Gear Meter & Viscosity

One of the greatest strengths of the oval gear meter is its ability to measure fluids of all viscosities, it can measure thin fluids such as organic solvents to very thick oils and greases. There are several factors to take into account when selecting a meter for fluids of different ranges of viscosity.


The calibration process is an important factor to consider to understand how Oval Gear Meters respond to different viscosities and how the accuracy and flow rate range are altered by changes in viscosity. Oval gear meters are usually callibrated agains a master meter due to their efficiency, a 3cP viscosity standard is chosen as it can be a good callibration point for fluids of both higher and lower viscosity. Single-point callibratoin can achieve ± 0.5% accuracy over the stated flow range, while a multipoint callibration can improve accuracy up to ±0.2%. However, when flow viscosities drop below 3cP or when the fluid has low lubricity, the effective flow range and accuracy of Oval Gear Meters may be affected. In such cases, it's advisable to consult the manufacturer for assistance to ensure optimal performance and accuracy.


Oval gear meters are very robust and reliable. Having been commercialized for nearly a 100 years they are now a standard measurement tool in the fuel distribution industry. From small self-bunded tanks to delivery trucks to distribution terminals, positive displacement meters are a standard metering tool all along the fuel distribution chain. With only two moving parts and a very solid, simple mechanism and measuring system, oval gear meters are a long-accepted, industry solution for fuel measurement due to the reliable and accurate delivery. With a lubricating clean fluid such as diesel or motor oils, oval gear meters will work for years.



Oval gear meters are indeed well-suited for delivering small volumes, especially in applications where flow needs to start and stop precisely. When combined with a high-quality batch controller, they form an excellent batch control system. Oval gear meters have a high turndown rate, meaning they can accurately measure both high-bulk volumes and low-finishing flow rates. This versatility allows them to deliver volumes repeatedly and accurately across a wide range of flow rates. When designing a batching system, it's essential to match the meter to the full flow range expected in the process. For larger line sizes or when a two-stage batch control system is needed, careful consideration should be given to selecting the appropriate meter to ensure accurate measurement across the entire range.

Oval gear meters find widespread use in batching applications, such as dispensing water and oil into dry mixes for bakeries or food processing. They are also suitable for demineralized or reverse osmosis water batching, especially in situations where small volumes are required or where the presence of conductivity makes magnetic flow meters less suitable.

Oval gear meters excel in batch control applications where precise measurement of small volumes is required, and their high turndown rate and accuracy make them a reliable choice for a wide range of industries and processes.


Low Pressure Drop

Oval gear meters, when appropriately sized and installed, exhibit a low pressure drop compared to other positive displacement meters. This characteristic makes them particularly well-suited for gravity-fed fuel applications, where minimizing pressure drop is essential for efficient operation. Oval gear meters have a large turndown ratio, allowing them to accurately measure varying flow rates and pressures experienced between a tank being full and empty. This versatility ensures reliable measurement performance across different operating conditions.

The low pressure drop also enables them to handle high viscosity fluids effectively. While some applications involving thick greases and oils may pose pumping challenges, oval gear meters can still perform well due to their low pressure drop characteristics.
Oval gear meters offer efficient and accurate flow measurement solutions for a wide range of applications, including gravity-fed fuel systems and high viscosity fluid handling. Their low pressure drop and large turndown ratio make them particularly advantageous in various industrial settings.