Understanding how a Positive Displacement Flow Meter Works

We use cookies to enhance your experience. By continuing to browse this site you agree to our use of cookies. More info.

Positive Displacement flow meters are the only flow measuring instrument that can directly measure the volume of liquid passing though the flow meter. These meters achieve this by trapping small amounts of fluid between rotating components within a high precision chamber. Repeatedly filling a beaker with liquid, and pouring the contents downstream, while counting the number of times the beaker is filled, is a comparable method. Gas Pd Meters

Understanding how a Positive Displacement Flow Meter Works

As the flow of fluid is causing the rotation, the rotational velocity is directly proportional to the flow rate.

Electronic flow meters contain magnets within the rotating components, which can activate various sensor options outside of the fluid chamber. On the other hand, mechanical positive displacement flow sensors use the rotation to drive a magnetic coupling, or a direct gear train, connected to a mechanical counter.

PD flow meters do not need a power supply to operate, nor do they need straight upstream and downstream pipe runs to be installed. Positive displacement flow meters range in sizes from in to 12 in and operate with turndowns up to 100:1, although ranges of 15:1 or lower are normal. As slippage between the flow meter components becomes reduced, the metering accuracy increases with the increase in the viscosity of the process fluid.

The process fluid must also be free from contaminants. Particles greater than a hundred microns in size have to be removed using filtering methods. Positive displacement meters operate with minor clearances between their precision-machined parts and wear can rapidly destroy their accuracy. For this reason, PD meters not often recommended when measuring slurries or abrasive fluids. However, in clean fluid services, their precision and wide range make them an excellent choice for custody transfer and batch charging. They are commonly used as household water meters and millions of these units are produced every year. Positive displacement flow sensors are commonly used across industrial and petrochemical applications for batch charging of liquid and gaseous matter.

Even though the slippage through the positive displacement flow meter decreases (i.e. the accuracy increases) as fluid viscosity increases, there is also an increase in the pressure drop through the meter. As a result, the maximum (and minimum) flow capacity of the flow meter is reduced as the viscosity is increased. In short, the greater the viscosity, the lower the amount of slippage and the lower the measurable flow rate becomes.

One of the key benefits of using a PD flow meter is the level of accuracy that can be obtained, and the high precision from the internal components means that clearances between sealing faces is minimal. How small these clearances are, is relative to how high the accuracy will be. Only the fluid that can bypass this seal is not counted, this is known as ‘by-pass’ or ‘slippage’.

Rotating Positive Displacement flow Meters

Another benefit of these flow meters is their ability to process a wide range of viscosities. It is not uncommon to experience greater levels of accuracy when processing more viscous fluids, simply because of the by-pass. When comparing flow meter accuracies, it is crucial to be knowledgeable of both ‘linearity’ i.e. the positive displacement flow meters ability to accurately measure across the complete turndown ratio, and ‘repeatability’ – the ability to maintain high accuracy over a number of cycles. This is another area where positive displacement flow meters excel, with a repeatability of 0.02% and 0.5% linearity being standard.

If the right flowmeter has been chosen for a given application, it is expected to perform without error for many years. PD flow meters are often sent for servicing and recalibration, with many instruments having been in the field for 10, and in a lot of cases, 20 years continuous use. This reliability is largely due to this proven technology being in use for over 60 years and has allowed for the significant advances to be concentrated within the fields of tribology, as well as achieving the required precision at an affordable cost.

The required amount of maintenance is heavily reliant on the application. For example, if a flow meter processes a fluid that displays lubrication properties, i.e. oil, then the requirement for maintenance is virtually non-existent. However, if the fluid has poor lubricating properties, then the maintenance requirements will need to be discussed with your distributor and/or expert. It is not common that maintenance to a positive displacement flow meter would be more frequent than other instruments within the same system. Any such maintenance can be scheduled to be carried out at the same time as the other components, thus reducing the downtime.

Positive Displacement Flow Meter for Industrial Processes

Positive Displacement Flow Meter for Fuels and Oils

Although positive displacement flow sensors are very robust, they do possess some downsides. Firstly, they should not be used for fluids containing large particles, unless they undergo a pre-filtering process before entering the measurement chamber. They are also unsuitable for applications where large amounts air are present within the fluid; however, air eliminators are available for such applications. One factor that requires serious consideration, is the pressure drop caused by the PD meter; even though it is minimal, they should be factored in for system calculations. As mentioned above, when using processing fluids with poor lubricating properties, it is recommended that advice is first sought from your distributor; various options are available for these applications.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

Please use one of the following formats to cite this article in your essay, paper or report:

OMEGA Engineering Flow & Level. (2019, July 24). Understanding how a Positive Displacement Flow Meter Works. AZoM. Retrieved on November 19, 2022 from

OMEGA Engineering Flow & Level. "Understanding how a Positive Displacement Flow Meter Works". AZoM. 19 November 2022. <>.

OMEGA Engineering Flow & Level. "Understanding how a Positive Displacement Flow Meter Works". AZoM. (accessed November 19, 2022).

OMEGA Engineering Flow & Level. 2019. Understanding how a Positive Displacement Flow Meter Works. AZoM, viewed 19 November 2022,

Do you have a review, update or anything you would like to add to this article?

AZoM speaks with Seokheun “Sean” Choi, a Professor in the Department of Electrical & Computer Engineering at the State University of New York. His new research details the production of a prototype circuit board printed on a sheet of paper.

Dr. Anne Meyer / Dr. Alyson Santoro

In our latest interview, AZoM spoke with Dr. Anne Meyer and Dr. Alyson Santoro who are currently collaborating as part of Nereid Biomaterials. This group is creating a new biopolymer that can be broken down by bioplastic-degrading microbes in marine environments, bringing us closer to reducing the i

This interview discusses how ELTRA, as part of Verder Scientific, produces elemental analyzers for the battery component space.

TESCAN launches its brand-new TENSOR system, a dedicated, near-UHV 4-STEM for multimodal characterization of nanoscale particles.

Discover 3D-Micromac's microFLEX™, a roll to roll laser system that can be used to process flexible substrates.

Spectrum Match is a powerful software program that allows the user to search through a custom-built spectrum library to find similar spectra.

This article provides an end-of-life assessment of lithium-ion batteries, focusing on the recycling of an ever-growing amount of spent Li-Ion batteries in order to work toward a sustainable and circular approach to battery use and reuse.

Corrosion is the degradation of an alloy caused by its exposure to the environment. Corrosion deterioration of metallic alloys exposed to the atmosphere or other adverse conditions is prevented using a variety of techniques.

Due to the ever-increasing demand for energy, the demand for nuclear fuel has also increased, which has further created a significant increase in the requirement for post-irradiation examination (PIE) techniques. - An AZoNetwork Site

Understanding how a Positive Displacement Flow Meter Works

Tcs Lc Displacement Flow Meter Owned and operated by AZoNetwork, © 2000-2022