A Guide to Positive Displacement Flow Meter Calibration

March 20, 2022

A positive displacement flow meter is the only flow measuring technology to directly measure the volume of the fluid that passes through it. It works by trapping pockets of fluid between rotating components housed within a high precision chamber.

The volume that passes through the flowmeter is recorded with a geared mechanical register or used to give an output of so many pulses.

In this write-up, we’ll be covering in detail about positive displacement flow meters, and also list steps to calibrate this device.

How It Works

As mentioned, positive displacement flow meters depend on moving parts to displace fluid through them, and these parts must be locked against each other to avoid leakage. Although the parts in individual flow meters can vary based on the design, all such devices are built with the following components:

• High precision measuring Chamber
• Rotating Components
• Inlet & Outlet

Here the fluid enters the measuring chamber through the inlet and proceeds to rotate the rotor. Based on the speed at which the rotor rotates, the subsequent volumetric flow rate is calculated. Once it’s done, the fluid exits the meter through the outlet.

Typically, in positive displacement meters, the rotors are made up of two gears that are aligned to each other either horizontally or vertically. The gears here create a seal within the chamber and thereby block the fluid to be measured for a short period before it leaves the chamber.

Why to Calibrate PD Flow Meters

All our meters are tested at the factory for repeatability & to ensure they comply with published accuracy specifications. However, final calibration must be performed upon installation to compensate for difference in the characteristics of the actual metered liquids and the test liquid, the test standards used, the test methods employed, the test personnel, the local regulatory standards, and the system characteristics, including flowrate. This is true of all flowmeters, irrespective of make/model. In industries, flow meters are periodically calibrated to ensure that measurements are accurate allowing operations to proceed in a safe and timely manner.

For example, a weighing scale that indicates 2 kg when nothing is kept on it, needs to be recalibrated & reset to indicate an initial zero value.

It is not possible to define general rules for the required frequency of calibration. If operated under good working conditions, our meters do not require periodic recalibration nor should they need to be re-calibrated over the operational life of the meter.

However, good working condition is a matter of proper use and maintenance. Some meters may experience an accuracy drift over time due to environmental or application related factors. If a meter is suspected to have drifted outside of its published accuracy range, it can be sent back to us for local third party calibration.

For most applications however, it is the relevant Weights & Measures local authority or the client/user who must define the calibration interval and the policy to determine when to calibrate.

How to Calibrate PD Flow Meters

Calibration is the process by which the measurements made by an instrument is compared with those made by a more accurate instrument, typically a master meter or Prover. A calibrated flowmeter can provide accurate results and ensures the efficient operation of the system. However, there are different calibration methods widely used for fluids.

To perform Master Meter Calibrations:

• Place the master meter in series with the flow meter under test.
• Compare the readings of the master flow meter and flow meter using a measured volume of liquid.
• Turn the adjuster (for mechanical meters) or change the K-factor (for electronic meters) of the flowmeter under test to conform with the master flow meter calibration.

To perform Prover Calibrations (or Calibration Cans):

• Place a measured volume of process fluid into the prover.
• Run product from the prover into the flow meter under test.
• Compare this value to the measurement obtained from your flow meter and adjust your flow meter’s calibration accordingly.

Advantages of Positive Displacement Flow Meters

• As mentioned, Positive displacement flow meters are highly useful for measuring varying fluid viscosities.
• These flow meters can also adjust for temperature-induced variations.
• There is no requirement for flow conditioning.
• PD meters are ideal for remote sites that demand low maintenance as its versatile design permits operation with both voltage pulse signals or mechanical registers.
• PD Meters are robust, can operate with low levels of noise, and can provide a high turndown ratio at an economical cost.

Drawbacks of PD Flow Meters

Although these meters are robust in nature, there are certain limitations on how they can be used:

• Firstly, PD Meters should not be used for fluids that contain solid particles, unless they can be filtered out before the liquid reaches the measuring chamber using a strainer.
• PD Meters should also not be used for fluids with large pockets of air present in them, but you can overcome this problem by using air eliminators & air check valves.
• Another factor to consider is the pressure drop caused by the flow meter, although these are minimal, one should consider such variations during system calculations.
• Variations may occur when:
  • The size of the meter differs from the size of the pipeline.
  • Not using the right class of construction for the medium being measured.
  • Not using the additional necessary devices such as air check valves, strainer, etc.