How should a positive displacement flow meter be selected? For oil products, lubricants, chemical liquids, and other liquids with certain viscosity, a positive displacement flow meter is a flow measurement instrument that determines flow by counting repeated fixed-volume displacement cycles.
Unlike flow meters that primarily determine flow from changes in fluid velocity, the basic concept of positive displacement measurement is straightforward: a certain volume is repeatedly measured, and the number of completed cycles represents the total volume that has passed through the instrument.
Proper selection requires consideration of the medium, minimum/normal/maximum flow, operating pressure, temperature, viscosity, allowable pressure drop, and pipeline connection conditions. American VTON can evaluate these parameters together to determine a suitable positive displacement flow meter configuration.
1. What Is a Positive Displacement Flow Meter?
A positive displacement flow meter measures flow by repeatedly filling and discharging a defined volume.
Its internal structure generally contains moving measurement components capable of forming measuring chambers. As liquid enters the meter, it drives the internal measuring mechanism. A defined volume of liquid is repeatedly admitted into and discharged from the measuring chamber.
By detecting the movement, rotation, or displacement of the measuring mechanism, the instrument can determine the accumulated volume passing through the meter.
The principle can be summarized as:
Liquid enters → a defined volume is formed → the measuring chamber repeatedly fills and discharges → cycles are counted → accumulated flow is calculated.
Because the measurement process is directly related to volume, positive displacement flow meters can be considered for applications requiring liquid flow indication, accumulated measurement, or quantitative control.
For oil, lubricants, and certain higher-viscosity liquids, viscosity and flow conditions are particularly important during selection.
2. How Does a Positive Displacement Flow Meter Work?
The operating principle can be understood in three steps.
Step 1: Liquid enters the measuring chamber.
The measured liquid enters the meter and drives the internal measurement mechanism under the process pressure.
Step 2: A defined volume repeatedly completes filling and discharge cycles.
The measuring mechanism causes the chamber to go through a repeating sequence of filling, isolation, and discharge. Each completed cycle corresponds to a relatively defined measurement volume.
Step 3: Cycles are accumulated and converted into flow.
The meter detects the movement of the measuring mechanism through mechanical components or sensors. The number of cycles is converted into a volume signal, from which instantaneous and accumulated flow can be obtained.
The core logic can therefore be summarized as:
Defined volume × number of cycles = accumulated volume.
When the volume is considered over a unit of time, the corresponding instantaneous flow rate can also be obtained.
3. How Should a Positive Displacement Flow Meter Be Selected?
① Check the medium: viscosity and cleanliness matter
The first step is to identify the measured medium.
For oil products, lubricants, chemical liquids, and similar media, density, viscosity, temperature, and corrosiveness should be considered.
Because positive displacement flow meters contain internal moving measurement components, the presence of large amounts of solid particles, impurities, or materials that can settle should be evaluated carefully.
For higher-viscosity liquids, the actual viscosity range and viscosity changes caused by temperature variations should also be considered.
② Check flow, pressure, and temperature
It is recommended to provide the minimum, normal, and maximum flow rates.
The flow range affects the meter specification and measurement capability, while operating pressure and temperature affect the actual operating conditions of the medium and equipment.
For liquids with significant temperature variation, the influence of temperature on viscosity should also be considered.
The allowable pressure drop of the process system should also be confirmed. If the process is sensitive to pressure loss, selection should not focus only on measurement range. The internal meter structure and system pressure conditions should be evaluated together.
American VTON can analyze measurement range, medium viscosity, pressure, temperature, and related structural parameters according to the actual application.
③ Check the pipeline and output requirements
The pipe nominal diameter, connection type, installation location, and available space should also be confirmed.
At the same time, determine the required output method.
If the main requirement is local viewing of accumulated flow, the display configuration should be considered. If the instrument needs to connect to an automation or control system, the required pulse, analog, or communication output should be specified.
For long-term industrial operation, maintenance access and installation space for related accessories should also be considered in advance.
Customer Application Example: Lubricant Transfer System at a Certain Location
A lubricant transfer system at a certain location required continuous flow measurement in a liquid pipeline. During the technical evaluation, the lubricant properties, viscosity, operating temperature, operating pressure, and minimum/normal/maximum flow were first established.
The pipe diameter, connection arrangement, available installation space, and system output requirements were then considered when evaluating the positive displacement flow meter configuration.
American VTON’s selection approach considers the medium viscosity, flow range, pressure and temperature, pipeline conditions, and output requirements together rather than selecting the instrument solely from pipe diameter.
For liquid accumulated-flow measurement, this comprehensive approach helps establish a more suitable relationship between the instrument and the actual process conditions.
Final Recommendation: Define the Liquid Properties Before Selection
Before selecting a positive displacement flow meter, prepare the following information:
Medium name, density, viscosity, minimum/normal/maximum flow, operating pressure, operating temperature, medium cleanliness, pipe diameter, connection type, installation space, and output requirements.
For oil products, lubricants, or other high-viscosity liquids, viscosity data at different operating temperatures should also be provided whenever possible.
American VTON’s positive displacement flow meter selection process evaluates the medium, flow range, pressure and temperature, allowable pressure drop, and installation conditions step by step.
For industrial liquid measurement, the core concept of positive displacement measurement is not complicated. It divides continuous liquid flow into measurable fixed-volume cycles and determines flow from the number of cycles.
The key to selection is therefore not simply comparing instrument names, but ensuring that the medium properties, measurement range, pressure conditions, viscosity, and installation arrangement correspond properly.
