science

Positive displacement meter

Positive displacement meter is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Positive displacement meter rather than just read about it. In short: A positive displacement meter is a type of flow meter that requires fluid to mechanically displace components in the meter in order for flow measurement. Positive displacement (PD) flow meters measure the volumetric flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes (finite increments or volumes of the fluid).

Key takeaways

Positive displacement meter belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Positive displacement meter to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Positive displacement meter from memory before moving on to harder problems.

Reference excerpt

A positive displacement meter is a type of flow meter that requires fluid to mechanically displace components in the meter in order for flow measurement. Positive displacement (PD) flow meters measure the volumetric flow rate of a moving fluid or gas by dividing the media into fixed, metered volumes (finite increments or volumes of the fluid). A basic analogy would be holding a bucket below a tap, filling it to a set level, then quickly replacing it with another bucket and timing the rate at which the buckets are filled (or the total number of buckets for the “totalized” flow). With appropriate pressure and temperature compensation, the mass flow rate can be accurately determined.

These devices consist of a chamber(s) that obstructs the media flow and a rotating or reciprocating mechanism that allows the passage of fixed-volume amounts. The number of parcels that pass through the chamber determines the media volume. The rate of revolution or reciprocation determines the flow rate. There are two basic types of positive displacement flow meters. Sensor-only systems or transducers are switch-like devices that provide electronic outputs for processors, controllers, or data acquisition systems. Complete sensor systems provide additional capabilities such as an integral display and/or user interface. For both types of positive displacement flow meters, performance specifications include the minimum and maximum measurable flow rate, operating pressure, temperature range, maximum allowable material viscosity, connection size, and percent accuracy (typically as a percentage of actual reading, not full scale). Suppliers indicate whether devices are designed to measure fluid or gas.

Types

Screw meter A screw flowmeter is composed of a set of screws (also called spindles) which form with the internal structure of the flowmeters' casing a measurement chamber. The screw will get into rotation thanks to the medium passing through the device, which will then be transferred by the-said screws from one end to the other end of the measuring device. For this to be done, the pressure drop is essential and seen as a "necessary evil". This rotation can then be recorded by a sensor which, combined with the processing unit (software and hardware), will be able to deliver a measurement according to the flowrate, viscosity and size of the measurement chamber. ft Screw flowmeters are well-acknowledged for their excellent linearity (±0.001%), excellent repeatability (up to 0,006%) and accuracy (±0.1%). They have the propensity to be used as metrological international reference and/or standard by metrological institutes, due to their outstanding features and reliability. Thanks to screw meters, public and independent institutes of metrology worldwide can compare their respective work, facilities, or calibrate other flowmeters (e.g., master metering) or compare flowmeters' performance according to different measurement principles.

List of public and independent institutes of metrology using screw flow meters as international reference and/or standard:

Australia Austria Belgium Canada Czech Republic Denmark France Germany Japan Mexico Scotland Sweden Switzerland Taiwan R.O.C. The Netherlands The United Kingdom Vietnam

Reciprocating or oscillating piston Each piston is mechanically or magnetically operated to fill a cylinder with the fluid and then discharge the fluid. Each stroke represents a finite measurement of the fluid (can be a single or multi-piston device).

Gear Gear flow meters rely on internal gears rotating as fluid passes through them. There are various types of gear meters named mostly for the shape of the internal components

Oval gear Two rotating oval gears with synchronized teeth “squeeze” a finite amount of fluid through the meter for each revolution. With oval gear flow meters, two oval gears or rotors are mounted inside a cylinder. As the fluid flows through the cylinder, the pressure of the fluid causes the rotors to rotate. As flow rate increases, so does the rotational speed of the rotors.

Helical gear Helical gear flow meters get their name from the shape of their gears or rotors. These rotors resemble the shape of a helix, which is a spiral-shaped structure. As the fluid flows through the meter, it enters the compartments in the rotors, causing the rotors to rotate. Flowrate is calculated from the speed of rotation.

Nutating disk

A disk mounted on a sphere is “wobbled” about an axis by the fluid flow and each rotation represents a finite amount of fluid transferred. A nutating disc flow meter has a round disc mounted on a spindle in a cylindrical chamber. By tracking the movements of the spindle, the flow meter determines the number of times the chamber traps and empties fluid. This information is used to determine the flow rate.

Rotary vane A rotating impeller containing two or more vanes divides the spaces between the vanes into discrete volumes and each rotation (or vane passing) is counted.

Flow = volume of measuring chamber × RPM × 4

Diaphragm Fluid is drawn into the inlet side of an oscillating diaphragm and then dispelled to the outlet. The diaphragm oscillating cycles are counted to determine the flow rate.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Positive displacement meter

Start with the simplest possible case. Write down what Positive displacement meter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Positive displacement meter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Positive displacement meter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Positive displacement meter

In research: Positive displacement meter appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Positive displacement meter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Positive displacement meter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Fluid dynamics, so understanding it makes those chapters shorter.
In everyday life: Look for Positive displacement meter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Positive displacement meter in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Positive displacement meter means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Positive displacement meter out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Positive displacement meter in simple terms?

Why does Positive displacement meter matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Positive displacement meter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Positive displacement meter.