Briefing of Unit Measurement

Both gas and liquid flow can be measured in terms of physical quantities such as volume or mass flow in liters per second or kilograms per second respectively.These measurements are related to the density of the material.The density of a liquid is almost independent of conditions.This is not the case with gases, whose density depends largely on pressure, temperature, and to a lesser extent on composition.When a gas or liquid is transferred because of its energy content, as in natural gas sales, flow can also be expressed in terms of energy flow, such as gigajoules per hour or BTUs per day.Energy flow is volume flow times energy content per unit volume or mass flow times energy content per unit mass. Energy flow rate is usually derived from mass or volumetric flow rate by using a flow computer.

Gas 

Gases compress and change volume when they are pressurized, heated or cooled.A certain volume of gas under one set of pressure and temperature conditions is not equivalent to the same gas under different conditions.Will refer to the "actual" flow through the meter and the "standard" or "basic" flow through the meter in acm/h (actual cubic meters per hour), sm3/sec (standard cubic meters per second), kscm/h ( thousand standard cubic meters per hour), LFM (linear feet per minute) or MMSCFD (million standard cubic feet per day).Gas mass flow can be measured directly, independent of pressure and temperature, using ultrasonic flow meters, thermal mass flow meters, Coriolis mass flow meters or mass flow controllers.

Liquid

For liquids, different units are used depending on the application and industry, but may include gallons (US or Imperial) per minute, liters per second, bushels per minute, or when describing river flow, cumecs (cubic meters per second) or acres per day . In oceanography, a common unit for measuring volumetric transport (such as the amount of water transported by currents) is the sverdrup (Sv), which corresponds to 106 m3/s.

Primary flow element

A primary flow element is a device inserted into a flowing fluid that produces physical characteristics that are accurately related to flow.For example, an orifice creates a pressure drop that is a function of the square of the volume flow through the orifice.The primary flow element of a vortex flowmeter produces a series of pressure oscillations.Often, the physical properties produced by the primary flow element are easier to measure than the flow itself. The characteristics of the primary flow element, and the fidelity of the actual installation to the calibration assumptions, are key factors in flow measurement accuracy.

Mechanical flow meters

A positive displacement flow meter can be compared to a bucket and a stopwatch.The stopwatch starts when traffic starts and stops when the bucket reaches its limit.Volume divided by time gives flow rate. For continuous measurement, we need a system that constantly fills and empties the bucket to divert the flow without letting it flow out of the pipe.These continuously forming and contracting volumetric displacements can take the form of pistons reciprocating in cylinders,gear teeth meshing with the inner wall of a gauge, or progressive chambers formed by rotating oval gears or helical screws.