The Operating Principle of a Mass Flow Meter

The Operating Principle

Alicat Laminar Flow Element instruments have improved upon many of the challenges set by classic orifice plate measurements of volumetric flow being utilised within mass flow calculations. Similarly, they also eliminate the problems inherent within thermal flow designs by addressing hot-wire drift, micro-flow calculations, their response time and the numerous examples given above. Alicat mass flow meters operate on the same principles as many larger laminar transfer standards, but within smaller, easily integrated packages.

To view our range of mass flow meters you can navigate here.

Volumetric flow is based on the Poiseuille Equation

The operating principle of the volumetric flow sensor is based on the physics of the Poiseuille Equation that quantifies the relationship between pressure drop and flow.

Q = (P₁ – P₂) ηr⁴/8ηL

Where:

In it more simplified form, the calculations related to the geometry of the restriction can be replaced by a constant factor. Therefore:

Q = K(ΔP/η)

This now shows the linear relationship between volumetric flow rate (Q), differential pressure (ΔP), and absolute viscosity (η).

To put this mathematics into practical use, an internal restriction is created. This restriction is known as a Laminar Flow Element and it forces the gas molecules to move in parallel paths along the length of the passage. This eliminates turbulence and creates a state of laminar gas flow beneath the accepted Reynolds threshold of 2000. Next, the differential pressure drop is measured within the laminar region. Finally, the gas temperature is measured as the viscosity of the gas must be calculated by the microprocessor in relation to that gas temperature.

We now calculate the mass flow rate

We now have the volumetric flow rate upon which we can apply additional measurements and calculations to determine the actual mass flow rate. Ideal Gas Laws show us that the density of a gas is affected by its temperature, absolute pressure and its compressibility. Using non-Ideal Gas Laws requires a reference to a standard temperature and pressure (STP) condition for "normalizing" the mass flow calculation. Essentially this is a determination of the density of the gas at sea level and a pre-determined temperature as related to the actual flow conditions. In order to determine the mass flow rate, these two known laws of physics need to be applied; the temperature effect on density and the absolute pressure effect on density. Thus:

M = Q(T_s / T_a) (P_a / P_s) (Z_s / Z_a)

Where:

M = Mass Flow
Q = Volumetric Flow
T_s = Absolute Temp at Standard Condition (Kelvin)
T_a = Absolute Temp at Flow Condition (Kelvin)
P_a = Flow Absolute Pressure
P_s = Absolute Pressure at Standard Condition
Z_a = Compressibility at Measured Conditions
Z_s = Compressibility at Standard Conditions

In an Alicat mass flow instrument a discrete absolute pressure sensor and a temperature sensor are placed in the laminar region of the flow stream. The sensors send information to the microprocessor which determines the mass flow. A series of calculations is performed and flow rate data is updated an average of 1,200 times per second. This allows for extremely fast, real time measurements of flow that are sensitive enough to report pulsations in flow as well as step changes.

Popular MFM's used across a range of industries

Gas Mass Flow Meter

M Series

Low Pressure Drop Meter

MW Series

Portable Gas Mass Flow Meter

MB Series

Intrinsically Safe Meter

ISM Series

Process applications and common uses

Common uses and applications present throughout Research, Academia, Industry, Analytical and the Environmental Industries to name just a few. Some examples would be: