Gas Flow Meters

Fixed Orifice Flow meters require a high flow compared to their size and are smaller than the pipe they are in with instant changes in flow area making them the most constrictive of the group.
Venturi Tubes can operate with the lowest flow rates relative to their size. this and their gradual change in flow area allow these meters to be the least constrictive.
Flow nozzle Meters are really just a hybrid of the two previous meter types and cover the spectrum form one extreme two the other.
The Rotameter is an inverted cone with a light object in it that is simply pushed to a height by the pressure of the gas flow through it. Rotameters have trouble with intermittent flows due to the momentum of the floating object making it bounce around, and have to be read by sight, not electronically.

In a velocity flowmeter the flow is calculated by measuring the speed in one or more points in the flow, and integrating the flow speed over the flow area.

The pitot tube are one the most used (and cheapest) ways to measure fluid flow, especially in air applications as ventilation and HVAC systems, even used in airplanes for the speed measurement. The pitot tube measures the fluid flow velocity by converting the kinetic energy of the flow into potential energy. While this method measures pressure it is not a differential pressure meter as there is ideally no flow restriction and thus no pressure change in the flow. instead the difference between the static pressure and the dynamic pressure is measured.
The use of the pitot tube is restricted to point measuring.

In a hot wire flowmeter one of the two sensors is constantly heated and the cooling effect of the flowing fluid is used to monitor the flowrate. In a stationary (no flow) fluid condition there is a constant temperature difference between the two temperature sensors. When the fluid flow increases, heat energy is drawn from the heated sensor and this heat reduction is measured. The reduction is proportional to the velocity of the fluid, and then must be integrated over the flow area to achieve a flow rate.
Response times will vary with the ratio of the thermal mass of the sensors to the thermal conductivity of the fluid. In general lower thermal conductivity require higher velocity for proper measurement. The hot wire anemometer can achieve relatively high accuracy at low flow rates, but still is a point velocity measurement system

Turbine flow meters are based on a simple principle. If a fluid moves through a pipe and acts on the vanes of a turbine, the turbine will start to spin and rotate. The rate of spin is measured to calculate the flow. The turbine meter usually measured the entire flow area and therefore has different accuracy concerns than the other common types of velocity flow meters.

The positive displacement flowmeter measures fluid flow with rotors or pistons as flow measuring elements. Known and fixed volumes are displaced between each lobe of the rotors or each stroke of the piston. The rotational speed of the rotors, or the velocity of the piston are both proportional to the volume of the fluid being displaced.

The positive displacement rotor construction can be done in several ways:
Reciprocating piston meters are of single and multiple-piston types.
Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth.

Nutating disk meters have movable disks mounted on a concentric sphere located in spherical side-walled chambers. The pressure of the fluid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.

Rotary vane meters consists of equally divided, rotating impellers, two or more compartments, inside the meter's housings. The impellers are in continuous contact with the casing. A fixed volume is swept to the meter's outlet from each compartment as the impeller rotates.

The best gas flow meter are dry types that do not contaminate the measured flow in any way. These meters use a viscous seal so no external contaminates are introduced.