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Mechanical Flow meters

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Mechanical flowmeters measure flow by using an arrangement of moving parts, either by passing specific, known volumes of a fluid through a series of gears or chambers (in the case of positive displacement meters) or by means of a spinning turbine or rotor in an arrangement called a turbine flowmeter.

Positive Displacement Flowmeters 
Positive displacement flowmeters or PD meters operate by isolating and counting known volumes of a fluid (gas or liquid) while feeding it through the meter. By counting the number of passed isolated volumes, a flow measurement is obtained. Each PD meter has its own distinct mechanism that goes through a specific number of cycles for counting fluid volumes. Every cycle of the meter’s mechanism displaces a precisely defined (“positive”) quantity of fluid, so that a count of the number of mechanism cycles yields a precise quantity for the total fluid volume passed through the flowmeter. Many positive displacement flowmeters are rotary in nature, meaning each shaft revolution represents a certain volume of fluid has passed through the meter. Some positive displacement flowmeters use pistons, bellows, or expandable bags working on an alternating fill/dump cycle to measure off fluid quantities. Positive displacement flowmeters are applicable only to clean fluid flow streams.

Turbine Flow Meters
Turbine flow meters use a free-spinning turbine rotor to measure fluid velocity, much like a miniature windmill installed in the flow stream. The fundamental design goal of a turbine flowmeter is to make the turbine rotor as free-spinning as possible, so no torque will be required to sustain the turbine’s rotation. If this goal is achieved, the turbine blades will achieve a rotating (tip) velocity directly proportional to the linear velocity of the fluid which is then used to infer the volume of fluid passing through the flowmeter.

In the design of a turbine flowmeter, the diameter of the turbine rotor is very close to the inside diameter of the metering chamber, and its speed of rotation is
proportional to the volumetric flow rate. Turbine rotation can be detected by solid state devices or by mechanical sensors. Turbine flowmeters are commonly used to track total fuel gas (e.g. natural gas) consumption used by a commercial or industrial facility for billing.