Inferential (non PD) Meters
Inferential meters operate on the basis of deducing total volumetric flow rate by measuring some property of the flow stream. There are two basic types of inferential meters, turbine and orifice.
In a turbine, the basic concept is that a meter is manufactured with a known cross sectional area. A rotor is then istalled inside the meter with its blades axial to the product flow. When the product passes the rotor blades, they impart an angular velocity to the blades and therefore to the rotor. This angular velocity is directly proportional to the total volumetric flow rate.
In a turbine (inferential) meter, the product to be measured is pumped through the meter where it drives a turbine suspended within the flow. The rotation of the turbine is proportional to the volume passed through it for a given set of temperature, pressure and flow rate values. As these values change, so too can the proportional rotational value of the meter. For these reasons, the output of the turbine meter is usually coupled to an electronic indicating meter which then makes the necessary corrections and outputs a volume indication. Turbine meters are best suited to large, sustained flows as they are susceptible to start/stop errors as well as errors caused by unsteady flow states.
Turbine meters are very susceptible to flow disturbances. For this reason, they are usually installed in a meter run with flow straighteners to help control swirl (+/- erors depending on direction), jetting (+ errors) and pulsations (+ errors).
See Other Meters for more information on Orifice Meters.