Reference Flow Measurement

Flowmetering terms can often seem cryptic. Here are definitions of some of the most commonly used. ACCURACY Accuracy is a quantity defining the limit that errors will not exceed. This value should include the combined effects of conformity, hysteresis, deadband, and repeatability errors. When applied to flowmeters, accuracy is specified in one of two different ways; % of full scale or % of rate. % OF FULL SCALE A % of full scale accuracy specification defines an expanding error envelope. For a flowmeter with a flow range of 0 to 100 gpm and accuracy of 1%, FS will read +/-1gpm anywhere in its operating range. This corresponds to +/-1% of rate at full scale and +/-10% of rate at 10% of full scale. % OF RATE A % of rate accuracy specification defines a constant error envelope. For the previous flowmeter with the same flow range and an accuracy of 1% of rate, the reading will be +/-1% of actual flowrate anywhere in the meters operating range. As a result, at full scale the reading will be +/-1 gpm and at 10 gpm it will be +/0.1 gpm. Figure 1 contains graphs that illustrate the difference in % of rate and % of span accuracy statements. REPEATABILITY Repeatability is the closeness of agreement between consecutive measurements of the same flow. This can also be specified as a % of full scale or a % of rate. RANGEABILITY Rangeability refers to the minimum and maximum measurable flowrates. For example, if the maximum flowrate is 100 gpm and the minimum flowrate is 10 gpm, the rangeability is 10% to 100%. TURNDOWN Turndown conveys the same information as rangeability but in a slightly different way. Turndown is the ratio of maximum flow to minimum flow. For the same flowmeter we used in the rangeability example, the turndown would be 100/10 to 1 or 10 to 1. VISCOSITY For a newtonian fluid, viscosity is the same as consistency and is the resistance offered by the fluid to deformation. Viscosity will be stated as either absolute or kinematic viscosity. Absolute viscosity is the fundamental viscosity measurement of a fluid and has units of Poises in the cgs system. Kinematic viscosity is equal to absolute viscosity divided by the fluid density and has units of Stokes in the cgs system. REYNOLDS NUMBER Reynolds Number is dimensionless quantity that is proportional to the ratio of inertia forces to viscous forces in a flow system. The proportional constant is the characteristic length of the system. For Pipe Reynolds Number, the characteristic length is the pipe diameter. Reynolds Number is a convenient parameter by which to compare different flow systems. Pipe Reynolds Number can be found by using the following equations: R = [DV/v]ρ or [DV /u] where R = Reynolds Number D = inside pipe diameter (ft or m) V = fluid velocity (ft/s or m/s) v = kinematic viscosity (ft2/s or stokes) ρ = fluid density (lb/ft3 or kg/m3) u = absolute viscosity (lb/ft s or poises) Many convenient hybrid forms of the Reynolds Number equation exist. One such equation for liquid is: R = C*Q(GPM) v(cSt) Where C is the constant from Table 1. Table 1 is valid for schedule 40, pipe.