Viscosity
In a turbine meter, the one factor that affects the linear range the greatest is viscosity . The skin friction (viscous affect of
the boundary layers) on the blades of the rotor and adjacent surfaces, is known to be a function of the Reynolds number
(a dimensionless parameter) . At a sufficiently low Reynolds number, the boundary layer is completely laminar . At a high
Reynolds number, the boundary layer is turbulent . In the transition region, there is a gradual change from laminar to
turbulent flow . At low viscosities, the Reynolds number is high, so that at the minimum operating frequency the flow is still
turbulent . As the viscosity is increased, the Reynolds number decreases and the meter (at the same minimum frequency) is
operating in the transition region . At this point, the drag actually decreases and the K-Factor (cycles per gallon) increases .
A further increase in viscosity and the Reynolds number decreases to a point where the flow is completely laminal and the
K-Factor decreases . In effect, as the viscosity increases, the range in which the flow is turbulent decreases . In low capacity flow
meters, the viscosity effect may be of such an order that the entire flow range will be in the laminar flow region .
Mounting for Calibration
Turbine flow meters are calibrated with the axis horizontal and the pickoff on top . Flow meters with ball bearings may be
mounted in any attitude with nil affect on the linearity range or calibration . Pipe configuration, such as valves, tees and
elbows immediately preceding the meter, can produce swirl in the fluid with erroneous results . A minimum of 10 diameters
of tubing the same size as the meter is recommended . For maximum precision, external flow straighteners are available for all
size meters .
Pressure Drop
Pressure drop across turbine flow meters is substantially constant for a given gravimetric flow rate, but varies in approximate
proportion to the square of the volumetric flow rate . This variation is proportional to a liquid’s density . The values shown
under range characteristics are based on a liquid specific gravity of 0 .760 and a viscosity of 1 centistoke .
Specific Gravity
Changes in the specific gravity of a liquid in a linear shift in gravimetric calibration can be plotted as a function of specific
gravity . These changes have no measurable effect on the volumetric flow rate but will cause a shift in the pressure drop across
the flow meter .
Pressure
Pressure changes have no measurable effect on volumetric flow rates .
Temperature
Large temperature changes cause an area change within the flow meter . Higher temperature will result in decreased fluid
velocity while depressed temperature will result in increased fluid velocity . This change will cause a variation of the K-Factor
that is supplied with the turbine flow meter . Turbine flow meters calibrated at one temperature and operated at another
require correction of their K-Factor . Cox Precision Turbine Flow Meters can operate from –350…500° F, and up to 800° F using
a special high temperature pickoff .
Associated Equipment
Electrical leads from the flow meter to remote associated equipment should be carefully chosen to be compatible with
the flow meter output and the impedance values of the components used . Distance between flow meter and associated
equipment is then a negligible factor . Use good quality coaxial cable or twisted pairs, with or without shielding, as required
by environmental factors . If a shielded lead is required, it must not be grounded at the flow meter since neither pin of the
standard pickoff is grounded . Ground at some other point to eliminate ground loops in the associated equipment .
Filtration
Filtration is recommended as follows:
• LoFlo meters, meter sizes 84 through 08, and flange sizes 84, 86, 8 and 10 flow meters should have filters with a rating of
25…40 microns .
• Size 10 through 32 and flange sizes 12 through 48 flow meters should have filters with a rating of 40…75 microns .
Precision Meters, Turbine Flow Meters
Page 22
May 2014
