Depending on the accuracy requirements and flow conditions, primary standard flow meter calibration techniques alone may meet the application requirements. When accuracies to ±0.012% reading are needed, advanced flow electronics and calibration techniques can be used to provide volumetric or inferred mass flow outputs.

The combination of a precision turbine flow meter and advanced flow electronics provides a solution for measuring bi-directional flow in hydraulic and pneumatic testing. Flow measurement performance is optimized by compensating for viscosity changes from variations in ambient fluid operating temperature, which affects the accuracy of liquid flow data.

In hydraulic test stand applications, a turbine flow meter with a bi-directional Universal Viscosity Curve calibration can be equipped with pickups for determining flow direction and sensing temperature. The turbine meter may be paired with a flow computer, which performs linearization and temperature compensation to ensure high accuracy over the meter’s full extended range. The unit’s electronics can be configured to display volumetric or mass with temperature compensation for variations in liquid viscosity and density due to temperature. Such flow computers simplify test procedures by storing temperature versus viscosity and density data for multiple fluids — saving time to boost productivity.

In order to correct for viscosity changes in the hydraulic fluid, the relationship of temperature vs. viscosity for the fluid and the operating temperature range of the measurement must be known. With the operating kinematic viscosity range known, the turbine flow meter is calibrated at multiple viscosities spanning the operating range. The flow meter will require a calibration for every factor of ten that the viscosity changes. hydraulic test stand applications

For example, a fluid with an operating range of 1 to 100 cSt would require a multiple point calibration at 1, 10, and 100 cSt. These data are then calibrated on a universal viscosity curve, which is formed by plotting the flow meter’s K factor on the linear scale and frequency divided by kinematic viscosity on the logarithmic scale. The result of this plot is one continuous curve covering the range of possible meter frequencies and viscosities.

Conclusion
Aircraft equipment manufacturers require state-of-the-art flow instrumentation for use in component test environments. Turbine flow meters have long been the choice for aerospace and defense industry testing. They are accurate, have a wide turndown ratio and fast speed of response, and, when paired with advanced flow electronics using universal viscosity calibration techniques, can be employed over a wide range of temperatures and viscosities.

Ladd Howell is aerospace applications engineer at Flow Technology Inc., Tempe, Ariz. For more information, visit www.ftimeters.com.

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