Calibration of flowmeters for use in HVAC systems monitoring
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Affordable multi-channel, microprocessor-based data loggers are beginning to make it possible for engineers and building operators to cost effectively monitor energy savings from HVAC retrofits. Affordable thermal energy monitoring, requiring bulk fluid velocity (flow) measurements, is usually required for an accurate assessment of retrofit savings. This paper reports on efforts to establish a full-scale dynamic-weight liquid flowloop for measuring the accuracy of flowmeters in pipe sizes ranging from 7.6 to 30.5 cm (3 to 12 inches) in diameter, with varying flow velocities up to 3.05 m/s (10 ft/s). Preliminary tests indicate that magnetic-type paddlewheel insertion flowmeters appear to be reasonably accurate for fluid velocities above 0.91 m/s (3 ft/s), but tend to significantly overstate velocities below 0.91 m/s. Non-linear, velocity-dependent corrections will probably need to be developed for each combination of sensor-transducer, pipe size, and insertion depth in order to obtain higher accuracy thermal energy usage measurements. The preliminary results also have several important implications for building researchers. First, both the fluid velocity (flow) and thermal usage (i.e., kJ or Btu) readings should be recorded so that post-calibration can be applied to the thermal usage measurements. Second, fluid velocities should be ascertained prior to the selection of the flowmeter (for example by using an ultrasound meter). When making fluid velocity measurements in pipes where the velocity is less than 0.91 m/s, magnetic paddlewheel flowmeters are not the best choice. In the worst cases, metering legs (with reduced diameter piping) may be required in cases where velocities are below 0.91 m/s range (0 to 3 ft/s).
