Design and validation of a calibrated orifice meter for sub-500 liter per minute flow rate applications

Accurate and reliable ventilation rate is an essential input and can be a major source of uncertainty in gas emission studies applied to controlled livestock systems. High flow and low operating pressures in animal emission research applications present a unique and costly challenge for volumetric flow measuring instruments. Orifice meters are differential pressure flow meters that provide accurate, economical, and reliable flow measurement and may be found in many industrial processes and engineering applications. Six orifice meters were designed and fabricated following ASHRAE and ASME guidance for flow measurement to provide a maximum of 510 lpm based on ventilation requirements for beef cattle in open circuit respiration chambers. The orifice meters were constructed with a 50.8 mm (2 in.) diameter PVC pipe for the entry and exit sections, a 3.175 mm thick acrylic orifice plate with a 20.6 mm diameter concentric beveled orifice, and four flange taps on the upstream and downstream sides of the orifice plate. A calibration reference was constructed with a wind chamber and nozzle using the same components of ventilation system. Concurrent differential pressure measurements across the calibration reference and the orifice meter were recorded and the corresponding flow rates were calculated using the differential pressures, environmental conditions, and design parameters of each instrument for flow rates from 281 to 510 lpm. Regression analysis showed a linear relationship between the calibration reference and orifice meter regression demonstrated that each orifice meter required an individual calibration curve for best accuracy. Linear regression coefficients (standard error of coefficient) varied from 0.09244 (2.48 E-03) to 1.0699 (2.50 E-03) for all six orifice meters. The standard error of each regression provides an idea of the overall uncertainty over the range of calibration. At a nominal 481 lpm the relative error was from 0.02% to 0.04%. Custom designed, constructed, and, calibrated orifice meters are accurate and cost effective for volumetric ventilation rate measurement in livestock calorimetry and emission studies.