Design and Validation of a Precision Orifice Meter for Ventilation Rate Control in Open-Circuit Respiration Chambers

Abstract

Abstract. Accurate and reliable measurement of ventilation rate is an essential input and can be a major source of uncertainty in animal emission studies. Open-circuit respiration chambers and calorimeters requiring low ventilation rates operated at low static pressures present a unique and potentially costly challenge for volumetric flowrate measurement. Orifice meters, when properly calibrated, are differential pressure flowmeters that provide accurate, economical, and reliable volumetric flow measurement and are found in many industrial processes and engineering applications. In this study, orifice meters were designed and fabricated following ASHRAE and ASME guidelines for flow measurement to accurately provide ventilation to six environmentally controlled respiration chambers. The orifice meters were constructed with 50.8 mm (2 in.) diameter PVC pipe for the entry and exit sections, a 3.175 mm (1/8 in.) thick acrylic orifice plate with a 20.6 mm (13/16 in.) diameter concentric beveled orifice, and four pressure flange taps on both the upstream and downstream sides of the orifice plate. Individual calibration of each orifice meter was completed using a reference system comprised of two sections with a nozzle for flowrates from 279 to 510 lpm (9.9 to 18 cfm). The calibration procedure described here is unique because it was conducted under the low static pressure operation used in the respirations chambers. Concurrent differential pressure measurements across the calibration reference and the orifice meter were recorded, and the corresponding flowrates were calculated using the differential pressures and orifice flow theory. A unique method of calibration was developed. Analysis showed that the bias (non-unity slope) was significant, distinct for each fabricated unit, and eliminated by this calibration method. Linear regression coefficients (standard error of coefficient; ±SE) varied from 0.92 (±2.48 E-03) to 1.07 (±2.50 E-03) for the six orifice meter calibrations. At a nominal 500 lpm (17.65 cfm), the relative standard uncertainty with an applied coverage factor of 2 (approximately 95% CI) ranged from 3.6% to 4.9% across the six orifice meters. Custom designed, constructed, and calibrated orifice meters can be accurate and cost effective (approximately $250 including 5 h of labor each) for volumetric ventilation rate measurement in individual animal emission measurement systems.