Integrity Assessment of Open-Circuit Respiration Chambers for Ruminant Animal Indirect Calorimetry

Abstract

Abstract. Open-circuit chambers have been widely applied to gas exchange measurement for indirect calorimetry and greenhouse gas emission. The reliability of these systems is crucial and is commonly evaluated through recovery tests. These tests simulate gas exchanges of animals through the injection or consumption of known quantities of gases, while monitoring the gas flux with the measurement system, and then comparing the difference between the known mass and the measured mass to assess the system integrity. Although the recovery test approach is widely accepted, the method of how to choose recovery tests has rarely been documented, and no single source has been found that summarizes a complete process for evaluating a system and correcting post-evaluation systematic errors. In this study, three recovery methods were applied: (1) the alcohol combustion method (ACM) burns a known amount of pure ethanol to simulate both consumption (O2) and production (CO2); (2) the gravimetric gas injection method (GRAV) directly measures the weight change of a compressed CO2 gas cylinder during the injection process; and (3) the constant gas injection method (CGIM) steadily injects sulfur hexafluoride (SF6) gas into the chamber. An experiment was conducted that included eight repeated trials for each of the three methods in six identical chambers (144 total tests) of the Ruminant Emission Measurement System (REMS) at the Beef Cattle and Sheep Field Laboratory at the University of Illinois in Urbana-Champaign. The recovery percentages of the respiratory quotient for the six ACM chambers varied from 72.58% to 77.76%, indicating that measurement and calculation errors occurred in the system. The CGIM and GRAV methods were used for identifying the errors from ventilation measurement and calculation of exhaust airflow rate. Correction factors were generated from the results and proved effective, as recovery percentages were improved to an acceptable level (100% ±5%) for all chambers. This approach can be applied to other studies and other systems to assist in selecting appropriate integrity tests and applying correction methods. Keywords: Cattle, Emissions, Gas exchange, Metabolism, Recovery method, Tracer gas measurement.