Design and evaluation of open-circuit respiration chambers for beef cattle

Methane (CH4) generated by cattle is a major source of greenhouse gas emissions and an indicator of feed efficiency; thus, accurate and confident quantification of CH4 production is required for addressing future global agricultural requirements without the neglect of environmental impacts. One of the most common techniques for quantifying CH4 emissions from cattle is the chamber technique, known as the respiration chamber or indirect calorimeter, which houses the head or the whole body of the animal in a chamber. Psychrometric properties and gas concentration measurements at the inlet and exhaust of the chamber, in conjunction with fresh air flowrate, are used to calculate the Emission Rate (ER) of CH4 produced by the animal inside the chamber. Reliability and accuracy of estimated ERs with the chamber technique is a concern and often only verified through release-recovery methods, but should also include documentation and quantification of measurement uncertainty. Research with the chamber technique is primarily focused on the application for understanding relationships between ruminant CH4 emissions and nutrition, genetics, and rumen biology. There is limited work regarding estimates of confidence in computed CH4 emissions that goes into understanding these relationships. Individual measurements as well as the integrity of the system introduce uncertainty that affects the computed ER; therefore, confidence in the results obtained by the chamber technique can be described through an evaluation of each source of uncertainty and the release-recovery test. Identifying and quantifying the uncertainty from each source and its contribution to the overall uncertainty associated with ER will lead to greater confidence in the results of CH4 emission research and provide insight to which measurements are most critical. The overall aim of this work was to document the design, including construction and uncertainty analysis, and evaluate performance through bias significance testing for a version of the chamber technique named the Ruminant Emission Measurement System (REMS). The REMS was developed to quantify eructated CH4 emissions from beef cattle and consists of six positively pressured, ventilated hood-type, open-circuit respiration chambers installed at the University of Illinois Urbana-Champaign Beef and Sheep Field Research Unit. The design and construction documentation for each chamber included specifications for the thermal environmental control subsystem, fresh air supply and measurement subsystem, and gas sampling subsystem. Measurement standard uncertainty was quantified and propagated through the computation of ER, which was derived from mass flow balances on CH4 and air. A sensitivity analysis simulated the normal operation of the REMS and variations in gas analyzer and ventilation measurement uncertainties to assess the relative contributions of each source of uncertainty to the combined ER standard uncertainty. Expanded uncertainty (~95% confidence level) associated with ER was approximately 5.9% for CH4 emission rates between 3.5 to 17.2 g h-1. Ventilation rate and concentration measurements contributed approximately 69% and 29% to the ER standard uncertainty, respectively. The ER standard uncertainty provides insight to the sensitivity required to detect differences between computed ERs. The fresh air supply measurement was separately evaluated and consisted of six orifice meters designed and fabricated for accurate measurement fresh airflow to each chamber. Calibration of each orifice meter was completed using a reference comprised of two chambers with a precision nozzle for flowrates from 279 to 510 lpm (9.9 to 18 cfm). Regression analysis showed a linear relationship with slope significantly different from unity (P < 0.05) between the calibration reference and orifice meter, demonstrating that each orifice meter required an individual calibration for best accuracy. At a nominal 500 lpm (17.65 cfm), the relative expanded uncertainty ranged from 3.6% to 4.9%. Custom designed, constructed, and calibrated orifice meters are accurate and cost effective (approximately $250 for materials plus 5 h of labor each) for volumetric ventilation rate measurement in animal emission studies. The commissioning of the REMS included incorporating the propagation of measurement uncertainties, systematic errors, and the variability in repeated release-recovery tests. In addition, each subsystem was evaluated to ensure design criteria were met. A whole system verification experiment (mass recovery test) compared the mass flow recovered by the REMS to the total mass flow injected by a reference integrated over the steady-state regime. An uncertainty analysis was applied to the mass recovery test, including eight replications of the test over time, to establish the confidence in the release-recovery method. Mean mass recovery percent for the six chambers ranged from 92.0% to 96.6%, with SSMRP absolute expanded uncertainties (~95% confidence interval) ranging from 10.4% to 13%. Mass recovered uncertainty contributed from 70.1% to 90.7% to SSMRP uncertainty, mass injected uncertainty contributed from 2.5% to 4.0%, and reproducibility contributed from 5.6% to 27.3%. Significant (P < 0.05) SSMRP systematic error was detected for five of the six chambers; therefore, emissions measured with these chambers should be corrected for bias following the methods and guidelines presented here. Mass recovery rates should include a documented stated standard uncertainty to establish a confidence level for whole system verification and subsequent emission rate measurements, as demonstrated in the discussed commissioning results. The documented methodology for the design and evaluation of the REMS fulfills the need for establishing the best estimate of confidence in accumulated emissions reported from the chamber technique. The analysis described here, although specific to the REMS, can be applied to quantify confidence and performance of other respiration systems or indirect calorimeter. Reliable estimates of uncertainty associated with the chamber technique will lead to development of improved understanding of the relationships between CH4 production and nutrition, genetics, and different management strategies