Improving the State-of-the-Art in Flow Measurements for Large-Scale Oxygen Consumption Calorimetry

The accuracy of the exhaust flow measurement contributes significantly to the uncertainty of calorimetry measurements for large fire testing. Less than ideal flow characteristics such as skewed velocity distributions are typical of these large-scale flows and make it difficult to achieve the desired measurement accuracy. Consensus standards for fire testing recommend either bi-directional probes or orifice plates to determine exhaust flow. Both have limited accuracy in the presence of less than ideal flow conditions. Averaging pitot probes are an off-the-shelf technology widely used to monitor flows for industrial processes. They have been utilized in a system of large fire calorimeters to demonstrate differences of less than 5% between heat release rate measurements by oxygen consumption calorimetry and the theoretical heat output from a gas burner. Differences exceeded 5% for a small set of conditions but were still less than 10%. Both levels of agreement are within the confirmation requirements of the consensus standards and were achieved without a system calibration as recommended by the standards. Including this technology as an alternate method to measure exhaust flow would be an improvement to relevant fire testing standards and to the overall accuracy of calorimetry measurements for large fire testing.