Characterizing inward leakage in a pressure-demand, self-contained breathing apparatus

An analytical model of the flow across a resistive flow path such as an orifice or pipe was applied to predict the inward leakage in the facepiece of a self-contained breathing apparatus (SCBA) during a steady below-ambient facepiece pressure. The model was used to estimate leakage rates with respect to the size of the leak and for below-ambient (negative) pressure conditions reflective of measured occurrences. Results of the model were also used to make quantitative estimates of the protection level of the respirator. Experiments were designed to induce a continuous below-ambient pressure inside the facepiece of a pressure-demand SCBA mounted on a headform. Negative facepiece pressure measured in the presence of a leak correlated with the measured particle concentration ratio. Results show that the analytical model generated reasonable estimates of leakage rates during conditions of negative pressure inside the facepiece. Thus, the analytical model performed well for constant flow conditions, demonstrating the capability to predict a momentary compromise in respirator protection during momentary negative facepiece pressure conditions.