The disclosed concept maintains that Qp=Qc\u2212Qleak, where, Qp is the estimated patient flow, Qleak is the estimated leak and Qc is the measured total circuit flow. The data utilizing apparatus is configured to send an identification information-sending request to the data providing apparatus, output identification information sent from the data providing apparatus, send the identification information to the data providing apparatus, and utilize data sent from the data providing apparatus. The SVF may be a function of interest rates derived from a yield curve.