Inline dissolved-gas-derived oxygen saturation reference for dynamic blood phantoms at adult-head optical properties

Dynamic liquid blood phantoms are often used to evaluate performance of near-infrared spectroscopy (NIRS) oximeters, but require a continuous and independent method to determine the "true" oxygenation (SO₂), which serves as a reference. We developed IPCR (Inline PhysicoChemical Reference), which derives continuous SO₂ from the inline partial pressure of oxygen (pO₂), partial pressure of carbon dioxide (pCO₂), pH, and temperature sensors using the hemoglobin-oxygen dissociation curve. We achieved a median propagated uncertainty of 2.1%. We tested IPCR in blood phantom experiments. IPCR agreed well with the ISS OxiplexTS (FD-NIRS) at low optical load (RMSE 3.5-4.2%, CCC > 0.99). During eight deoxygenation runs (reduced scattering coefficient between 6.0-23.0 cm^-1, total hemoglobin concentration between 35-230 μM), both OxiplexTS and Masimo O3 (CW-NIRS) agreed well with IPCR at high SO₂, where they provided reliable outputs. The signal quality of both NIRS devices degraded at higher optical loads, while IPCR maintained a measurement range of ∼0-100%. We conclude that IPCR provides a continuous, optically independent physicochemical reference for dynamic blood-phantom experiments.