Blood-pH Optical Measurement: A Model to Compensate for the Effects of Temperature

Under physiological conditions, the human body maintains blood pH within [7.36, 7.44] pH. Small deviations from this range can reveal the onset of pathological states and worsen the patient’s condition. This article reports the performance analysis of a real-time, noninvasive pH-measuring system for extracorporeal circulation (ECC). In particular, this study focuses on the analysis of the effects that the measurand temperature may have on the error in estimating blood pH. Even if the blood temperature in ECC is often thermostated at 37 °C, there are treatments in which the blood temperature is varied by a few Celsius degrees, and the exploited measurement principle—fluorescence—is known to be affected by temperature. First, we verified that the temperature-induced error could exceed the maximum permissible measurement error of ±0.04 pH. Hence, a linear-correction factor for temperature compensation was proposed. The results obtained showed how the simple addition to the measuring system of a temperature sensor and the use of a linear-correction factor can effectively allow maintaining the measurement error within the ±0.04-pH range, even when the fluid—phosphate buffer saline (PBS) and blood—temperature is varied in the range [30 °C, 39 °C].