Problems of calibration and stabilization of tcPO2 electrodes.

The stirring effect factor phi, used to correct tcPO2 readings for gradient of PO2 induced in skin by electrode O2 consumption, was measured with 20 micrometer cathode electrodes at 44 degree C for various membrane, spacer and electrolyte combinations. Two in vitro models closely resemble skin phi values: (1) 50% ethylenge glycol/water equilibrated with air at 44 degree C, compared to air above this liquid; (2) a mock skin consisting of 25 micrometer Telfon on foam rubber, in air, dry. phi varied from 1.01 with 22 micrometer polypropylene over a 12 micrometer cellophane spacer, with either aqeous or non-aqueous, chloride-free alkaline electrolyte, to more than 1.10 when 25 micrometer Telfon was tested without a spacer. phi may be predicted: phi = 1 + 0.44 i/(M + 10C + 20) where i is pA/mmHg electrode sensitivity, and M and C are membrane and cellophane thickness, micrometer. Pressure sensitivity, tested agains foam rubber, was minimized by use of a spacer, and often increased with age of Telfon membrane. Drift may result from water vapour movements across membranes if calibration media have P(H2O) either too high or too low. Miminum drift on skin is also caused by a slow effect of PCO2 on reference electrode potential, and this is minimized by buffering electrolyte with K2B4O7.