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Abstract

An in situ probe failure detection method for probes which deliver an electric signal on account of a change of the concentration of a substance to be measured. An example of such a probe failure may be caused by, for example, fouling of the membrane as the relative change in signal may be interpreted as a change in the concentration of the substance to be measured. More particularly, the method involves detecting the need for calibrating probes, which probes deliver an electric signal on account of a change of the concentration of a substance to be measured, characterized by: (a) subjecting the probe to a stepwise change of the concentration of the substance to be measured; (b) calculating the value of the probe response time constant t.sub.c of the obtained measurement curve with the help of the following main equation (1): t.sub.c dy/dt=-y+.alpha..sub.0 +.alpha..sub.1 t wherein t=time (time), t.sub.c =probe response time constant (time), y=substrate probe signal (mass volume.sup.-1), .alpha..sub.0 =substrate concentration at start of response (mass volume.sup.-1), .alpha..sub.1 =rate of change of substrate concentration (mass volume.sup.-1 time.sup.-1), and (c) comparing the calculated t.sub.c values of at least two different measurements with each other, and in case the calculated t.sub.c values do not match, calibrating the probe.

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