For the correct evaluation of the physiological parameters of medical interest it is necessary that the measures are repeatable and reliable. We performed an analysis of the main error components that may influence the measurements:
- It is necessary to avoid the formation of spurious potentials that would contaminate our measurements. In particular, the measurement technique must not depend on the skin ph and on the internal resistance of the characteristic point. This can be achieved using electrodes that minimize the overpotentials, such as the silver/silver chloride ones.
- The tip of the probe must have a proper shape in order to minimize pain for the patient and to keep a costant contact area on the skin. The contact area will vary as a function of the pression exerted by the operator whereas the measured current will be a function of pressure. Therefore a pressure compensation system must be introduced such that measurements are virtually insensitive to fluctuations in pressure over a wide range of values.
- The current emitted by the characteristic point, when measured in a short circuit, decreases asymptotically from a peak value to a lower steady state value as a function of many physiological parameters and conditions. Measured data validation is therefore demanded to a software procedure to calculate the standard deviation of the datum in a given number of samples. When the standard deviation is lower of a preset threshold, the current value is accepted and displayed.