To realize the measuraments, a noninvasive probe has been designed. The probe is illustrated in Fig.7 and its electrode is a rod 2 mm in diameter made of casting of Ag-AgCl powder. The rod must minimize the skin pain to the patient during the measurements; to achieve this goal it has been decided to use a semisphere shaped rod instead of a cylindrical one because the last one could injure the skin at some probe-skin angles.
Figure 7. The realized probe
The contact surface will increase or decrease according to the pressure and pressure unbalance will introduce an error at a rate proportional to the unbalance itself. Also the contact resistance will be influenced by the pressure. It can be easily seen that an error of more than 100% of the measured current can be introduced for a pressure variation of about 300 grams. Assuming that the reliability of the measure must be as high as possible, the above mentioned error cannot be tolerated, hence a pressure compensation system must be introduced to make the measure almost insensitive to pressure changes for a wide span of values. It was then studied a pressure compensation system to allow the operator to push the probe on the measure point to a well defined and repetitive pressure.
Figure 8. Probe dimensions
The presence of a pressure sensor allows the operator to work in a linear range of pressures, namely between 100 and 250 grams, in order to get the contact surface variation in a linear range. The probe tip holder can slide in its guide for about 1mm to transmit the force on the pressure gauge, but at the same time, a perfect guide sealing must be provided in order to avoid the saline solution to penetrate the inner space of the housing of the probe. A sealing grease should have in any case very low friction, in order to assure a complete repositioning [reset] of the tip after measures. The residual pressure on the sensor must be at any rate checked by the application software before any current measure. Figure 8 shows the probe dimensions.