The realized biological sensor and the pressure compensation system have been tested and evaluated by a group of representative users. The probe has been then used to measure currents emitted from different points of the body on various subjects. The measurements were done holding the probe in an upright position, at an angle of 80 degrees with the skin, with the tip in contact with the characteristic point to be tested. All the measurements shown that the pressure compensation system allows the operator to work on the measure point to a well defined and repetitive pressure obtaining reliable and repeatable measures that are virtually insensitive to fluctuations in pressure over a wide range of values.
Figure 7: Complete electronic scheme of the electronics interface.
The pressure compensation system parameters can be checked and modified by the user through the Graphical User Interface. Moreover the user can configure different kinds of current measurements, in a range of 30 to 3,000 nA, saving and displaying data in graphical or textual formats and generating a report . The computational unit calculates the standard deviation of the data coming from the acquisition circuit in a given number of samples. When the standard deviation is lower of a preset threshold, the current value is accepted and displayed in the results mask. A vocal synthesis system provides also an audio output of the results. The software package manages different kinds of current measurements, in a range of 30 to 3,000 nA, saving and displaying data in graphic or text formats. All data either in text or in graphics form are stored in a data base together with all the information related to the test session. The measured current and the pressure exerted on the probe during the measurement can be displayed as a function of the time in a specific pattern.
Figure 8: Displaying of the measured current and of the pressure exerted on the probe during the measurement.
In order to minimize the measurement error, the developed algorithm is able to automatically compensate the measurement as a function of the pressure exerted on the probe. As can be seen in Figure 8, thanks to the action of the compensation algorithm, the current (red line) remains unaffected by the large variations in the pressure exerted on the probe (blue line). One of the main issues has been the replicability of the measurements. To analyse this question the consistency of the currents emitted by different characteristic points over time for 10
minutes has been studied.
Table 1: Results of measuring for 10 times the currents emitted by point A.
Table 1 shows the results of measuring for 10 times the currents emitted by point A with a time step of 60 seconds. Point A is the point LU11 (Shao Shang) of the Lung Channel and is located on the hand on the radial side of the thenar eminence, 0.15 cm posterior to the nailbed. All the indicated currents are in nA and the table also provides the standard deviation (SD) and the mean value (MV). Figure 9 illustrates the
standardized values of the current (in p.u.) showing that no significant change in current amplitude was found during the 10 minutes period with regard to the point or its averaged value.
Figure 9: Standardized values of the currents emitted by point A every 60 seconds.