Chemistry Reference
In-Depth Information
10.2
Description of the cell configuration
Owing to the perpendicular orientation of the electrodes, the electrode con-
figuration discussed in Chapter 9 cannot be used for the type of analysis
discussed in this section. Therefore, an alternative configuration needs to be
developed, which is much flatter and easy to use at the surface of the human
body or to implant in a nappy. This can be achieved by using two yarns
instead of two sheets, positioned linearly against each other (Fig. 10.1). In
practice, two yarns of stainless steel were used for this investigation. To
immobilise them for performing a characterisation study, these yarns were
put on a PVC sheet with some glue as schematically presented in Fig. 10.1.
According to the theory, this type of electrode generates errors because the
surfaces of these yarns are not flat; thus the local distance between the yarns
will vary from position to position giving rise to variation in local electrolyte
resistance. The actual resistance measured will be dependent on the mag-
nitude of the difference between highest and lowest local resistance and
therefore the resistance will be dependent on the diameter of the yarns and
their regular structures. However, this effect can be minimised by optimis-
ing the values for the distance between the electrodes (
d
) and the diame-
ter of the electrodes (2
r
). The difference (D
R
) between the highest and
lowest local impedance will decrease for decreasing yarn diameter and
increasing distance between the electrodes (Fig. 10.2). Therefore, it can be
assumed that for a certain maximum value of 2
r
/
d
, the system behaves like
a system with flat electrodes, because the errors generated caused by imper-
fections are smaller than the experimental error of the sensor system itself.
3
1
2
4
10.1
Scheme of the electrode configuration and the electrical
equivalent circuit: (1) electrode, (2) PVC sheet, (3) area protected
by glue and (4) electrolyte solution.
