Frans_M_Everaerts_Isotachophoresis_378342.pdf

CONDUCTIVITY DETECTION 149
N.Y., U.S.A.). The lowest position of both of these dials corresponds with an output
voltage of ICs and amplification of IC4 of zero. The circuit with IC1 represents the
comparator. The 1-pF capacitor and the 1000-kQ resistor are included so as to ensure that
the oscillation of the oscillator formed by ICI , IC2 and IC3 is always guaranteed. The
47-kS2 resistor in series with the 47-pF capacitor prevents undesirable oscillation of ICI
during triggering.
Th~s circuit for resistance determination was developed for use with micro-sensing
electrodes. The volume of the conductivity cell is approximately 16 nl. The output
voltage IC1 is attenuated 11 or 110 times, depending on the ‘Range’ switch, if the ‘Range’
switch is in the open position, resistances can be measured between SO kS2 and 1 MQ,
while if it is closed, resistances can be measured between 1 and 10 MQ, less accurately
than in the open position. The proportions and construction of the transformer are
mainly determined by the measuring range chosen. If the inductance of the primary coil
is too high, the quality factor of the resonance circuit is too small, which is particularly
inconvenient if small resistances have to be measured. The measurement of these resistances
are no longer accurate because the square-wave voltage from IC, is fdtered badly.
If the inductance of the primary coil is too low, the transformer is already saturated if
the resistance between the micro-sensing electrodes is high. Consequently, the voltage
over the primary coil is low, if the resistance between the micro-sensing electrodes is low.
A high capacitance of the capacitor results in rapid saturation of the primary coil, but
a low capacitance makes the influence of parasitic capacitances too great.
Of many possibilities, a capacitor of 2.2 nF, a self-induction of the primary coil of
about 800 mL and a ratio of number of turns of unity were chosen. The resonance
frequency is then CQ. 4000 Hz and the quality factor is about 1, if the resistance to be
measured is of the order of 20 kQ. Thls value is acceptable for a sufficiently accurate
measurement of the resistance. The quality factor is, moreover, dependent on the quality
of the capacitor applied, and a mica capacitor is therefore recommended as the dielectric
losses are small.
It should be noted that the circuit does not work efficiently with resistances below
1 kQ, as the coupling of the coils can no longer be assumed to be unity. If one measures
a resistance that is, for example, a factor A smaller, the number of turns of the secondary
coil must decrease with a factor of t/x; the number of turns of the primary coil remains
unchanged.
The core material is P 36/22. 3B7 or 3H1, p, (permeability) = 2030. This potcore
is provided with a gap by applying a foil of insulating material between the two parts in
order to limit the temperature drift of the inductance. A potcore P 33/22, pe = 220,
can also be used; this is already provided with an air gap.
The micro-sensing electrodes can have a maximum potential of approximately 6 kV
towards earth, otherwise the leak current towards earth is intolerable. In order to attain
this value, the secondary coil must be well insulated, eg., by constructing both the
primary and secondary coils in a PTFE housing. A possible construction is shown in
Fig.6.19.
The extra PTFE ring is mounted around the secondary coil. The wires leaving the
PTFE via a small hole made in the PTFE are insulated with an extra PTFE narrow-bore
tube. This transformer must be mounted as near as possible to the micro-sensing
electrodes so as to diminish losses of electricity towards earth.