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Using the Model Membrance
Test Procedures
In the following testing procedures, allow a tolerance of ±1% on the readings taken. For
example, if the test response is indicated as 100 mV, a reading from 99.0 to 101.0 mV
would be within tolerance.
Offset Controls
Vm OFFSET (VOLTAGE ELECTRODE section): The full range of this control is ±200
mV. This can be verified by rotating the control first fully clockwise and then fully
counterclockwise while observing the VOLTAGE ELECTRODE METER. The displayed
readings will indicate off scale at the extremes of the control’s manipulation since the
meter is only capable of displaying ±199.9 mV.
Vm x10 output BNC (VOLTAGE ELECTRODE section): This output can be monitored using
an oscilloscope. The reported voltage will swing between ±2 V as the Vm OFFSET control
is manipulated throughout its full range. Set the Vm OFFSET to 0.0 reading on the meter
and verify that the Vm x10 reading on the scope also reads 0 V.
Ve OFFSET control (CURRENT ELECTRODE section): The Ve OFFSET control is tested
in the same manner as the Vm OFFSET control. This control is adjustable when the
CLAMP MODE switch is set to off. The voltage of the Ve OFFSET is read from the BATH
ELECTRODE METER when the CLAMP MODE switch is set to off.
Ve x10 output BNC: This BNC is located on the rear panel of the instrument and reports
the setting of the Ve OFFSET control when the CLAMP MODE switch is set to off.
Return all controls to their initial settings when done with this test.
Voltage Electrode Test
In actual practice, the voltage electrode test is used prior to entering the cell and
indicates the resistance of the electrode. When used in conjunction with the model cell,
it measures both the electrode and membrane resistance (2 MΩ). This test is performed
using controls in the VOLTAGE ELECTRODE section.
With the model cell in place, depress the Vm ELECTRODE TEST pushbutton and observe
a reading of 20 mV on the VOLTAGE ELECTRODE METER. This corresponds to a 2 MΩ
reading at a calibrated response of 10 mV/MΩ.
On the oscilloscope, the Vm x10 output will read 200 mV which is x10 the applied test
voltage. Since the test current is being passed through 1 MΩ (Rm), the I MONITOR
output will indicate 1 mV, which corresponds to 10 nA of current.
With the CLAMP MODE switch set to off, the BATH ELECTRODE METER monitors Ve
(voltage at the current electrode). In this case, Ve will be a measure of the voltage across
Rm and the meter will indicate 10 mV (1 MΩ). The Ve x10 output BNC on the rear panel
can also be checked to see that it reads 100 mV (meter reading x10).
