8 World Precision Instruments
INSTRUMENT DESCRIPTION
EVOM™ Manual Meter
Fig. 3—USB port, Electrode connection
Fig. 4—Foot switch, Future expansion, reset, power connection, power switch
Theory of Operation
The EVOM™ Manual system uses an electrode assembly made of four sub-
electrodes, two for current sourcing and two for voltage measurement.
The EVOM™ Manual passes a known constant current through the membrane on
two electrodes and measures the voltage needed to pass that current on the other
two electrodes and computes resistance using Ohms law. The polarity of the current
changes from positive current to negative current 12.5 times per second. This polarity
change avoids leaving a charge on the membrane and negates any voltage osets due
to the membrane potential or from the electrodes. The amount of current and voltage
is low (2, 4 or 10µA) to minimize any tissue stimulation and to prevent the migration
of metal ions.
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