WPI EVOM - Theory of Operation

10 World Precision Instruments
The back panel has the following connection ports and switches:
• Foot Switch Port – Plug the foot switch (WPI #13142) into this port for a simple,
hands-free way to take measurements.
• USB Port for Secure Data Transfer to PC – This USB type B port is used for
secure data transfer. Use the included USB cable (WPI #503535) to connect the
EVOM™ Manual to your PC. The type A end plugs into a port on the computer,
and the type B end plugs into this port on the EVOM™ Manual.
• Programming Button – FOR FACTORY USE ONLY. Do NOT push this button
without having specic instructions from WPI.
• Power Connection Port – Plug the A/C power cord (WPI #803025) into this port
to charge your EVOM™ Manual.
CAUTION: Always connect power when the warning messages appear. NEVER
let the battery get completely discharged, or it may not charge again. See
"Power Level Messages" on page 32.
• Power Switch – Toggle this switch to turn the unit o (O) or on (|).
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 Ohm's 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 osets due
to the membrane potential or from the electrodes. The amount of current and voltage
is low (2, 4 or 10µA) to minimize any tissue stimulation and to prevent the migration
of metal ions.