WPI EVOM3 - Page 35

EVOM3
World Precision Instruments 31
Issue
Possible Cause Solution
Voltage reading is not zero in plain
culture media or blank cell
Electrode not
eiliated
The Ag/AgCl electrodes (inner voltage measurement
electrodes) on the probe may become polarized due to
an uneven distribution of ions between the electrode pair.
This ionic charge can be neutralized by immersing the
probe into 0.1M KCL for 24 hours with the probe elec-
trodes shorted electrically. The probe is shorted electri-
cally when it is connected to the meter with the power
OFF, and the FUNCTION switch in the OHMS position. See
"Probe Null" on page 14 for information on how to null
the electrodes.
Dirty electrode Clean the electrode. (See "Cleaning/Maintaining the
STX2-PLUS Electrodes" on page 26.)
Contamination
between
electrodes
Inspect the inter-electrode surface areas for material
which could form an insulative coating on the surface
of the inner and outer electrodes. If the material cannot
be removed, the electrode should be replaced. (See
"Cleaning/Maintaining the STX2-PLUS Electrodes" on
page 26.)
Corrosion has
formed on con-
ductive traces
An insulative transparent coating protects the copper
conductors that connect each electrode to the main
wiring in the handle. When the probe is very old, this
coating can deteriorate, allowing the salts in the media
solution to leech through to the copper conductors.
This is observed as a black discoloration on the copper
circuit traces underneath the conformal coating. If this
is observed, replace the electrode.
Resistance
reading un-
usually high
Contaminated
electrode con-
tacts
With use, the chopstick electrode probe contacts may
aie a ild  tein  the eign mateial
that eetiel ineae the aeline eitane  the
electrodes. The electrodes can be cleaned using various
methods. (See "Cleaning/Maintaining the STX2-PLUS
Electrodes" on page 26.)
Resistance reading unusually
low
Cell culture or
media problem
If the ell lte ha een gien ient time t ahiee
nene, and the eading i tale t ignifiantl
lower than expected, then the problem is probably related
to the cell culture. Electrode failure will not generally cause
a lower than expected yet stable reading. Use the test
resistor to verify the meter is functioning correctly. The
mete dila hld ead Ω ee ntment
Diagnostics and Calibration" on page 21, "Testing the
STX2-PLUS Electrodes" on page 28 or refer to the ap-
propriate electrode manual for instructions on testing
the electrode. See "APPENDIX E: Improving the Accuracy
and Repeatability of the System" on page 39.