WPI EVOM - Page 43

EVOM™ Manual
World Precision Instruments 39
Issue
Possible Cause Solution
The reading appears to be
drifting* (see the drift denition
below).
The electrodes
are coated with
deposits from the
use of media or
buer.
The electrodes require enzymatic cleaning.
In a 5% CO
2
environment, a
loss of CO
2
causes
the media pH to
change, and the
resistance reading
may increase
sharply.
A 5% CO
2
environment can help in reducing pH media
changes.
The electrode
reading is unstable.
(See the stability
denition below).
The electrodes are
coated.
The electrodes require enzymatic cleaning.
Radio frequency
interference
Turn o or move any cellular phones farther away from the
experimental setup.
---- appears on
the screen
Resistance reading
is out of range
An out of resistance range will display ---- on the screen. Adjust
the range to correct the error. A saved well will read a space
(“ ") if the display is out of range. Go to Setup , Mode Units and
increase the resistance range. See page 11.
NOTE: The EVOM™ Manual will read only about 5% above
100K Ω.
NOTE: If you have a problem/issue with that falls outside the denitions of this
troubleshooting section, contact the WPI Technical Support team at (941) 371-1003 or
wpi-ts@wpiinc.com.
*Drift–Readings that continuously increase or decreases a signicant value (either
voltage or resistance) over time. Example: At 1000 Ω the reading is increasing 100
Ω/minute. (A drift of 10 Ω/minute is acceptable.) Excessive drift may be caused by
unwanted deposits on the electrode, pH changes or temperature variance.
Instability–At 500 Ω, the reading jumps from 450 to 550 Ω and will not settle down (an
instability ±5 Ω is acceptable in the 500 Ω range. In the higher ranges, up to ±1000Ω is
acceptable at the 100K range. Electrodes showing instability may be coated with media
or buer constituents and may require enzymatic cleaning.