WPI EVOM3 - Appendix E: Improving Accuracy and Repeatability

EVOM3
World Precision Instruments 39
APPENDIX D: 24 MM DIAMETER (6-WELL) INSERTS
Note that the resistance readings from 24 mm diameter tissue culture inserts (6-
well plates) obtained by using WPI’s STX series of electrodes will be 1.8 to 2.2 times
higher than that obtained using the ENDOHM-24G, because the STX electrode
cannot deliver a uniform current density over the relatively large membrane through
the small gap between the membrane and the bottom of the well. Therefore, the
resistance reading of a 24 mm (6-well) diameter insert obtained by STX electrodes
should not be used to calculate the unit area resistance. STX electrodes are only
intended  iding a alitatie meaement  ell mnlae health and
antitatie meaement  ell nene hi i nt a lem ith malle
inserts, such as the 12 mm and 6 mm diameter inserts, because of the relatively
smaller size of the membrane compared to the electrode. When the EVOM3 is used
with the EndOhm, it gives a repeatable and accurate value of membrane resistance
consistent with that obtained by using a well-designed Ussing chamber. It is also
consistent with that obtained by the STX electrode for the 12 mm and 6 mm diameter
filte inet
NOTE: The unit area resistance is independent of the area of the membrane and may
e ed t mae data tained m inet  dieent ie
For 6-well plates (24 mm inserts), the STX2-PLUS can be used to take readings at the
12 o’clock, 3 o’clock, 6 o’clok and 9 o’clock positions to look for low resistance points in
a  mm inet t tet  nene
APPENDIX E: IMPROVING THE ACCURACY AND
REPEATABILITY OF THE SYSTEM
Whenever possible, operate the EVOM3 meter with the battery charger disconnected.
This ensures maximum electrical isolation for the most accurate readings. When
the charger remains connected to the meter during measurements, electrical noise
can be introduced into the system. For the highest accuracy in measurement, WPI
recommends disconnecting the unit from the AC power source.
For 24-well or 12-well measurements the STX2-PLUS electrode is capable of making
precise measurements. The STX2-PLUS is designed primarily for 24-well inserts, and it
can be used with 24-well HTS plates* and 12-well inserts* with good results as long as
the placement is consistent. Consistent electrode placement and mechanical stability
ae eential  nitent eitane elt  t ee the id leel and id
temperatures constant. The use of a warming plate can add to measurement items
and reduce CO
2
out gassing during readings. CO
2
out gassing can change the pH of
the media and sharply increase a resistance reading. See "Tips and Advice" on page
6 and the notes on page 6.
*For 24-well HTS plates, keep the basal electrode centered in the basal access slot.
*For 12-well inserts, the STX2-PLUS will have to be hand held.