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World Precision Instruments
APPENDIX F: RESISTANCE CALCULATIONS
Resistance
The value of the blank (control well) always adds to the total resistance measured
across a tissue culture membrane. The blank resistance must be measured and then
subtracted from the resistance reading across the tissue in order to obtain the true
tissue resistance. For example, suppose the resistance through a 0.15 M KCl solution
and across the membrane support (with no tissue present) of a 12-well cell culture
insert measures 130 Ω. This is the blank or control reading for that cell culture insert.
(Resistance may vary for culture cups made by other manufacturers.) In this example,
using 800 Ω as the sample measurement, the calculated resistance for the tissue itself
(R tissue) is:
R
Total
= 800 Ω
R
blank
= 130 Ω
R
blank
+ R
true tissue
= R
Total
R
true tissue
= R
Total
- R
blank
R
true tissue
= 800 Ω - 130 Ω = 670 Ω
Resistance Value of the “Blank” Insert
When using an
EVOM3
with an
STX2-PLUS
to measure a blank insert, the resistance
value is typically between 60 to 180 Ω, depending on the specific brand of the insert,
and lower in EndOhms. This is not background resistance due to the resistance of the
blank filter. Rather, if the filter membrane is removed from the insert, the resistance
reading of the insert will remain nearly the same, because the background resistance
reading is due mainly to the electrode gap, fluid resistance, the small gap between
the bottom of the cell culture insert and the bottom of the cell culture plate. See Fig.
54. For a Millipore insert this gap is about 1 mm, with some insert brands having a
slightly larger gap than others. The variation in this gap is the cause of the difference
between blank readings of different brands. The smaller the gap, the higher the
electric resistance. The resistance of the filter membrane itself is actually negligible.
If an
ENDOHM-24G-SNAP
or
ENDOHM-12G
chamber is used, the blank resistance
becomes near zero, because the external electrode is directly underneath the filter
and the gap does not exist.
Summary of Contents for EVOM3
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