EVOM3
World Precision Instruments
43
Fig. 54—Electrodes in solution
Unit Area Resistance
As the resistance is inversely proportional to the area of the tissue, instead of
reporting resistance, typically the product of the resistance and the area is calculated
and reported. The unit area resistance is independent of the area of the membrane
used and may be used to compare data obtained from inserts of different sizes.
NOTE
: Resistance readings for 24 mm or larger diameter inserts obtained by using
the
EVOM3
with the STX electrodes should not be converted to unit area resistance.
The EndOhm is recommended for these larger inserts. The unit area resistance is
obtained by multiplying the meter readings by the effective surface area of the filter
membrane. The dimension is Ω-cm
2
. The resistance is inversely proportional to the
surface area. Thus, the larger the membrane, the lower the resistance.
Resistance of a unit area = Resistance (Ω) x Effective Membrane Area* (cm
2
)
* See manufacturing specifications for the particular insert
Unit Area = 1 cm
2
The unit area resistance is independent of the area of the membrane used and may
be used to compare data obtained from inserts of different sizes.
Continuing with the previous example, in which the R
true tissue
= 670 Ω, if an effective
membrane diameter (d) was 6.5 mm, the unit area resistance would be:
Resistance x Effective Membrane Area = 670 Ω * π r
2
= 670 Ω * 3.14* 3.25* 3.25 / 100
= 670 Ω * 0.331 cm
2
= 221.77 Ω-cm
2
.
221.77 Ω is the resistance of a unit area of 1 cm
2
(TEER).
The larger the membrane, the lower the resistance. The dimension is Ω-cm
2
, not Ω / cm
2
.
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