A Volt Meter
A volt-ohm meter may apply a constant voltage of unknown current across
the membrane and damage the cells electrically and also leave a chemical
imbalance in the electrodes. WPI experimented with two volt-ohm meters:
- An expensive Fluke meter applies a DC voltage of 50 to 80mV, which
(in a 1000Ω membrane) will pass 80µA. With a 200Ω membrane, this is
- A discount meter puts out 500mV DC, which will pass 500µA through the membrane. With a 200Ω membrane, this is 2.5mA.
Volt Meter Charges or Electrocutes Cells
When you use a volt ohm meter to test a cell layer, the Ag/AgCl electrodes accumulate an unbalanced
chemical charge. If the cells are not electrocuted in the process, they
also accumulate a charge. The unbalanced electrodes can now act as a
source of voltage and current that has to be overcome to make an
If a volt ohm meter was employed in a TEER (Trans Epithelial
Electric Resistance) measurement and left measuring the cells for more
than a few seconds, then the TEER measurement value would probably drift
downwards as the cells and electrodes are changed by the voltage and
current applied. In a perfect world, a measuring device minimizes its impact on what it is attempting to measure. These meters are best left to
measuring fixed resistors and circuits.
EVOM - Perfect for TEER
The EVOM² passes a constant current of 10µA through membrane and
reverses the polarity 12.5 times per second so that it does not leave a charge
behind on either the electrodes or the membrane. The voltage that is
present at 10µA on 1000Ω is 10mV. (On a side note, even this is too
much for some tissues, like retinal.) Typically, this value on a 200Ω
membrane is 50µV. At either setting, that is much less energy to
dissipate in the membrane or to charge the electrodes.
The EVOM passes the 12.5Hz constant current 10µA
signal through the two current electrodes (I1 and I2) on the STX
through the membrane. The companion electrodes (V1 and V2) measure the
voltage that was required to reach the 10µA current and sends this to
the processor. The processor converts this to ohms via Ohm's Law (E=I/R)
and displays the signal on the digital meter. Since the current is fixed
at 10µA, the processor can easily convert the measurement.
Built in Averaging
The EVOM and EVOM² both have averaging built in so that spurious
readings are “sampled out.” The EVOM² has a recording output connector
so that it can be fed into a data recorder. The EVOM system requires the
use of a conductive liquid to make the measurement. If the STX
electrodes are in non-conductive air, the reading is invalid. The older
model had an alarm in it to indicate a broken electrode or measurements
in air or non-conductive media.
Electrode Stabilizing for Balancing
The EVOM² has an electrode stabilizing function built into
the meter. When the meter is powered off, the V1 and V2 electrodes are
short-circuited together so that if the researcher places them in a
conductive liquid, then those electrodes are balanced at 0mV. That is,
they are “equilibrated.” This electrode balancing is critical to
membrane potential measurements. It is not as critical with TEER
measurements, because the EVOM electronics are specifically designed to
compensate for electrode imbalances. The EVOM and EVOM² are recognized worldwide as a standard measuring devices for this science.
The EVOM² qualitatively measures cell monolayer health and quantitatively measures cell confluence. The EVOM² is best suited for TEER measurement and
monolayer confluence detection.