Gamry ECM8, Operator'S Manual

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Chapter 2 -- Introduction--Switching and Inactive Cell Control
2 - 6
You have a choice of 3 different control schemes for inactive cells. These are tabulated in Table 2-1 along
with the active mode.
Table 2-1
ECM8 Control Modes
The inactive control mode labeled "open" is the simplest. In this mode the cell is disconnected and isolated
from both the system potentiostat and the ECM8. No cell current can flow between any electrodes. This is
the most common inactive mode in corrosion studies, where the cell behavior at open circuit is of interest.
The inactive control mode labeled "local potentiostat" is used to maintain potential control between
readings made with the system potentiostat. If you know something about operational amplifiers and
potentiostats, you may recognize that the op amp in Figure 2-2 is wired as a simple potentiostat. With S3
closed, the working electrode is connected to the ground on the ECM8 circuit board and the counter
electrode is connected to the output of an op amp. The feedback for this amplifier is taken from the cell's
reference electrode. This mode can be used whenever it is important to maintain potential control
between readings. One experimental example is tests on amperometric chemical sensors.
NOTE: The ECM8's local potentiostat can be disabled via jumpers inside the ECM8 chassis. This
is done to allow operation with cells that contain a grounded electrode. See Appendix E for
details. If this has been done, the local potentiostats will not work.
Unlike a typical system potentiostat, the local potentiostat is very limited in capability. The amplifier in this
local potentiostat is not a large power amplifier. Its output is typically capable of outputting  30 milliamps
at up to  12 volts. The local potentiostat has no cell switch other than S3 and it cannot measure current.
Notice that the reference electrode is always connected to the local potentiostat amplifier. You must make
sure that the reference electrode potential does not go beyond  12 volts with respect to the ECM8 chassis
voltage. This is true in all operating modes including the active mode. You can damage the local
potentiostat's amplifier if the reference electrode potential exceeds  24 volts versus the ECM8 chassis.
The cell can be potentiostated at a voltage that is set via the D/A converter seen in Figure 2-2. This is a 12
bit converter operating at a resolution of 2.5 mV per bit. The maximum voltage available is  5 volts.
Notice that the D/A voltage is also brought out to a rear panel monitor point. The monitor point for each
channel is a BNC connector labeled "D/A Monitor" on the panel. The polarity of this monitor point is
inverted with respect to the local potentiostat's voltage. If you set the potentiostat for +1.2 volts (working
versus reference electrode potential) the rear panel monitor will show a voltage of -1.2 volts.
The final inactive mode is label "shorted" in Table 2-1. In this mode a relay shorts the working and counter
electrodes. This mode was added so that the ECM8 could be used in galvanic corrosion studies. In a
galvanic corrosion experiment the current flow between two electrically connected metals is measured.
The device that makes this measurement is called a zero resistance ammeter (ZRA).
In the shorted inactive mode the two metals are connected even when the system ZRA is measuring other
channels. The shorted mode is subject to some errors at high currents due to the resistance in relays, wires
and connectors. With a 1.5 m Gamry Instruments cell cable, the resistance between the two electrodes
Mode S1 S3 S4
Active Closed Open Open
Open Open Open Open
Local Potentiostat Open Closed Open
Shorted Open Open Closed