Notational Conventions - Gamry Instruments IMX8 Operator's Manual

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Quick start manual (4 pages)

Table Of Contents

Table of Contents

Mode

Active

Open

Local Potentiostat

Shorted

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 IMX8. 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 operational amplifier in Figure 2-2 is wired as a simple potentiostat. With S3 closed, the

working electrode is connected to the ground on the IMX8 circuit board and the counter electrode is

connected to the output of an operational amplifier. 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.

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 generating ±30 mA at up to ±10 V.

The local potentiostat has no cell switch other than S3, and it cannot measure current.

Caution:

potential exceeds ±24 V versus the IMX8 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 16-bit

converter operating at a resolution of 152.59 µV per bit. The maximum voltage available is ±4.9 V.

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 IMX8 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.

Both the local potentiostat and shorted mode have voltage inaccuracies caused by the cell current. The working

electrode is nominally at zero volts versus earth ground. When cell current is present, resistance in relays, wires,

and connectors causes the voltage to differ from zero. In the shorted mode the counter lead also has resistance

that contributes voltage error. With a 1.5 m Gamry Instruments cell cable, the resistance in each cell lead

should be less than 0.2 Ω. At 30 mA of current, the voltage error in the local potentiostat mode can be as large

as 6 mV. In the shorted mode and at the same current, the total voltage error can be 12 mV.