Zero Resistance Ammeter

Zero Resistance Ammeter Manual

Introduction

The ACM Instruments Zero Resistance Ammeter is frequently referred to as a ZRA. It is a high performance laboratory instrument designed to allow ease of use in the experimental configuration. Its purpose is to measure the galvanic current flowing between two electrodes, WE1 and WE2, which act as if they are coupled by a zero resistance wire and to monitor the potential of the galvanic couple.

INDEX

1/ Specification.

Figure 1. Line drawing of a typical front panel.

2/ Description of the front panel connectors and switches.
2.1 RE
2.2 WE1
2.3 WE2
2.4 ISO RUN
2.5 Range change knob and ranges 1 to 7.
2.6 Under over range leds.
2.7 Current and potential meter.
2.8 Current / potential switch
2.9 Buffered current output.
2.10 Buffered potential output.

Figure 2 Schematic diagram of a Zero Resistance Ammeter in use.

3/ General use of Zero Resistance Ammeter

4/ Direction of current, electron and ionic flow.

5/ Additional features or aspects.

1/ Specification.

Mass 3.8 Kg
Dimensions cm 30.5 W, 11.8 H, 27.3 D
Maximum current output 300mA minimum 500mA max
Maximum measurement voltage 14 Volts
Potential between WE 1 and WE2 10µV
Buffered outputs 2 mA, 14V
Current and potential meter 3½ digit, u, m, A and V
Range Select Led's 50 mV under
2 V over
Supply 220 to 240, VAC 50 or 60Hz

Battery Power
Serial No
Fuse 500 mA 325mA

Current Range Counter Resistor Approximate Maximum Current (Slightly
lower for Battery Powered Versions)

1 10_ 325 mA
2 100_ 140 mA
3 1,000_ 14 mA
4 10,000_ 1.4 mA
5 100,000_ 140 µA
6 1,000,000 14 µA
7 10,000,000 1.4 µA

Accuracy of resistors 0.1% 3,4,5,
0.3% 6
1% 2,7
5% 1

2/ Description of the front panel connectors and switches.

See Figure 1 for reference

2.1 RE Connect this terminal to your reference electrode. The reference electrode, if used, should have a constant stable potential and not pollute the test cell with unwanted ions.

2.2 WE1 Connect this terminal to the first working electrode, WE1. A working electrode is one on which the test is performed. This terminal is connected to the instrument ground within the ZRA. Instrument ground is electrically isolated from mains ground. In order to reduce mains interference a 0.1 micro farad capacitor is used to couple the instrument ground to the mains ground. The WE1 terminal is internally connected to all black reference or measuring terminals.

2.3 WE2 Connect this terminal to the second working electrode, WE2, which is to be Galvanically coupled, through the ZRA, to WE1. This terminal is a virtual earth. In normal operating conditions the virtual earth maintains the same potential as instrument ground or WE1.

2.4 ISO RUN This switch, when in the ISO or isolate position, disconnects the electrodes from the electronics within the instrument and isolates them from each other, such that no external path, to those already existing in the test cell, exists for electrons to pass between the electrodes. When the electrodes are required to act as if they are coupled by a zero resistance wire, then the switch must be positioned to the run position.

2.5 Range change knob. This rotary switch changes the current range of the instrument.

2.6 Under over range leds. These illuminating leds act as a visual aid in order to make a suitable range selection. When both leds are not illuminated the correct range has been selected.

2.7 Current and potential meter. This meter displays the current flowing between the two electrodes in µ_ and mA. It can also display the potential value of the reference electrode with respect to the galvanic couple, WE1 and WE2. The meter displays its result in mV’s between -1999 and 1999 mV’s. To obtain the potential of the couple with respect to the reference electrode, simply reverse the sign.

2.8 Current and Potential Switch. The position of this switch dictates the reading on the digital meter. Whilst the switch is positioned in the current position the digital meter displays the value of the galvanic current flowing between WE1 and WE2. When the switch is positioned in the potential position the digital meter displays in mV’s the potential of the reference electrode with respect to the Galvanic Couple, WE1 and WE2.

2.9 Buffered current output. This output is used to measure, with an external voltmeter or recorder, the current flowing between the two working electrodes. It is a voltage output. In order to translate the voltage output into galvanic current it is necessary to perform a small calculation. This calculation is shown in example 1.

Example 1 Calculation of galvanic current.

Current Output 1.435 volts
Range 10mA

Galvanic Current = Current Output * Range

= 1.435 * 10mA
= 14.35mA

2.10 Buffered potential output. This output is used to measure, with an external voltmeter or recorder, the potential of the reference electrode with respect to the galvanic couple, WE1 and WE2. If the potential of the galvanic couple is required with respect to the reference electrode, simply reverse the sign.

3/ General use of the Zero Resistance Ammeter.

Switch the instrument to ISO or isolate, select range 100mA and switch the mains power or auxiliary power on. Check to see the digital meter is illuminated with zeros or a single digit. If the meter does not illuminate, then check the fuses have not blown in the instrument and mains lead. Replace the fuses as required.

Connect the instrument to the cell as shown in figure 2. The reference electrode, if used, should have a constant stable potential and not pollute the test cell with unwanted ions. With the instrument in this mode, all electrodes are isolated. Switch the instrument to run, using the ISO/run switch. This action will connect all electrodes to the internal electronics within the instrument, Galvanically coupling WE1 and WE2 through the instrument and referencing the reference electrode to the galvanic couple, WE1 and WE2. Use the range change leds to assist selection of the correct range. If a range change diode regains its illumination after a short time, reset the range until the range change leds are not illuminated. Illumination of the range change leds, in this case, is likely to be due to the coupling polarisation current settling down and decaying or increasing in magnitude. Measurement of the galvanic current between electrodes WE1 and WE2 can be made using the buffered current output, or by viewing the digital meter. Measurement of the potential of the reference electrode with respect to the galvanic couple, WE1 and WE2 can be measured using the buffered potential output or the digital meter. If the potential of the galvanic couple, WE1 and WE2, with respect to the reference electrode is required, then simply change the sign of the output.

The digital meter is not perfectly linear in operation. Thus for high accuracy measurements, it is suggested that an external voltmeter is used to measure the buffered current and potential outputs.

For long term experiments or experiments in which large variations in current are expected, it is wise to set the instrument on the largest current capacity range at which the leds are off. This will reduce the chance of exceeding the maximum output, listed in the specification, for the selected range. If a range change led becomes illuminated during prolonged operation, this is probably not drastic, as the maximum current output for the selected range may not have been exceeded. If the maximum current output for the selected range has been exceeded, the electrodes, WE1 and WE2, will be partially coupled. If the buffered current output is only a few mV’s, accuracy will be reduced in the current measurement.

In test conditions the potential difference between WE1 and WE2 will be approximately 10µVolts. If currents are high and/or lead lengths are long, then due to ohmic drop down the leads the potential between the two electrodes will be greater. In normal laboratory circumstances with leads less than one ohm in resistance, then this effect will produce little error or potential difference between the electrodes, especially for low currents. ACM Instruments produce a long range ZRA which can be distinguished by a three pin connector at the WE1 and WE2 terminals. This overcomes ohmic drop by using sense lines down both working electrode leads. The long range ZRA is suitable for experiments which may involve high currents and large lead lengths, such as experiments on site.

4/ Direction of current, electron and ionic flow.

Case 1 Current displayed as 14.5 µA

Current flow from WE2 to WE1 through solution.
Electron flow from WE2 to WE1 through instrument.
Positive ionic flow from WE2 to WE1 through solution.
Negative ionic flow from WE1 to WE2 through solution.

Case 2 Current displayed as -14.5 µA

Current flow from WE1 to WE2 through solution.
Electron flow from WE1 to WE2 through instrument.
Positive ionic flow from WE1 to WE2 through solution.
Negative ionic flow from WE2 to WE1 through solution.

5/ Special Long Range ZRA

The long range ZRA has a few internal features which enable electrodes to be coupled together as if by zero ohm wire even though they are tens of metres away from the ZRA. Three sockets and plugs have been supplied on the front panel. They are to be wired as follows.

Wiring of the connections.

Instrumental effects have reduced the accuracy of all ranges by approximately 10 E-9 Amps. This is a small current which is insignificant compared to the currents that are expected to be measured at the dockside.

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