A-MSystems 1900 User manual

INSTRUCTION MANUAL
FOR
POLAROGRAPHIC AMPLIFIER
MODEL 1900
Serial #__________
Date____________
A-M Systems
PO Box 850
Carlsborg, WA 98324
U.S.A.
360-683-8300 ♦800-426-1306 FAX:
360-683-3525
http://www.a-msystems.com
Version 9.0
March 2020

Contents
General Description........................................................................................................................................1
Instrument Features .............................................................................................................................................1
Controls and Connectors......................................................................................................................................2
Operating Instructions...................................................................................................................................4
Polarographic electrodes......................................................................................................................................4
Instrument Set-Up and Calibration......................................................................................................................6
Problem Solving ..................................................................................................................................................9
Theory Of Operation....................................................................................................................................11
Specifications..................................................................................................................................................13
Warranty and Service..................................................................................................................................17
Each Polarographic Amplifier is delivered complete with:
Rack Mount Hardware
NOTE
This instrument is not intended for clinical measurements using human
subjects. A-M Systems does not assume responsibility for injury or
damage due to the misuse of this instrument.

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General Description
Instrument Features
The Polarographic Amplifier Model 1900 is designed primarily for use with
microelectrodes (such as oxygen and hydrogen electrodes) which produce a current
response to an applied polarization voltage in the presence of the sensed gas. The
Model 1900 converts the input current to an output voltage, scaled and offset by user
operated controls. The voltage is output for subsequent analysis by the researcher.
The polarization voltage applied to the electrode may be set internally via manual
controls or through an external signal source. In either case, the polarization may be
turned off by a separate gate signal without disturbing other settings.
No head stage is required to achieve very low bias current (<1 pA, nominal, before
adjustment) using an actively-driven shielded sensor cable. Standard BNC connectors
are used for all signal connections. The sensor input connector can be operated with a
driven shield to minimize cable and connector leakage currents for applications
requiring the highest sensitivity; or with a grounded shield for greater convenience in
less demanding applications. The reference input may be set to differential or ground.
In differential mode, negligible current flows through the reference electrode,
eliminating reference electrode IR (voltage drop) errors. In ground mode, the
requirement for a separate ground electrode is eliminated.
The output scaling can be selected to read out in absolute current measured in nA, or
in mmHg, expanded 10x mmHg, percentage, or kPa. The input “zero” current may
be set up to one fourth of the full scale current range. In this way, any constant
leakage current (especially that from electrode current which flows in the absence of
any reagent gas) may be readily ignored. The gain is adjustable with step and fine
controls. Adjustment of the gain does not require readjustment of the zero current.
System sensitivity is high, with a maximum resolution of 0.1pA when in 100 pA scale.
The output can be read directly using an internal digital meter (3 1/2 digits), and can be
connected to chart recorders, FM tape recorders, or computers. The digital meter can
also be used to measure the polarization voltage.

A-M Systems 131 Business Park Loop, P.O. Box 850 Carlsborg, WA 98324
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Telephone: [phone] * 360-683-8300 * FAX: [phone]
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Controls and Connectors
The Model 1900 has been designed to ease polarographic measurements, while
accommodating the imperfections of real electrodes. This section gives a brief
description of the controls and connectors.
INPUT
The INPUT section is directly associated with the electrodes.
The sensing electrode is connected to the SENSOR input.
The reference electrode is connected to the REF input.
While the shield of the REF input connector is permanently
attached to ground, the shield of the SENSOR input may be
grounded (GND) via the SHIELD switch or may be actively
driven (ACTIVE), in order to reduce leakage currents in the
interconnecting hardware. The REF input itself may be
connected to ground (GND) via the REF switch, or the
amplifier may be operated differentially (DIFF).
POLARIZATION
The POLARIZATION section controls are used to bias the
sensing electrode at the required potential relative to the
reference, whether the reference is at ground or operating
differentially. The polarization voltage source can be set to
EXTernal INTERNAL+, or INTERNAL -, with a rotary switch. An
external polarization voltage source is connected to the EXT
IN connector. Alternatively, an INTERNAL polarization
voltage level is set with the INTERNAL VOLTAGE control.
Whatever the source, the polarization voltage may be
turned on or off using a gate signal applied to the GATE IN
connector. The applied polarization voltage may also be
monitored externally at the MONITOR OUT connector.
SCALING
The SCALING section provides control over the gain and
offset settings. Offset is controlled by the FINE ZERO control,
which may be switched ON or OFF with the ZERO CURRENT
switch. The gain is controlled in steps by the NANOAMPERS
rotary switch, and continuously over a narrower range by
the FINE GAIN control.

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OUTPUT
The form of the output is controlled by the OUTPUT selector switch. In
the CURRENT mode, the measurements are absolute (not sign
adjusted). An additional x10 gain (expansion gain) is inserted in the
mmHg x10 and % x10 modes for increased resolution with small
currents. The decimal point of the panel meter is shifted, maintaining
the original sense of the measurement, while providing the higher
resolution. The output voltage is available externally via the BNC
connector for continuous recording with a chart recorder, or
computerized data acquisition equipment.
METER
The METER may be used to monitor either the
POLARIZATION voltage (unaffected by the GATE IN signal), or
the OUTPUT voltage. pA and nA indicators supplement the
decimal point location on the METER to provide a 5-decade
sensitivity range without loss of resolution.

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Telephone: [phone] * 360-683-8300 * FAX: [phone]
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Operating Instructions
The Model 1900 has been designed to be simple and intuitive to operate, while allowing a
high degree of flexibility and performance. A brief description of pertinent polarographic
electrode characteristics is included in this section along with operating instructions for the
instrument and solutions to common problems which are encountered in its use, in order
to assist with the effective use of this instrument.
Polarographic electrodes
The most important property of polarographic electrodes is that their current, instead of
their voltage, varies with a change in the concentration of the sensed substance (most
commonly a dissolved gas such as oxygen)1.
Figure 1. Typical oxygen electrode response
To use such an electrode, a bias voltage is applied which intersects the plateau
region(s), allowing the current response to be linear over a physiologically meaningful
range. The current amplitude is often extremely small, typically in the nano- or
picoampere range. These characteristics require that an amplifier with current-to-voltage
capabilities be used in order to measure the concentration of the sensed substance.
1. An excellent description of the theory, construction, and application of polarographic electrodes can be found in: Irving
Fatt, Polarographic Oxygen Sensor, Malebar FL: Robert E. Kreiger Publishing Co., 1982.

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The polarographic electrode can be used as part of a 2 electrode or a 3 electrode
configuration (see Figure 2 and Figure 3):
The 2 electrode configuration is simple and convenient. The polarization voltage is
impressed across the two electrodes. Notice, however, that the sensed current must flow
through the reference electrode. The reference electrode may have an impedance of many
MV. In such a case, the IR drop within the reference electrode may be sufficient to bias the
polarographic electrode out of the plateau region of the electrode. This problem can be
eliminated with the addition of another electrode.
Figure 2. Two-electrode configuration Figure 3. Three-electrode configuration
Using the 3 electrode configuration, the polarographic current now flows through the
separate ground return electrode. The placement of the ground return electrode can be
almost anywhere, as long as it is in electrical contact with the tissue.2The reference
electrode must be close to the tissue being sensed in order that the tissue voltage being
sensed by it be accurate. The current sensed is always that which is flowing in the
SENSOR lead. This allows flexibility in grounding arrangements.
For either the 2 or 3 electrode configuration, the voltage impressed on the polarographic
electrode is the sum of the reference voltage and the polarization voltage. For the 2
electrode configuration, this means that the polarization voltage is relative to ground, which
may be offset from the tissue potential by the IR drop in the ground/reference electrode.
For the 3 electrode configuration, the polarization voltage is relative to the reference
electrode voltage.
2. Some additional consideration may be necessary if the “ground” currents are allowed to be coupled through neural
tissues. Unwanted complications may result if these currents stimulate responses. The same considerations apply for
the 2 electrode configuration. The amount of current flowing through most polarographic electrodes is low enough that
this should not be a problem for nearly all researchers.

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Telephone: [phone] * 360-683-8300 * FAX: [phone]
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Instrument Set-Up and Calibration
Note: In most cases, the settings of the Model 1900 will not have to be changed from experiment to
experiment, except for calibrating the electrodes.
Connecting the Electrodes to the Input
The characteristics of polarographic electrodes make it necessary to exercise extreme
care in connecting the Model 1900 with the electrode set. The degree of difficulty
involved depends upon the quality of the electrode, and the magnitude of the response
relative to leakage currents and other sources of noise.
The polarographic electrode is connected directly to the SENSOR input. Many
commercially available polarographic electrodes have BNC connectors, and may be
used directly. Set the SHIELD and REF switches to the GND position for these
electrodes. For electrodes with low responses in the pA range, particularly when
maximal resolution is required, it may be necessary to drive the sensor cable shield,
with the SHIELD switch in the ACTIVE position. This reduces possible leakage
currents in the cabling by “bootstrapping” the cable shield to be at the same potential
as the inner conductor. This is especially important if the polarization voltage is to be
varied without readjusting the zero current. However, care must be taken to ensure
that the shield is not otherwise connected to ground (or any other potentials) for the
driven shield to work correctly. If this is not possible (or not necessary), theSHIELD
switch must be set to GND.
The reference electrode is connected directly to the REF connector. Some electrodes
incorporate a reference connection through the shield of the sensing electrode. In this
case, set the REF switch to GND; no other reference is required. For the 3 electrode
configuration, set the REF switch to DIFF. The return current path can be made either
through the outer shield of the REF connector, or via the front panel GND connector.
Some situations (particularly where space prohibits adding another electrode) require
that all shields be connected together, and to ground. The REF switch should be set to
GND in this case.
Controlling the Polarization Voltage
Polarographic electrodes need a bias voltage to ensure that they operate within their
respective plateau regions. The Model 1900 provides several options for solving this
problem. For most situations, the POLARIZATION selector switch is used. Set the
polarity to INTERNAL+ or INTERNAL-, and adjust the voltage using the INTERNAL
VOLTAGE control. The EXT IN signal is effectively disconnected in this mode. The
polarization voltage can be observed on the panel METER by setting the METER switch
to the POLARIZATION position. The readout is in mV. Oxygen electrodes are usually
set at -600 mV, and hydrogen electrodes at +250mV. The polarization voltage may
also be monitored via the MONITOR OUT connector.

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For some situations, it may be desirable to set the polarization voltage from an external
source, such as a computer controlled D/A converter. Simply set the POLARIZATION
selector switch to EXTERNAL, and connect the source to the EXT IN connector. The
INTERNAL voltage is disconnected in this mode. The polarization voltage may be
monitored at the METER or at the MONITOR OUT connector.
Leaving the GATE IN connector disconnected is sufficient to guarantee that the specified
polarization voltage appears at the polarographic electrode. If the GATE IN is driven low
(e.g. by shorting the input to ground, or with a low TTL signal), the polarization voltage
is turned off, and the voltage at the polarographic electrode is set to the same potential
as the reference electrode. The polarization voltage available at MONITOR OUT reflects
the state of the signal at GATE IN. However the METER,when set to POLARIZATION
mode, always displays the polarization voltage regardless of the state of the signal at
GATE IN so that the voltage can be set more easily without disturbing the electrode.
The OUTPUT will continue to indicate the current flowing in the sensor electrode.
Setting the Output Mode
The type of output signal desired should be selected before beginning to calibrate the
electrodes. You may switch between expanded and unexpanded modes (% and % x10;
or mmHg and mmHg x10) without any complication, but switching between output types
requires recalibration. This is necessary because no two electrodes are identical.
Therefore, in general there is no interconvertibility of signal types, such as CURRENT
and mmHg, except in the context of a specific electrode. However, switching between
expanded and unexpanded modes, increasing gain when the signal dwindles, or
decreasing gain to keep the signal within range may be done atany time. The % and %
x10 output modes may be used for scaling the output in kPa.
Calibrating the Electrode
If electrode current is of interest, no electrode calibration is necessary. Simply set the
ZERO CURRENT to OFF, turn the FINE GAIN fully clockwise, and set the NANOAMPERES
scale to the desired range. The Model 1900 is calibrated and ready.
If the signal is to be displayed in mmHg or %, the electrode must be calibrated, since
there are no “standard” electrodes. The Model 1900 has been designed to ease some
of the difficulty involved in calibrating electrodes. The following calibration procedure
example uses the case of measuring oxygen dissolved in an aqueous environment.
While a specific application may be different, the calibration procedure will likely be
analogous.
In addition to the electrode set, cables, and the Model 1900, a method must exist to
control the concentration of the substance being measured. The test solution should
mimic the features of the tissue to be tested later. Important features to mimic may
include both chemical and physical properties of the solution, such as temperature and
osmolarity. Which features are important depends on the specificity of your electrode;
that is, the degree to which the electrode is sensitive to factors other than those being