Rosemount 755A, Инструкция по эксплуатации

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E
LECTRONIC
C
IRCUIT
A
NALYSIS
245364-U Rosemount Analytical April 2000 6-9
Model 755A Oxygen Analyzer
counteracting magnetic force to the test body displacement caused by oxygen
concentration in the test assembly magnetic field.
Attached to the center arm of the test body dumbbell is a diamond-shaped mirror.
Attached to the mirror are two separate platinum wires in tension with the supports for
the test body. The supports are isolated from ground and are electrically connected to
the feedback loop and the electronics for that loop. The platinum wires form a fulcrum
around which the test body pivots.
The detector operates in the following fashion. If the sample gas contains oxygen, it
collects in the non-uniform magnetic field around the test body. Oxygen, because of
its paramagnetic qualities, gathers along the magnetic lines of flux and forces the
dumbbell of the test body out of the magnetic field.
A light source is focused on the test body mirror. As the test body moves out of the
magnetic field, the mirror distributes light unevenly on two photocells (BT1 and BT2).
The photocells create a potential at comparator U1. The output of U1 goes to U2.
The output of U2 causes current to flow through the feedback loop attached to the
dumbbell and mirror into the test assembly magnetic field until the mirror reflects light
almost uniformly on each photocell. A current proportional to the oxygen
concentration in the magnetic field of the test assembly has to be flowing through the
feedback loop in order to maintain balance and provide a reading of the oxygen
content of the sample.
Resistances R5, R17 and the resistance of the wire in the feedback loop determine
the gain of amplifier AR2. The mirror on the dumbbell is positioned by the amount of
current in the feedback loop. The mirror reflects light from the source (DS1) to the
photocells (BT1, BT2). This repositioning of the mirror is a form of mechanical
feedback to the input of the amplifier U1. The net result is that the output of U1 could
vary from 0 to -70 mV, or 0 to -7.0V, depending on the range of the instrument
On application of AC power, capacitor C6 has no charge. The current will have to flow
through R18. Initially the full 30V drop (the difference between the
±
15VDC power) will
appear cross R18. The cathode of CR2 will be initially at -15VDC. The anode of CR2
will be some value more positive than -15VDC. CR2 will conduct. The input terminal
of U1 will be negative and the current through the feedback loop around U2 will cause
the dumbbell and mirror to be positioned correctly in the test body.
As the charge on C6 increases, the cathode of CR2 becomes more positive. When it
exceeds that on the anode, CR2 ceases to conduct and isolates the
±
15VDC power
supply from the input circuit.
Coarse Zero Adjust R9 and front panel ZERO potentiometer R13 permit adding an
appropriate voltage to the input of U2 to counteract any electrical offset resulting from
imbalance in the detector and/or photocells BT1 and BT2.
The output current that U2 must provide to restore the dumbbell is a measure of the