Instruction Manual

245364-V May 2002

Model 755A

5-6 DETECTOR WITH FIRST STAGE AMPLIFIER AND PRESSURE COMPENSATION CIRCUITS

Refer to Figure 5-7 on page 5-10. The detector assembly consists of a test body suspended on a platinum wire and located in a non-uniform magnetic field.

The test body is constructed of two hollow glass spheres forming a dumbbell shape. They are filled and sealed with pure, dry nitrogen. Around the test body, a titanium wire is chemically etched in order to form a feedback loop that can create a 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 displacing force and thus is a function of both

(a)the % oxygen concentration of the sample and (b) the sample pressure.

The output from U2 is further amplified by U4 to provide a 0 to 10VDC output that constitutes signal Vx for the pressure compensation circuit described in Section 5- 6a on page 5-9 .

5-8

Circuit Analysis

Rosemount Analytical Inc. A Division of Emerson Process Management