Instruction Manual

748213-S April 2002

5-6 DETECTOR WITH FIRST STAGE AMPLIFIER

Refer to Figure 5-6, page 5-9. 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 nitro- gen. 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 mir- ror. 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 current proportional to light. This current is converted to a ± voltage by U1 and U2 located on the connector board in the detector housing. This voltage is then presented to comparator U1 on the controller board. The output of U1 goes to U2. The output of U2 causes current to flow through the feedback loop attached to the dumbbell.

This feedback current creates an electro -magnetic field that attracts the dumbbell and mirror into the test assembly magnetic field until the mirror reflects light almost uni-

Model 755R

formly 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 a sample.

Resistances R7, R8 and the resistance of the wire in the feedback loop determine the gain of amplifier U2. 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 -7.0 V, depending on the range of the instrument. R4, C3 and R5, C7 form damping circuits for the input amplifier U1 and to smooth out noise that might be introduced by the measurement source.

Diode CR2 is a low-leakage device. Its purpose in the circuit is to ensure that the dumbbell and mirror are positioned correctly with respect to the photocells on initial application of power.

If the dumbbell was out of position on start-up, the mirror might reflect light from the source onto one of the photocells. If the photocell output was positive, the current in the feedback loop would be in the wrong direction and its electromagnetic field would cause the dumbbell to be further repelled from the permanent magnetic field. The result would be error, not balance.

On application of AC power, capacitor C1 has no charge. The current will have to flow through R2. Initially the full 30 V drop (the difference between the +15 VDC and -15 VDC power) will appear cross R2. The cathode of CR2 will be initially at -15 VDC. The anode of CR2 will be some value more positive than -15 VDC. CR2 will conduct. The input terminal of U1 will be negative and the current through the feedback loop around U2

5-8 Circuit Analysis

Rosemount Analytical Inc. A Division of Emerson Process Management