Instruction Manual

748213-S April 2002

Model 755R

Shaded

Pole

Piece

Sphere

(Magnetic Susceptibility = ko )

Fk

Sample Gas

(Magnetic Susceptibility = k )

Note:

As percentage of oxygen in sample gas increases, displacement force (Fk ) increases.

Figure 4-2. Spherical Body in Non-Uniform Magnetic Field

4-3 ELECTRONIC CIRCUITRY

Electronic circuitry is shown in the Control Board schematic diagram, Drawing 652826, and is described briefly in the following sec- tions. For detailed circuit analysis, refer to Section 5 Circuit Analysis.

a.Detector/Magnet Assembly

A cross-sectional view of the optical bench and detector assemblies is shown in Figure 4-3B, page 4-7. Source lamp DS1, powered by a supply section within the Power Supply Board assembly (See Section 4-3c, page 4-5) directs a light beam onto the mirror attached to the test body. The mirror reflects the beam onto dual photocell BT1, BT2.

The difference between the signals de- veloped by the two halves of the photocell constitutes the error signal supplied to the input of amplifier U1 on the Control Board assembly. Amplifier U1 drives U2 which,

in turn, supplies the restoring current to the titanium wire loop on the test body (See Section 4-1, page 4-1).

Detector temperature is sensed by thermistor RT1, an integral part of the detector assembly (See Figure 4-3B, page 4-7). The thermistor provides the input signal to the detector temperature control section of the Power Supply Board assembly: HR1, mounted on the top of the magnet, and HR2, mounted permanently on the rear of the detector assembly.

b.Control Board and Associated Cir- cuitry

The Control Board consists of signal conditioning and control circuitry.

This circuitry includes the following:

4-4 Theory

Rosemount Analytical Inc. A Division of Emerson Process Management