C
IRCUIT
A
NALYSIS
6-12
June 1997 Rosemount Analytical 748213-P
Model 755R Oxygen Analyzer
6.9 ANALOG OUTPUT CIRCUITS FOR RECORDER AND
ALARMS
Refer to Figure 6-8. The analog output circuits utilize two amplifiers, first-stage
amplifier and second-stage amplifier.
6.9.1 F
IRST
S
TAGE
A
MPLIFIER
Permits selection of the desired fullscale oxygen range for the recorder via
jumper-selectable signal amplification for scale expansion. This amplifier permits
selecting the desired fullscale oxygen range for the recorder by an appropriate jumper
selection of one of seven recorder spans. The following recorder spans are available:
1, 2.5, 5, 10, 25, 50, and 100%.
6.9.2 S
ECOND
S
TAGE
A
MPLIFIER
Provides (a) a jumper-selectable output for a potentiometric recorder and (b) an output
to drive the voltage-to-current and/or alarm option(s), if used. This amplifier is an
inverting configuration that provides a signal attenuation of 2X, thus reducing the
10-volt fullscale input signal to obtain a 5-volt fullscale output. This output is routed to:
Recorder Output Resistor Network. It provides a jumper-selectable output of 0 to 10 mV, 0
to 100 mV, 0 to 1 V, or 0 to 5 VDC for a potentiometric recorder.
Current Output Receptacle J1. This connector accepts the optional plug-in current-output
board.
Dual Alarm Amplifier Circuit. This circuit drives the optional 654019 Alarm Relay Assembly.
Oxygen is strongly paramagnetic while most other common gases are weakly
diamagnetic. The paramagnetism of oxygen may be regarded as the capability of an
oxygen molecule to become a temporary magnet when placed in a magnetic field.
This is analogous to the magnetization of a piece of soft iron. Diamagnetic gases are
analogous to non-magnetic substances.
With the Model 755R, the volume magnetic susceptibility of the flowing gas sample is
sensed in the detector/magnet assembly. As shown in the functional diagram of Figure
5-1, a dumbbell-shaped, nitrogen-filled, hollow glass test body is suspended on a
platinum/nickel alloy ribbon in a non-uniform magnetic field.
Because of the “magnetic buoyancy” effect, the spheres of the test body are subjected
to displacement forces, resulting in a displacement torque that is proportional to the
volume magnetic susceptibility of the gas surrounding the test body.
