Model 755A

Instruction Manual

245364-V May 2002

a.Pressure Compensation Circuit

This circuit provides a pressure-corrected output signal conditioned in the range of 0 to 10VDC. The circuit solves the following equation:

Vo = k (Vx/Vz)

where

Vo = the corrected output signal

Vx = the amplified detector-output signal, which includes a pressure factor

Vz = the pressure signal derived from: (a) The pressure sensor and associated amplifiers, and (b) the positive reference voltage power supply

k = the constant that is characteristic of the circuit

Circuit function is such that, assuming a constant % oxygen concentration in the sample stream, the output from the analog divider U6 (and thus also the conditioned output from amplifier U8) will remain constant regardless of pressure variations within the specified range.

b.Pressure Signal Circuit

Refer to Figure 5-8 on page 5-10. The pressure signal circuit consists of the pressure sensor (transducer) and two associated amplifiers. The sensor provides a voltage output that is proportional to pressure. This signal is ratioed and combined with the negative voltage reference so as to provide a zero- based signal at the output of the first amplifier, where it is available for measurement and display for calibration and setup purposes.

The second amplifier adds the 10V reference back ;into the pressure signal. The output from the second amplifier is then appropriately attenuated to provide signal Vz for analog divider U6. Signal Vz is also made available to the display and measurement circuit for calibration purposes.

c.Positive and Negative Reference Voltage Circuits

These circuits provide precisely controlled voltages that set overall system performance and accuracy for the oxygen measurement system. While the absolute accuracy is of importance, more important yet is the ability of these circuits to remain stable despite temperature variations that may occur during normal instrument usage.

Rosemount Analytical Inc. A Division of Emerson Process Management

Circuit Analysis

5-9