Emerson ROSEMOUNT OCX 8800 - Manual (Page 13)

Instruction Manual
IM-106-880, Rev 1.0
January 2007
1-5
OCX 8800
5. An operator can calibrate and diagnostically troubleshoot the OCX 8800
using the HART Interface. Each of the OCX 8800's 4-20 mA output lines
transmit an analog signal proportional to oxygen or combustible levels
detected. The HART output is superimposed on the oxygen 4-20 mA
output line only. This information can be accessed through the following:
• Model 275/375 Handheld Communicator - The handheld
communicator requires Device Description (DD) software specific
to the OCX 8800. The DD software will be supplied with many
Model 275/375 units, but can also be programmed into existing
units at most Emerson Process Management service offices.
Refer to Section 4, Using HART Communications, for additional
information.
• Personal Computer (PC) - The use of a personal computer
requires AMS software available from Emerson Process
Management.
• Selected Distributed Control Systems - The use of distributed
control systems requires input/output (I/O) hardware and AMS
software which permit HART communications.
6. Optional Blowback System. The blowback system periodically blows
instrument air back through the sample line filter and out the sample
tube. This clears out particulate and keeps the sample line filter from
clogging.
System Operation
Figure 1-2 shows the relationship between the components of the OCX 8800.
The sensors and the electronics are contained in separate housings. The
sensor housing and probe mounts to a duct or process wall so that the probe
protrudes into the flue gas stream. An air powered eductor continuously pulls
samples of the process flue gas through the probe to a chamber in front of the
sensor housing where the sample passes the O
2
sensor and continues on to
the COe sensor. Dilution air is provided to the COe sensor and reference air
to the O
2
sensor. After the gas sample flows past the O
2
sensor and through
the COe sensor, it is drawn through the eductor where it mixes with the
eductor air and exits through exhaust back into the system. The electronics
housing contains the CPU and HART boards which convert the sensor inputs
into 4-20 mA analog output signals. The CPU can also initiate and perform
calibrations. Three test gasses and instrument air can be turned on and off by
solenoids. Test gas flow to the sensors is regulated by a flow meter between
the electronics and sensor housings. Instrument air is separated into eductor
air, reference air, and dilution air. The instrument air solenoid does not allow
air flow until the heaters are up to temperature. This minimizes the amount of
sampled process flue gas being pulled into cold sensors causing
condensation.