Table 7-1 Sensing Unit Cell Assembly Cable Connections - Honeywell 7866 Operation And Maintenance Manual

Measuring and Reference Thermistors
These thermistors are the two larger hex-head assemblies mounted in the top of the thermistor-chamber
assembly. Leadwires plug onto terminal pins on each hex-head. Unscrew the entire assembly using
appropriate care as the thermistors are mounted in small glass knobs at the end of each of these screw-in
plugs. (Thermistors can be cleaned with solvent or detergent, rinsed, and dried.) The two thermistor plug
assemblies are identical. The cell chambers are identified M for measure and R for reference. If new
thermistors are to be installed, replace both seals because leakage between the Measuring and Reference
Cell chambers will cause drift of the cell assembly output.

Table 7-1 Sensing Unit Cell Assembly Cable Connections

Circuit-
Card Post
A (2) Green/red
B (1) Blue/red from S
C (1) Blue/green
C (1) White/blue/red
D (1) White/brown from T
E (1) Green/light green
E (1) White/brown
Temperature-Sensing Thermistors
These thermistors are the small hex-head assemblies. The leads are permanently attached. Unscrew the
assembly and remove from the cell block. Coat the threads with silicone grease before reinstalling to insure
a good thermal contact. Replacements are complete units, including leadwire.
Cartridge Heaters in Cell Block
Use a large screwdriver to remove the screw-in retainer, then remove the heater. Replace a pair of heaters
rather than a single heater. Be especially careful not to damage the leads or insulation.
O-rings at Inlet and Outlet Connectors (Figure 7-2)
Unscrew each fitting at the bottom of the cell block. The O-ring can now be removed from the cell-block.
To remove the internal O-ring from the inlet or outlet fitting the outer casting, use a socket wrench to
remove the retaining nut inside the casting, then pull out the entire fitting assembly and remove the O-ring.
The external compression-fitting O-ring can be removed easily without taking out the entire assembly.
To remove O-rings and flame-arrestor parts from explosion-proof housing proceed as follows: Use a
socket wrench to remove the retaining nut inside the casting. Remove the fitting, the small inside O-ring
from the flame-arrestor, and withdraw the flame-arrestor fitting. The large outside O-ring can be removed
from the fitting and the three filter elements can be pushed through the flame arrestor body from outside
the casting with a length of tubing.
9/06 7866 Triple Range Digital H
Wire Cell-Block
Circuit
Bridge
Circuit
and CO Analyzer/Indicator - Operation and Maintenance
2 2
Circuit- Wire
Card Post
F White/black
J White/black
G White
H White
Maintenance
Cell-Block
Circuit
Heater
Heater
Thermistor
Thermistor
63