Sensing Unit - Honeywell 7866 Operation And Maintenance Manual

Introduction
1.2

Sensing Unit

The 7866 Thermal Conductivity analyzer's Sensor Assembly is supplied in an explosion proof housing.
The housing consists of a rugged cast aluminum construction that permits reliable operation under adverse
ambient conditions.
The Sensor Assembly consists of two sections—the cell block assembly and the electronic assembly
(Figure 1-2).
The Cell Block assembly is constructed of stainless steel with two identical internal cells, the measuring
cell and the reference cell. The highly stable thermistor is mounted in each cell. These matched thermistors
form the active arms of a bridge circuit; the unbalanced current of the bridge provides the means of
measuring the relative ability of the sample and reference gases to conduct the heat away from their
respective thermistors to the cell wall, which is held at a constant temperature.
The cells in which the thermistors are mounted are dead-ended so the sample gas enters only by diffusion,
minimizing the effect of sample flow variations. In addition, the entire cell-block assembly is maintained at
a constant optimum temperature through two heaters and a control thermistor that are located in the cell
block assembly.
The Sensing Unit's electronics assembly incorporates solid state electrical circuits. These circuits include:
• Current Regulator: supplies the constant current to the thermistor cell bridge circuit
• Proportional Action Temperature Controller: maintains the entire cell block at a constant
temperature
• Voltage to Current Converter/Amplifier: its current output is transmitted to the analyzer's control
unit
2 7866 Triple Range Digital H and CO Analyzer/Indicator - Operation and Maintenance
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