Instruction Manual
748441-D
May 2004
Rosemount Analytical Inc. A Division of Emerson Process Management
Description and Specifications 1-9
Model CAT
100
d.
Thermal Conductivity Method
To measure gases like Hydrogen (H
2
), Argon (Ar) or
Helium (He), the measurement method of thermal
conductivity (TC) will be used.
Sensor
Design
The sensor consists of four small PT
100 resistors arranged in a Wheat-
stone Bridge which is mounted into
a block made of either aluminum,
stainless steel or Hastelloy, depend-
ing on the application (e.g. stainless
steel / Hastelloy for corrosive
gases). The block is thermostat con-
trolled to suppress influence of ex-
ternal temperature change.
Analysis
Cell
Both the volume of the block and the mass of
the resistors have been minimized on order
to obtain short response time.
The block contains two gas paths for sample
and reference gas, whereat the reference
gas path is closed for standard applications.
Always two sensors are located in the sam-
ple and the reference gas path. The resistors
are full glass packaged to withstand aggres-
sive gases.
The material in contact with the gases are
glass, Gold, Aluminum, stainless steel and
Hastelloy, so a high resistance against cor-
rosion by aggressive gases is provided by
this cell.
Measurement
Method
The entire measurement cell is thermostat controlled
to a temperature of up to 75 °C. The four sensors
are electrically heated to a higher temperature and
the signal of the Wheatstone Bridge is monitored.
Depending on the thermal conductivity of the gases
that pass the cell, the temperature of the sensors in
contact with the gas changes and thus their electri-
cal resistance. This changes the output signal of the
Wheatstone Bridge and electronic circuitry proc-
esses this signal to obtain standardized signal ampli-
tudes, and transmits these to both an indicator
instrument and to the signal output connector.
Figure 1-7. Thermal Conductivity Sensor
