2-5
Thermal Conductivity Analyzer
Operational Theory 2
Teledyne Analytical Instruments
The Temperature Control Board keeps the temperature of the measur-
ing cell regulated to within 0.1 degree C. A thermistor is used to measure
the temperature, and a zero-crossing switch regulates the power in a car-
tridge-type heater. The result is a sensor output signal that is temperature
independent.
In the presence of dissimilar gases the sensor generates a differential
voltage across its output terminals. A differential amplifier converts this
signal to a unipolar signal, which is amplified in the second stage, variable
gain amplifier, which provides automatic range switching under control of
the CPU. The output from the variable gain amplifier is sent to an 18 bit
analog to digital converter.
The digital concentration signal along with input from the control
panel is processed by the CPU and passed on to the 12-bit DAC, which
outputs 0-1 V dc Concentration and Range ID signals. An voltage-to-
current converter provides 4-20 mA dc concentration signal and range ID
outputs.
The CPU also provides appropriate control signals to the Displays,
Alarms, and External Valve Controls, and accepts digital inputs for exter-
nal Remote Zero and Remote Span commands. It monitors the power
supply through an analog to digital converter as part of the data for the
system failure alarm.
The RS-232 port provides two-way serial digital communications to
and from the CPU. These, and all of the above electrical interface signals
are described in detail in chapter 3 Installation.
2.4. Temperature Control
For accurate analysis the sensor of this instrument is temperature
controlled to 60
o
C.