Teledyne 238 - Thermal conductivity analyzer - Manual (Page 6)

Teledyne Analytical Instruments 1-2
1 Introduction Model 235
Because analysis by thermal conductivity is not an absolute measure-
ment, standardization gases of
known composition will be required to fix
the upper and lower parameters of the range (or ranges) of analysis. These
gases will be used to periodically check the
accuracy of the analyzer.
The difference in thermal conductivity between the fixed reference gas
and the sample is sensed by hot wire elements. The elements are mounted in
a cell assembly so that one set is in the reference and the other in the sample
stream. Each set of elements is a component in an electrical bridge circuit.
During calibration, the bridge circuit is balanced in zero and reference
gas at one end of the measurement range, and sensitized in reference and
span gas at the other end, so that intervening points along the range (or
ranges) of interest will produce a DC electrical signal representative of the
analysis. The resulting electrical signal is fed to an amplifier and span pot,
which produce a standard 0-1V output signal. An E to I converter PC board
is also installed and produces an isolated 4–20 mA DC current output in
addition to the voltage output.
The temperature of the measuring cell is regulated to within 0.1 degree
C by a sophisticated control circuit. A thermistor is used to measure the
temperature, and a zero-crossing switch regulates the power in a cartridge-
type heater. Temperature control is precise enough to eliminate diurnal
effects in the output over the operating ranges of the analyzer.
The overall design of the instrument is intended to facilitate servicing
and troubleshooting, should that ever be necessary. The controls are all
mounted on the front panel, which can swing down, allowing access to the
cell compartment. The cell is enclosed in an insulated compartment that is
readily removable from the chassis; the electronics are mounted on a series
of circuit boards at the rear of the enclosure, accessible by removing the
back panel.
Explosion-proof models of the series use sealed explosion-proof
enclosures for the analysis section (Model 237) or both the analysis section
and control unit (Model 238). Model 235 is general purpose with remote
control unit, and Model 235 is general purpose with integral control unit.
1.1 Electronic Circuitry
The electronic components are mounted on a number of circuit boards
that plug into sockets on a larger board, dubbed the “Mother Board”. This
allows for rapid troubleshooting and repair of any defective parts, and also
for rapid field installation of optional features not ordered with the unit.