Teledyne 238 - Thermal conductivity analyzer - Manual (Page 10)

Teledyne Analytical Instruments 2-2
2 Installation Model 235 Series
Use 2-conductor shielded cable (nominally No. 22 wire size) to inter-
connect the analyzer output signal with the recording equipment. The shield
should be terminated on the appropriate terminal (see interconnection
diagram) at the analyzer—and be left disconnected at the recorder.
2.3 Gas Connections
Customer gas connection points are located on the underside of the
analyzer case. (Standard, basic instrument)
(See Outline Diagram for identification of each point.)
2.3.1 Reference and Zero Gas
A constant supply of gas, of a fixed composition, is needed as the
reference to which the sample gas will be compared. The reference gas is
normally selected to represent the main background of the analysis. For
certain applications, an optional sealed air reference is available where the
reference side of the detector cell is filled with air and sealed. This elimi-
nates the need to have reference gas constantly passing through the cell. For
instruments equipped with the optional sealed air reference, there will not be
reference inlet or vent ports.
A supply of gas, containing little or none of the components of interest,
is required to zero-standardize the analyzer.
In order to satisfy the requirements, both of these gases must be
supplied from purchased cylinder sources — as no other economical means
is readily available that will guarantee the user that impurities are maintained
at a low, fixed level.
Because most cylinder gases are supplied 99.95 to 99.98% pure, TBE
recommends that one cylinder of gas be used to fill both needs for most
applications (i.e., zero and reference.)
Specific recommendations as to the number and type of supporting
gases required will be found listed in the calibration section of the manual.
It is essential to the accuracy of the analyzer that the purity of the zero
gas be known. The zero control would be adjusted during zero standardiza-
tion, so that the recorder indicates the impurity content of the zero gas,
rather than zero.