Valco baby Vici TCD-3, Инструкция по эксплуатации

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8
Carrier Gas Selection
The detector’s response to a component is based upon the difference between
the thermal conductivities of the component and the carrier gas; the greater the
difference, the greater the response. The table below shows thermal conductivi-
ties for a variety of light gases, given in units of milliwatts per meter per degree
Kelvin. The values listed are given for test conditions at 300°K and 100 kPa.
Gas
1
Thermal Conductivity
(mW/m/°K)
Hydrogen 186.6
Helium 155.7
Neon 49.4
Methane 34.4
Oxygen 26.5
Air 26.4
Nitrogen 26.0
Carbon monoxide 25.0
Water 18.6
Argon 17.7
Carbon dioxide 16.8
Table 4:
Thermal conductivities of common gases
As an example, note in the table that the thermal conductivity of hydrogen
is 186.6 and helium is 155.7, while nitrogen is only 26.0. Argon is even lower,
at 17.7. Since the largest difference in thermal conductivity yields the best
response, detection of small amounts of hydrogen is better done with argon
or nitrogen carrier than with helium.
However, while nitrogen as a carrier yields excellent response to hydrogen,
the response to oxygen and carbon dioxide is diminished compared to the
levels that could be achieved with helium or hydrogen. Argon would yield
poor response to carbon dioxide and water, but would be adequate for other
components.
There is no absolute “best choice” of carrier gas. For any situation, the choice
must take into account all of the parameters involved: column characteristics,
components of interest and their concentrations, safety considerations, carrier
cost, etc.
System Requirements
1
Harvey, Allan H. & Huber, Marcia L., 06 June 2011, Thermal Conductivity of Gasses, CRC Handbook
of Chemistry and Physics, 92nd Ed., 6-240, www.nist.gov/node/599951