VICI TCD2 - Manual (Page 11)

7
System Requirements
Components Not Included with the Detector System
• Carrier gas (99.999% purity is recommended)
• Ultra high purity grade gas pressure regulator with stainless steel
diaphragm (recommended)
• Any special adapters required for connection to the gas regulator
• Flow measuring device
• Flow regulating device
System Purity
Since detection of low concentrations depends in part on the purity of the
carrier, the purest carrier available must be used in order to achieve the
lowest possible detection limit. To maintain carrier purity, extra care must be
taken to assure that the delivery system is clean and free of leaks. Use
stainless steel tubing (cleaned to removed manufacturing solvents) instead
of nylon or PTFE tubing, which can diffuse contaminants into the carrier.
Recommended Carrier Gas Purifiers
The Valco Helium Purifer (product number HP2) and Nitrogen Purifer (product
number NP2), which utilize a rare earth gettering alloy to effectively remove
contaminants, are recommended for ppm level analysis of permanent gases.
For other applications, economical and convenient VICI Mat/Sen purifiers are
appropriate. Order the P300-1 for nitrogen, P200-1 for hydrogen, or the
P100-1 for helium and other inert carrier gases.
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 conductivities for a variety of light gases.
Hydrogen 45.9
Helium 36.9
Neon 11.8
Methane 8.6
Oxygen 6.6
Air 6.4
Nitrogen 6.4
Carbon monoxide 6.2
Water 4.5
Argon 4.5
Carbon dioxide 4.2
As an example, note in the table that the thermal conductivity of hydrogen is
46 and helium is 37, while nitrogen is only 6 and argon is even lower at 4.
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.