Agilent Technologies 6890 Series, Service Manual, Page: 474 / 867

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Jun 2001
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Agilent 6890 Gas Chromatograph Service Manual
Gases 520
Cryogenic cooling requirements
Cryogenic cooling requirements
Cryogenic cooling allows you to cool the oven below ambient temperature. A
solenoid valve introduces liquid coolant, either carbon dioxide (CO
2
) or
nitrogen (N
2
), to cool the oven to the desired temperature.
CO
2
and N
2
require different hardware. You must replace the entire valve
assembly if you want to change coolants. The liquid CO
2
valve kit is
part no. G1565-65510 and the liquid N
2
kit is part no. G1566-65517.
Choosing a coolant
When selecting a coolant, consider these points:
• The lowest temperature you need to reach
• How frequently you will use cryogenic cooling
• The availability and price of coolant
• The size of the tanks in relation to the size of the laboratory
• Liquid N
2
cools reliably to –80°C
• Liquid CO
2
cools reliably to –40°C
CO
2
is the choice for infrequent cryogenic cooling because it does not
evaporate and is less expensive than N
2
. However, a tank of CO
2
contains
much less coolant than a tank of N
2
and more CO
2
is used for the same amount
of cooling.
Although liquid N
2
evaporates from the tank regardless of frequency of use,
N
2
tanks contain more coolant than do CO
2
tanks and therefore may be better
for frequent use.