Cryogens and their use during DC3
by Frank Flocke (NCAR/ACD)
Cryogens, such as dry ice and liquid nitrogen, are used in a number of ways on our aircraft during DC3. They are used for sampling air with our instruments as well as for keeping some heat sensitive equipment cold during operation.
Liquid nitrogen exists at a temperature of −196 °C or −321 °F, only 77 degrees K above absolute zero. It is often used for concentrating air samples for chromatography and mass spectrometry and we have one of these instruments on the NCAR/NSF GV aircraft. At liquid nitrogen temperature, almost all atmospheric trace gases are frozen out, but oxygen and nitrogen are not. This allows for extraction of gases of interest from the air without having to contend with oxygen and nitrogen, which comprise 99% of the sample volume. By drawing air through a small tube cooled with liquid nitrogen, trace gases from samples of several liters of air can be concentrated into a few milliliters or less. This small volume can be directly introduced into analytical instruments and the components analyzed.
Cryogens are also used as a coolant for some very sensitive infrared light detectors. Some of these detectors need to be cooled with liquid nitrogen, but on the GV we use some photomultiplier tubes that are sensitive into the infrared and are cooled with dry ice. The detector tubes are built aluminum housing, which is well insulated to the outside and filled with about 10 pounds of dry ice. Dry ice exists at a temperature of -78°C or −109 °F and does not melt but evaporates directly into carbon dioxide gas, which makes it an ideal cooling agent for this application.
Dry ice is also used to remove unwanted water vapor from air samples that are analyzed on the GV. The instrument used to measure carbon dioxide (CO2) and methane (CH4) needs dry sampling air to function accurately. A bath of ground up dry ice floating in a Flourinert liquid (a heavy, engineered liquid that is not flammable and non toxic, but still liquid at dry ice temperature) is used to cool a trap made from highly polished stainless steel through which the sampling air is drawn. At dry ice temperature, essentially all water vapor is removed from the air but the carbon dioxide and methane quantitatively pass through the trap. This way we can deliver a dry sample to our instrument without compromising the accuracy of our measurements.