While dry nitrogen is not necessary to use the Montana Instruments Cryostation, it is recommended. There is a port at the back of the Control Unit labeled ‘N2 Tank,’ which introduces dry nitrogen into the sample space when the vent valve is open (see figure below). This port fits ¼” tubing. When open, the vent valve allows 10psi of dry nitrogen to flow into the sample space. If less than 10psi of nitrogen is flowing through the port, the software will not detect the presence of nitrogen, but the nitrogen will still flow into the sample space at the lower pressure.
Nitrogen helps to keep the sample space clean. Nitrogen molecules will adhere to any moisture within the sample chamber, and when vacuum is pulled the nitrogen and moisture are evacuated out of the sample space and into the roughing pump. This allows the user to achieve a better vacuum. Nitrogen allows the charcoal adsorbers, which trap molecules during cryo-pumping, to be recharged with each cool down/warm-up cycle. If air is introduced into the sample space this will not recharge the adsorbers as well. The purity of the nitrogen introduced is not essential, high purity will work, as will ultra-high purity.
When the system begins to pump down with the roughing pump in the Control Unit, the software will do a brief leak up test, at which point dry nitrogen flushes the sample chamber. Nitrogen is not used again until the end of the warm-up cycle. Once all temperatures have reached 285K the system purges with dry nitrogen. Nitrogen can also be introduced in manual mode by manipulating the case/vent valves and the roughing pump when the system is warm only. Nitrogen should not be introduced into the sample space when the system is colder than 280K, and especially not at temperatures lower than 77K as this will flood the sample space with liquid nitrogen.
Nitrogen should always be used in a few instances:
- If you plan to keep the system cold for a week or more, you’ll want a high integrity vacuum, so pumping and purging the system with dry nitrogen before a long cool down is advised.
- If you are in a humid environment (>50% humidity) attach dry nitrogen to avoid getting moisture into the sample space when the system vents to atmospheric pressure.
- If you’ve had a leak in your system the charcoal adsorbers will need to be recharged by purging the system with dry nitrogen as there will be a lot of moisture and contamination trapped in the adsorbers.
Nitrogen is not used to assist in the cooling of the system – as such it is optional, but it is a good idea to use it to achieve the best vacuum possible.