The instrument needs to be refilled with helium at least once per day. Use the helium blowoff meter to monitor the blowoff rate. The capacity of the liquid helium can is 3 liters, which is equivalent to 2400 liters of gas passing through the meter. The hold time can be estimated by measuring the time in seconds for the meter reading to increase by 10 liters (1 revolution of the rightmost digit), then dividing by 48 to get the hours per liquid liter, then multiplying by 3 to get the hold time for 3 liters:
The hold time is usually between 18 and 24 hours, but could be shorter. Be sure to carefully monitor the blowoff meter to determine when the helium needs to be refilled.
The 15 liter liquid nitrogen can holds for 2 to 3 days when the nitrogen is being pumped. It is best to top off the LN2 can by adding about 3 to 5 liters twice per day so that the LN2 is kept nearly full. This reduces the thermal effects of adding 77 K liquid to the 60 K pumped nitrogen already in the can, and reduces the possibility of running out of nitrogen.
Because the LN2 is pumped, the can must be filled through the siphon, or ``slurpee'' tube. The best way to tell if the can is full is to close the pinch valve on the pump-out hose, then add nitrogen until the siphoning action stops. Do not add more than about 5 liters at a time to prevent the valve from icing excessively. Be sure to re-open the pinch valve to keep the nitrogen pumped down.