Connect to the recovery a LIQUID helium storage Dewar (contains clean oil free helium!). CLOSE main recovery valve.
Clean by pumping and flushing 5 times at least the long hose to the Dewar (Dewar's valve close!). Let Dewar pressurize local recovery system for a while.
Close recovery valve to the Dewar hose. Keep main recovery valve still closed.
Needle valves (1K and UHV-sock) closed.
Pump and flush several times (up to 5 or 6): helium-tank, 1K-pumping line/capillary, UHV-sock/capillary. You do this by pumping with the 1K-pump and the sock-pump (“Saskia”). You flush opening the valve to the helium storage Dewar, when the valves to the pumps are closed.
With capillaries and helium tank connected to helim, close the recovery speedy-valve located on the top of the helium tank. The helium tank is full with helium gas.
Pump on the 1K-pumping line.
Open the 1K-NV (with Oxford Objectbench software connected to ITC503 MAIN) 50 or 80 %. Watch the pressure on the 1K-pot P-gauge. Close the valve to the 1K-pot pump to see the pressure change (increase) faster. Do this 2-3 times to rinse the 1K-NV.
CLOSE 1K-NV again (0%).
Do same rinsing procedure with UHV-sock NV: open NV 1/2 or 3/4. Watch pressure on flow meter. Repeat 2-3 times. CLOSE uhv-sock NV again.
After cleaning all lines with dry helium, open 1K and UHV lines to recovery. Keep MAIN recovery valve closed and local recovery pressurized by the helium Dewar boil-off during the entire nitrogen transfer.
Close the recovery speedy valve on the helium tank!
Quickly insert the nitrogen transfer siphon inside the helium tank, till it touches the bottom.
Quickly fit the restrictor onto the blind flange of the helium recovery.
Start transferring LN2 from 2 separate LN2 Dewars inside both helium and nitrogen tanks. It can be done at the same time. Start first slowly (~300-500 mbar) than you can speed up (0.8 to 1 bar). Normally this can take up to 2 hours. You can monitor the magnet temperature with a multimeter: ~ 300 Ω at RT, ~ 400 Ω at 77K.
Leave the nitrogen inside for 1 night. After 1 night the liquid nitrogen can be expelled from the helium tank:
Use the gas outlet of one of the 2 nitrogen Dewars to pressurize the helium tank and kick the liquid out. Connect a long hose from this gas outlet to the exaust of the helium tank - in the meantime you have replaced the constrictor with a conventional “X-mas tree”.
The long hose that was used for the initial refill should be left to air or better to an empty LN2 Dewar, so that you don't waste the LN2 coming out of the cryostat. It is very important now that the siphon is really touching the bottom of the helium tank, otherwise some LN2 will remain.
Open the gas outlet of the LN2 Dewar, slowly first than increase the pressure. LN2 should immediately start coming out of the tank. When no liquid comes out anymore, flow some dry N2 gas for 5-10 minutes to be sure that no liquid remains. The magnet temperature might/should rise a bit too (380 Ω or so).
Remove transfer line and close the tank. Remove “Xmas tree” piece and blind the tank with a flange. Open the valve to recovery, after having switched to the helium pump (1K-pot pump). Pump the tank and keep an eye on the pressure (use 1K-pot P gauge): if the recovery tube freezes and the pressure does not drop below 10mbar, STOP PUMPING. It means that there is still LN2 in the tank, and it should NOT be removed by pumping but by flowing more warm gas longer. In any case, if the siphon was lowered fully till the bottom in the previous steps and warm nitrogen gas was flown for 10 minutes after all nitrogen came out, it's very unlikely that liquid nitrogen is still inside.