See the flow charts for operating the cryocoolers in our manuals.
When the cryocooler first cools down, the pumps cool mainly by heat conduction through the heat switches, and the cold heads cool by gas convection inside the gas tubes. Generally the 3He pump cools most rapidly because it is small, followed by the heat switches and the 4He cold head. The 4He pump and the 3He cold head will usually remain warm for longest.
When the switches reach about 15 to 20K they will turn OFF, because the exchange gas inside them is absorbed onto the charcoal inside the small absorber pod at the end of the small side tube. Once the switches are OFF the pumps will stop cooling and may even heat up. The 4He pump will often remain at about 20 to 40K when its switch turns OFF, but the 3He pump is always very cold when its switch turns OFF. The 3He head will stop cooling once the 3He pump is colder than about 25K, because the gas inside is absorbed into the pump, and so cannot cool the 3He head by convection. This is why you must wait until the 3He switch has turned off, and then warm up the 3He pump above 30K to allow the 3He head to cool down. Once the cold heads and the Film Burner are all at around 4 to 5K you need to heat the pumps up. You should get the 4He pump to around 45 to 55K, but the 3He pump only needs to be heated to about 45K.
When the pumps are at these temperatures you should turn off the heater power to the 4He pump, and keep only a small stabilisation heater power on the 3He pump. Allow around 10 to 15 minutes for the heads to cool down. This is the time when the 4He gas is turning into liquid. After the 10 or 15 minutes have passed, the 4He head temperature should have dropped to a stable value below ~4K and the 4He should be mostly liquid inside the 4He cold head. Now you can apply heater power to the 4He switch. As the switch absorber pod warms up to 15 or 20K, the gas inside is released into the switch body, causing heat conduction. The gas release increases the switch heat conduction from a few milliwatts to several watts, and it is this that allows the 4He pump to cool down. At this time you will see the 4K stage get hotter as heat is dumped from the 4He pump, but the 4He cold head and Film Burner will start to cool, slowly at first, but then more quickly as the 4He pump gets colder.
While the 4He head cools it also makes the 3He head get colder. You should now turn OFF the 3He pump heater power, and you will see the 3He pump cool slowly. This is OK. You now must wait until the 3He head temperature is well below the critical temperature of 3He (<3.32K) before turning ON the 3He switch. It is best if you can wait until the temperature is below 2K, or even down to 1K, before turning the 3He switch ON. This will then cool the 3He pump rapidly in the same way that the 4He switch does for the 4He pump.
The cryocooler should now run with the 4He head at a temperature of less than 1K, and the 3He head will cool to below 300mK, provided there are no excess thermal load present. If all is well the cryocooler will stay cold for many hours with the cold heads at these temperatures.