Dilution refrigerator

A dilution refrigerator comprising:

• a still;
• a mixing chamber;
• a pump to pump coolant from the still through a still outlet port;
• a heat exchanger connected between the still and mixing chamber whereby coolant flows under the assistance of the pump from the still to the mixing chamber and from the mixing chamber to the still through respective first and second adjacent paths in the heat exchanger;
• means for defining an access path extending to the mixing chamber, wherein the means for defining an access path comprises an access tube which extends through the still;
• a probe to insert along the access path, the probe having a displacer which substantially fills the cross-section of the access path in use;
• and
• means for allowing any coolant from the mixing chamber which flows along the access path past the displacer to flow from the access path into the still, wherein the still outlet port is separate from the access path and
• wherein the means for allowing coolant to flow from the access path into the still comprises an aperture extending through the access tube.