Abstract: A dilution refrigerator includes a still; a mixing chamber; a pump to pump coolant from the still through a still outlet port and 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. An access path extends to the mixing chamber. A probe is provided for insertion along the access path, the probe having a displacer which substantially fills the cross-section of the access path in use. Any coolant from the mixing chamber which flows along the access path past the displacer can flow from the access path into the still. The still outlet port is separate from the access path.