We describe a device to conveniently measure the positions of the phase
boundaries in a dilution refrigerator. We show how a simple modification of a
standard capacitive level gauge (segmentation of one of the electrodes) permits
a direct calibration of the capacitance versus phase boundary position. We
compare this direct calibration with the indirect procedure that must be
adopted for a conventional capacitive level gauge. The device facilitates the
correct adjustment of the 3He/4He fraction in the dilution refrigerator.Comment: 3 page