Some Curious Results with a Gallium Fixed Point Cell

In 2009, most of the gallium fixed-point cells in use in different INRIM laboratories were compared with Italy’s national standard. The comparison has uncovered problems with one of the commercial devices, realizing a temperature about 0.7mK too low which initially was even changing linearly with time. An additional series of measurements was undertaken to find out the reason for this behavior, but not being allowed to open the cell, only a suspicion on the possible cause has remained. A way is suggested that might give users an indication of such misbehavior of their cell. The results underline the importance for those NMIs with only a single cell, for any fixed point, to undertake regular comparisons with another cell as a check on its behavior.