The YSI 17725 is a free-standing, mechanically refrigerated cryostat. The cell is placed in its well, which is operated in the form of an essentially gradient-free heat pipe. A unique control system provides a precise means for regulation of the cell and well temperature over the range from -36°C to -42°C, allowing the mercury triple point to be approached optionally from the freezing or melting direction. Two auxiliary wells are pre-chilling thermometers to be calibrated. The cell containment is a plasma-arc welded stainless steel shell. The mercury contained has impurities of less than 15 parts per billion. The purity of the metals such that triple point plateaus are flat to 100 microkelvins (0.0001°C).