The Density Apparatus allows measurement of aqueous liquids, e.g., ultrapure water and seawater, under the metastable supercooled conditions, i.e. at temperatures below the equilibrium freezing point. The setup can currently operate at pressures up to 200 MPa and temperatures down to -40°C.

Liquid sample is contained in two fused silica capillaries of inner diameter of about 0.3 mm. The capillaries are surrounded with a low-freezing heat transfer liquid providing a good thermal coupling to the stainless steel (SS) pressure cell. The menisci of the capillaries are pressurized with nitrogen. Alternatively, a mercury plug long a few millimeters is used to separate the water sample from nitrogen to prevent its dissolution and bubble formation upon pressure reduction. Nitrogen also pressurizes the heat transfer liquid so that the stress in the fused silica is only hydrostatic.

The left parts of the capillaries of lengths a₁ and a₂ (varying between 200 and 600 mm) are enclosed in a SS tube of outer diameter 6.35 mm and inner diameter 2.1 mm. The tube is located in a special heat exchanger connected alternatively to a cooling circulation thermostat delivering ethanol at desired temperature (e.g., -30°C) and to another circulation thermostat providing ethanol at a reference temperature (e.g., +20°C). The heat exchanger has a low thermal capacity allowing that the temperature stabilizes to about ±0.02 K in several minutes. This allows measurement of deeply supercooled samples. The right parts of the capillaries of length b are contained in an optical cell equipped with a pair of sapphire windows. The optical cell is a heavy-walled pressure vessel (figure 1) kept at the reference temperature. The distances z₁ and z₂ of the menisci of water in the capillaries from a mark are read optically. The capillaries are attached to a glass reticle grated by 0.1 mm. A high-resolution camera allows determining the distances z₁ and z₂ with uncertainty smaller than 0.01 mm by pixel-wise interpolation between the gratings. The advantage of using two capillaries, one long and the other short, is that the effect of temperature transition zone shown in figure 2 as lengths e and f is eliminated by taking the difference of readings of both capillaries. The pressurized parts of the apparatus are covered by safety containment.