Isotech calibration baths employ sophisticated designs to ensure calibration to the smallest of uncertainties. In 1986 Isotech purchased the interests of H N Irving & Sons Temperature division which had specialised in providing high quality baths to NPL and other international laboratories. The knowledge and concepts from this scientific instrument manufacturer were used in the development of the Isotech 815 and 915 Parallel Tube Liquid Baths. Whilst there is a higher cost compared to simple designs the quality and pedigree is unmatched. The key feature is the physical separation of the chamber in which the liquid is heated or cooled from the chamber that contains the calibration volume in which the thermometers are placed. The geometry of parallel tube is also important, with the shape and angle of the curves optimised for temperature uniformity. The new Libra Model 785 achieves this with the same parallel tube assembly as used in the 915 packaged into a different case and now with internal refrigeration can be used to lower temperatures. The cooling power varies dynamically with the benefit of giving greater stability at lower temperatures.