The calibration of thermometers requires a stable environment. In the range 90°C to 200°C normally comparison calibrations are performed in liquid baths with silicone oil as the fluid medium. This range is difficult to cover with sufficient temperature stability using commercially available baths. Therefore a calibration bath was constructed in our laboratory and presented at the 21st Nordic conference on Measurements and Calibration in 1999. The experience from this has led to this further improved version. The principle of the bath is based on a closed end aluminum tube, with an inner open concentric tube placed near the bottom and allowing free circulation of the oil around it. The circulation is achieved by a stirring propeller in the lower end, and the calibration zone is located inside the inner tube. The heating power is supplied by two different heaters, one main heater wrapped on the outside of the outer tube and one placed in the space between the cylinders. The main heater acts as a guard heater and is supplied with constant power to give the outer wall a stable temperature, just below the set point. The second smaller heater performs the regulation with the aid of a commercial controller. The bath is housed in a floor cabinet and has a working space of diameter 100 mm and a depth of 500 mm. Temperature stability and uniformity achieved in the bath is around +-1 mK in the whole range up to 200°C, and this is without the use of any kind of equalising block. This is an improvement from the earlier version in the upper temperature range. For example temperature instability and nonuniformity has decreased from +-3 mK to +-1 mK at 200°C. The performance is now comparable with the best water baths used at lower temperatures. Data and diagrams are presented in this paper.
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