Gas-controlled heat-pipes for accurate temperature measurements

Several gas-controlled heatpipes are operating at IMGC for primary temperature measurements from 100 to 962 °C. One consists of a stainless steel heat-pipe with a single thermometer well and uses 7N-pure mercury as working fluid; another is made in Inconel with six thermometer wells and uses 3N5-pure sodium as working fluid. Both heat-pipes are connected in parallel to the same accurate helium pressure control system made at IMGC. These heatpipes realize a very uniform and stable temperature zone, adequate for measurement at the millikelvin level with platinum resistance thermometers. The paper describes the two heat-pipes and their thermal characteristics, such as the temperature uniformity along the thermometer wells and their temperature response to a pressure change. For the study on the capillary structure, the heating system and the working fluids, heat-pipes in Pyrex glass with a conical connection for their opening and with three thermometer wells have been made. Some results, using pure water and a 3M™ speciality fluid for heat transfer applications, showed that a stability and uniformity of a few millikelvin can be obtained for short-term periods (about 10–15 min) without any pressure control, around the boiling temperature.