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New low-temperature fixed point

In the range of liquid helium temperatures between 1,5 K and 4 K, which is also important from the technical point of view, the necessary stability control of thermometers has so far been a complex undertaking. Now PTB has developed a temperature fixed point which can be easily managed using a lambda-point cell. The new device will enable a large number of users to carry out (cross) check measurements.

The liquid helium range of low temperatures is no longer of interest only for investigations related to solid-state physics and materials investigations but also for technical applications such as cooling the superconducting magnets of nuclear magnetic resonance tomographs or particle accelerators. This further increases the demand for reliable low-temperature measurements. However, the long-term stability of thermometers presently available is generally not sufficient for medium and high accuracy requirements. Therefore, the stability must be checked in regular intervals. For this purpose, temperature fixed points based on well-defined phase-transition temperatures of suitable substances are used. For liquid helium temperatures between 1,5 K and 4 K, however, the fixed points available until now relied on sophisticated measurements of other physical quantities such as, for example, vapour pressure.

Within the scope of an EU project PTB has developed and investigated a compact and easy-to-handle lambda-point cell for a wide variety of applications. At the lambda point, i. e. at Tl = 2,1768 K, the quantum liquid 4He passes from the superliquid into the normal liquid state. This is, however, a second-order phase transition at which no latent heat appears which could be used for temperature stabilization as in the case of melting points. The new cell nevertheless allows the lambda point to be used as a fixed point. For this purpose, the cell is first cooled down to just below Tl by cooling an upper heat sink. Next, a lower copper disk is electrically heated to just above Tl. Inside the cell a phase boundary between 4He in the normal liquid state (bottom) and 4He in the superliquid state (top) is formed. The heat flow Q can be set to provided steady-state conditions over a long period of time, provided the upper heat sink has a sufficiently long working time. Due to the extremely high thermal conductivity of the superliquid phase, the copper volume of the actual cell practically has the temperature Tl of the phase boundary. In tests using a commercial measuring set-up, the temperature could already be stabilized within a few mK for up to 18 hours.

The new lambda-point cell thus can provide ideal conditions to control the stability of thermometers by simple means readily accessible to a large number of users. In view of the uncertainty level achieved, the cell can probably also be used for the realization of the International Temperature Scale.

Contact at PTB:

Phone: +49-531-592-0