The invention relates to a temperature calibrating system (1) comprising a thermo siphon, a heat pipe or an equivalent (2) connected between a cooling unit (4) and a temperature calibration unit (3), where the system (1) further comprises an external chamber (8) connected to the heat pipe/thermo siphon or equivalent (2) for controlling the thermal conductivity between the two units and where the temperature of the external chamber (8) is held at a certain temperature, for example ambient temperature or a temperature controlled by external means. The external chamber (8) can be connected to the heat pipe/thermo siphon (2) via a conduit (9) which connection point is placed above liquid level at a evaporating end of the heat pipe/thermo siphon (2) and in that the external chamber (8) is arranged below the connection between the conduit (9) and the heat pipe/thermo siphon (2).