A hot wire sensor 10, fig. 3, comprises a hot wire module 14, wherein a measuring hot wire 26 and a compensation hot wire 28 can be detachably connected to a circuit of the hot wire sensor. A first contact pair 52, 54 is associated with the measuring wire 26 and a second contact pair 56, 58 is associated with the compensation wire 28. The contacts of the second contact pair are configured as leading contacts in relation to at least one of the contacts of the first contact pair, such that the contacts of the compensation wire are contacted electrically chronologically before at least one of the contacts of the measuring wire during connection. This may be achieved by different length contact pins 52, 54, 56, 58. This ensures no flow of current through the measuring wire before the compensation wire is electrically contacted and prevents undesired premature energisation. Anaesthesia apparatus and a ventilator comprising the hot wire sensor and hot wire module are also claimed.
