Capillary thermometers operate on the basis of expansion of fluid in a closed system, with which is filled capillary subassembly, consisting of membrane, capillary and the bulb.

The basic family of capillary thermometers is the TK 281.
 
The basic technical characteristics of the thermometer TK 281 are:

   -         fixing: snap-in cut out for fixing 26 x 59 mm
   -         temperature indication range: from 20 dgr.C to 120 dgr.C
   -         capillary length: 600 mm
   -         diameter and length of bulb: diam. 6 x 110 mm
   -         operating temperature: T 80