In these transducers the change of resistance of wire or tape is used from platinum for determination of temperature. They are named the capacitance-resistance detectors of temperature. It is not necessary from here, that other metals can not be used for taking temperature, however most often platinum pickoffs are used in such transducers.
The sensitiveness of capacitance-resistance temperature transducers is very subzero, and a dynamic reaction is enough slow (by virtue of construction of device). They are subject to destruction at vibrations and shots.
Dependence of resistance on a temperature was first fully certain for the industrial platinum thermometer of resistance in BOFS in 1904 for the range of temperatures from - 220 to + 1050°С.
There are two basic types of platinum wire capacitance-resistance transducers : the probe and pickoff assembled on a surface immersed on Wednesday. Wire elements are usually set on ceramic basis with a minimum pull and, as a rule, covered by protective material, preventing them from influence of environment.
The construction of model platinum wire probe is shown on a fig. 1(а), and the temperature pickoff assembled on a surface on a fig. 1(б). Transducers with pellicle pickoffs (fig. 1, в), metallic foil, located on an insulating base, is used in which, are not so widespread, as wire, although the sphere of their use broadens constantly, as they have small sizes, improved dynamic reaction, higher sensitiveness and relatively subzero cost.
Platinum capacitance-resistance transducers usually join in one of shoulders of bridge of Уитстона, which high exactness of measuring is provided by means of. Certainly, not high resistance of device (about 100 Ohm) creates problems during his commutation with a measuring apparatus, as resistance of wires, connecting transformer with a measuring chart, can be commensurably with resistance of transformer.
Fig. 1. Constructions of platinum wire capacitance-resistance transducers :
and is a wire probe; б is a wire transformer, assembled on surfaces of pickoff; in is a thinly pellicle transformer, set on the surface of pickoff