Fig. 1. Descriptions of dependence of resistance of some metals from a temperature:
1 is a nickel; 2 is a tungsten; 3 is a copper; 4 is platinum
Resistance sometimes also changes at fluctuation in a temperature. For a metal this change has linear dependence
R = Ro(1+ aT),
where Rо is resistance at a temperature 0 °С; Т is a temperature, °С; an is a temperature coefficient of resistance.
Model dependences of resistance of some metals on a temperature are shown on a fig. 1. They testify to the high degree of linearness of connection between resistance and temperature. For creation of temperature measuring transducers of such type a wire is usually used from platinum.
Other basic group of temperature-sensitive transducers, used in thermometric devices, is known under the name termistors. They have very nonlinear description, however can be effectively used in the systems for taking temperature. Resistance of termistor is determined by next expression:
Rt = Aexp(B/T)
where Rt is resistance; And - permanent, value of which for different materials differently; In is a characteristic temperature of device; Т is a temperature, К.
Fig. 2. Dependence of resistance of typical termistor on a temperature:
Typical description of termistor is presented on a fig. 2. Comparison of descriptions of capacitance-resistance transducers (fig.1) with descriptions of termistor allows to draw conclusion that last:
1) are more steep, i.e. the temperature coefficient of resistance substantially more than in metals for them, at least in basic part of curve;
2) fall with the increase of temperature, i.e. the temperature coefficient of resistance for them is negative.
Termistor transducers with the negative temperature coefficient of resistance are known anymore as NTC - termistors. It is necessary to notice that exist and termistors with a positive coefficient resistances which are designated as РТС- termistor. The last more frequent apply temperatures not for measuring, and, will say, for warning of overheat.
Other and by more comfortable formula for description of description of termistor in the case when his resistance of R1 is known at some temperature of T1, there is expression
R = R1expB( 1/T - 1/T1),
which turns out by a substitution in the before brought formula over of next obvious correlation :
R1 = Aexp(B/T1)
Termistors is substantially less than on sizes, than metallic capacitance-resistance transducers, and they quicker react on the change of temperature. On the other hand, the small sizes of termistors result in that for their self heating a small current is required. Consequently, it is possible to consider that a current influences on exactness of measuring.
SENSORS OF PHYSICAL PARAMETERS
Thermistor ( thermo resistance )
A thermistor is called a semiconductor component with temperature-dependent electric resistance. At the same time called thermistors and resistors made from other materials (copper, Nickel, etc.), whose resistance depends on the temperature so that it can be used to measure (control) temperature. Invented in 1930 by scientist Samuel Ruben, to this day, this component finds the widest application in engineering.
Thermistors are made from different materials, the temperature coefficient of resistance (TCR) which is quite high.
Directly the main resistive element, generally produced by powder metallurgy processing chalcogenides, halides and oxides of certain metals, giving them various forms, such as disks or rods of various sizes, large washer, medium tubes, thin plates, small beads, with dimensions from a few microns to tens of millimeters.
The nature of the correlation between the resistance element and its temperature, thermistors divided into two large groups - the PTC resistors and thermistors. PTC resistors have a positive TCR (for this reason, the heating cables are also called PTC thermistors), and thermistors — negative (called, therefore, NTC thermistors). Thus, with increasing temperature of the PTC increases and its resistance, with increasing temperature of the thermistor its resistance decreases accordingly.
Materials for RTDs are: mixture of polycrystalline oxides of transition metals such as cobalt, manganese, copper and Nickel, compounds III-V-type and doped, glassy semiconductors such as silicon and germanium, and of some other substances. Noteworthy, the heating cables of solid solutions based on barium titanate.