The measurement of temperature by thermoresistance,
is based on the property that metals have to vary their
ohmnic resistance with temperature. These change is
more accentuated deppending on the metal used.
The change of relative resistance with temperature
is known as temperature coefficient.
Amongst metallic conductors, pure metals present a
greater change in resistance, that is, they have a higher
temperature coefficient.
Despite to select a metal for a thermoresistace, it's
not enough to base the selection on its temperature
coefficient, there are other factors that have the same
importance as its repeatability of electrical constants
and its stability.
Metals used in industrial applications are mainly two:
•
Níckel
MEASUREMENT RANGE
-60.....a.....180ºc
•
Platinum
MEASUREMENT RANGE –220...a...630ºc
Construction:
In general we use nickel
or platinum probes with pyrex crystal or ceramic isilation
placed in a stainless steel tube filled with magnesium
oxide.
The prolongation between
the sensor and the connection terminal can be made with
copper or silver threads, all them under ceramic beads.
other current models
consist in the use of conductors embedded and isolated
with magnesium oxide, with metallic continuous sleeve.
|
