Page 272 - Instrumentation Reference Book 3E

256 Temperature measurement

The resistivity of a conductor is temperature-
dependent. The temperature coefficient of resist-
ivity is positive for metals, that is, the resistance
increases with temperature, and for semiconductors
the temperature coefficient is negative. As a general
guide at normal ambient temperatures the coeffi-
cient of resistivity of most elemental metals lies in
the region of 0.35 percent to 0.7 percent per C.
Table 14.12 shows the resistivity and tempera-
ture coefficients for a number of common metals:
both elements and alloys.
The metals most used for resistance measure-
ment are platinum, nickel, and copper. These
metals have the advantage that they can be manu-
factured to a high degree of purity and conse-
I quently they can be made with very high
I !
reproductibility of resistance characteristics.
Figure 14.15 Helical bimetal strip. Copper has the disadvantage of a low resistivity
resulting in inconveniently large sensing elements
and has the further disadvantage of poor resist-
Due to its robust construction, this instrument
ance to corrosion resulting in instability of elec-
is used on many industrial plants, and a slightly trical characteristics. The main area of application
modified form is used in many homes and offices of copper for resistance thermometers is in elec-
to indicate room temperature. It can be made for tronic instrumentation where it is in a controlled
a large variety of temperature ranges and is used environment and where an essentially linear tem-
in many places where the more fragile mercury- perature characteristic is required.
in-glass thermometer was formerly used.
14.4.1.1 Platinum resistance thermometers

14.4 Measurement techniques: Platinum is the standard material used in the
electrical resistance thermometer which defines the Inter-
national Practical Temperature Scale, not
because it has a particularly high coefficient of
14.4.1 Resistance thermometers
resistivity, but because of its stability in use. In
All metals are electrical conductors which at all fact, a high coefficient is not, in general, necessary
but very low temperatures offer resistance to the
passage of electric current. The electrical resistance
exhibited by a conductor is measured in ohms. The Table 14.12 Resistivities of different metals
proportional relationship of electrical current and
potential difference is given by Ohm's law: Metal Resistivity Tempemture
at 20°C coejfieient of'
R = E/I (14.10) microhms. resistivity
where R is resistance in ohms, E is potential meter ("C ')
difference in volts, and I is current in amperes. Aluminum 282.4 0.0039
Different metals show widely different resist- Brass (yellow) 700 0.002
ivities. The resistance of a conductor is propor- Constantan 4900 10-5
tional to its length and inversely proportional to Copper (annealed) 172.4 0.00393
its cross-sectional area, i.e., Gold 244 0.0034
L Iron (99.98%) 1000 0.005
R=p- (14.11) Mercury 9578 0.00087
A Nichrome 10 000 0.0004
780
or Nickel 11 060 0.0066
Platinum (99.85%)
0.003 927
A Silver
p=R- (14.12) 159 0.0038
L Tungsten 560 0.0045
__
where R is resistance of the conductor, p is resist- 1. Resistivities of metals dependent on the purity or exact
ivity of the material, L is length of the conductor, composition of alloys. Some of the above figures represent
and A is cross-sectional area of the conductor. average values.
The units of resistivity are ohms. meter. 2. Temperature coefficients of resistivity vary slightly with
temperature. The above values are for 20°C.