Page 288 - Instrumentation Reference Book 3E

272 Temperature measurement

Table 14.13 Thermocouples to British Standards
Tjp Conductors Manufactured to Temperature tolerance Output for indicated Service temperature
(positive conductor firsf) BS 4937 Part No. class 2 therniocouple temperature (cold (mar intermittent
BS 4537: Part 20: 1591 junction ai 0 "C) servicer)
Platinum: 30% Part 7: 1974 (1981) 600-1700 "C * 3°C 1.241 mV at 500°C 0-1500 "C (1700 'C).
Rhodiumlplatinum: 6% Better life expectancy at
Rhodium high temperature than
types R & S.
Nickel: chromium/ Part 6: 1974 (1981) -40 +333"Ck 3'C 6.317mV at 100°C -200 to +850°C
constantan (chromell 333-900°C f 0.75% (1100°C). Resistant to
constantan) (chromeb oxidizing atmospheres.
advance)
Ironiconstantan Part 3: 1973 (1981) -40 to +333"C 5 2.5"C 5.268 mV at 100 "C -280 to f850"C
300 - 750°C * 0.75% (1100°C). Low cost:
suitable for general use.
Nickel: chromium/ Part 4: 1973 (1981) -40 to +333"Cir2,5"C 4.095mV at 100°C -200 to +11OO"C
nickel: aluminum 333-1200°C f 0.75% (1300°C). Good
(chromelialumel) (CIA) general purpose. Best in
(TliT2) oxidizing atmosphere.
Nickel: chromium: Part 8: 1986 -40 to +333"C f 2.5"C 2.774mV at 100°C 0-1100°C (-270°C to
siliconinickel: silicon: 333-1200°C ir 0.75% +130O"C). Alternative
magnesium (nicrosili to type K.
nisil)
Platinum: 13% Part 2: 1973 (1981) 0-600"C* 1.5"C 4.471 mV at 500 "C 0-1500°C (1650°C).
rhodinmiplatinum 600-1600°C ir 0.25% High temperature.
Corrosion resistant.
Platinum: 10% Part 1: 1973 (1981) 0-600"C* 1.5"C 4.234mV at 500°C Type R is more stable
rhodium/platinum 600-1600°C f 0.25% than type S.
Coppericonstantan Part 5: 1974 (1981) -40 to +375 "C f "C 4.277mV at 100°C -250 to 400'C
1
(copperiadvance) (500°C). High
(CuiCon) resistance to corrosion
by water.
settle on, and react with, the platinum limb, causing specified in BS 4937. These three are based only
a fall in e.m.f. output. This is a comparatively slow on platinum and rhodium. Gold, iridium, other
process and is therefore only of significance in "platinum metals," and silver are also not uncom-
installations where the maximum stability and monly used. Figure 14.35 shows the characteris-
repeatability are required. Type B thermocouples tics of some of the options available.
are less susceptible to rhodium drift than types R
or S, but type B has a lower e.m.f. than R and S 14.5.3 Thermocouple construction
and is subject to higher errors.
Noble metal thermocouples may also be used Thermocouples, like resistance thermometers and
for measuring cryogenic temperatures. Iron- other temperature sensors, are available in a wide
goldhickel-chromium or iron-goldlsilver (nor range of mechanical constructions.
mal silver with 0.37 atomic percent gold) may
be used for temperatures from 1K to above 14.5.3.1 Plain wire thermocouples
300 K.
Noble metal thermocouples are often used in For use in protected environments, such as for
the "metal-clad" form with magnesia or alumina laboratory use or inside otherwise enclosed
powder as the insulant. This form of construction equipment, plain wire thermocouples can be used.
is described in Section 14.5.3.2. They are also used in plants where the fastest
The following sheath materials are used: nickel, possible response is required. However, they suffer
stainless steel. inconel in 1.6 and 3.2mm sizes, from the obvious disadvantage that they are
and 5 percent rhodium-plated and 10 percent both fragile and liable to chemical attack. The
rhodium-platinum both in l.Omm sizes. For wires are available insulated with PVC or glass
high-temperature work other special thermo- fiber sleeving, or for use with higher temperatures,
couples have been developed, tungsten 5 percent the wires can be insulated with refractory ceramic
rheniumhungsten 20 percent rhenium for use in beads or sleeves.
hydrogen, vacuum, and inert gas atmospheres
up to 2320 "C, and tungsten/molybdenum and
tungstenhridium for temperatures up to 2100 "C. 14.5.3.2 Sheathed thermocouples
There is quite a wide range of precious metal Thermocouples for use in plant situations, where
theirnocouples available. Types E, R, and S are robust construction is required or where they