Page 247 - Fluid mechanics, heat transfer, and mass transfer

TEMPERATURE MEASUREMENT 227

& Chromel–constantan
& Chromel–alumel
& Platinum–platinum þ 13% rhodium
& Platinum–platinum þ 10% rhodium
. What are the advantages and the disadvantages of
thermocouples?
Advantages
& A thermocouple is capable of measuring a wider
temperature range than an RTD.
& Simple, rugged, easy to use, inexpensive, self-
powered, wide temperature ranges.
Disadvantages
& If the thermocouple is located at some distance away
from the measuring device, expensive extension
grade thermocouple wires or compensating cables
have to be used.
Internal construction of a typical thermocouple.
FIGURE 8.2 & Thermocouples are not used in areas where high
radiation fields are present. Radioactive radiation
& Figure 8.2 shows the internal construction of a typical
(e.g., beta radiation from neutron activation) will
thermocouple. The leads of the thermocouple are
induce a voltage in the thermocouple wires. Since
encased in a rigid metal sheath. The measuring
the signal from thermocouple is also a voltage, the
junction is normally formed at the bottom of the
induced voltagewill cause an error in the temperature
thermocouple housing. Magnesium oxide surrounds
transmitter output.
the thermocouple wires to prevent vibration that
& Thermocouples are slower in response than RTDs.
could damage the fine wires and to enhance heat
transfer between the measuring junction and the & If the control logic is remotely located and temper-
medium surrounding the thermocouple. ature transmitters (millivolt to milliampere transdu-
& Thermocouples cause an electric current to flow in cers) are used, a power supply failure will cause
the attached circuit when subjected to changes in faulty readings.
temperature. & Other disadvantages include nonlinearity, low-
& The voltage generated is dependent on the temper- voltage output, requirement of reference junction,
ature difference between the measuring and the poor stability,and poor sensitivity; connections result
reference junctions, the characteristics of the two in additional junctions and voltage depends on com-
metals/alloys used, and the characteristics of the position of metals in the wires.
attached circuit. . Give the characteristics of different types of standard
& Figure 8.3 illustrates a simple thermocouple circuit. thermocouples.
& Table 8.2 gives the characteristics of thermocouples.
. What are the different metal combinations used in
thermocouples? & Types R and S are expensive and are resistant to
& Iron–constantan corrosion and high temperatures. Not very sensitive.
& Type T thermocouples are not expensive, and very
& Copper–constantan
sensitive but not suitable for corrosive and high-
temperature environments. Neither wire is magnetic.
Copper–constantan thermocouples are very suscep-
tible to conduction error due to the high thermal
conductivity of the copper and should not be used
unless long runs of wire (100–200 wire diameters)
can be laid along an isotherm.
& Type K thermocouples are more widely used for their
moderate cost and have reasonable corrosion and
high-temperature resistance. The alumel wire is
magnetic. Chromel–alumel thermocouples generate
Simple thermocouple circuit. electrical signals, while the wires are being bent and
FIGURE 8.3