In order to maintain the
efficiency and prolong the
service life of temperature
meters, proper selection
strategy must be followed.
The
wire
metals can
be an alloy or
a very pure metal,
and the measurement
technique works on the
principle of a thermo voltage
being created when two different metals
are brought into contact with one another.
A variety of temperature limits and
materials of construction are available
to measure temperature for different
applications. They are: copper/constantan;
chromel/alumel; platinum/rhodium-
platinum; and iron/ constantan. To
achieve high accuracy, thermocouples
used for process measurements must be
protected by a thermowell. For thermal
contact with the thermowell, a physical
contact or thermally conductive lubricant
is placed between the thermocouple and
the thermowell. For installations at low
temperatures, extreme precautions must
be taken to reduce sources of moisture.
With their ability to measure a wide range
of temperatures, thermocouples are
used in furnaces, salt baths, heat treating,
molten metal, and ceramics applications.
I nfrared
Infrared (IR) thermometers are non-
contact temperature measurement
devices that have many industrial
applications. They measure temperature
by detecting the amount of radiation
emitted by a surface. To function properly,
an IR instrument must take into account
the emissivity of the surface being
measured. The most basic design consists
of a lens to focus the infrared energy onto
a detector, which converts the energy to
an electrical signal that can be displayed
in units of temperature after being
compensated for ambient temperature
variation. With the ability to measure
temperature measurement from a
distance without contact with the object,
an IR thermometer is useful for measuring
temperature under circumstances where
thermocouples or other probe type
sensors cannot be used or do not produce
accurate data. The non-contact advantage
is particularly relevant for food industry
applications.
R esistance (RTD)
Resistance thermometers or RTDs
(resistance temperature devices) work on
the principle of the resistance of a material
changing with temperature. Platinum,
nickel and copper are regularly used for
RTDs because the change in resistance
with these metals is large. RTDs are used
in applications where faster response
and greater accuracies are required than
may be obtained with thermocouples.
Their relatively high electrical output also
makes them suitable for direct connection
to indicators, controllers, recorders, etc,
and RTDs may also be more economical
than thermocouples in some installations
since the extension wires can be made
of copper. In addition, a reference
temperature source is not required for
calibration
S election strategy
In order to maintain the efficiency and
prolong the service life of temperature
meters, proper selection strategy must
be followed. Indeed, improper selection
strategy is regarded as the most likely
cause of measurement failure, which can
also result in significant plant downtime.
In a worst case scenario, an improperly
specified temperature meter can fail to
measure temperature during an over and
under temperature event, compromising
plant safety and damaging valuable
equipment.
In order to improve temperature meter
operating efficiency and service life, as
detailed below, there are certain basic
selection criteria end users should
consider:
A ccuracy
The closeness of a reading of the
measurement device to the actual value
of the quantity be ing measured, accuracy
goes a long way in prolonging the
efficiency of a process. Usually expressed
as ± percent of full scale output or reading,
accuracy specifications can be misleading
because they are values usually published
by the OEM for ideal boundary conditions.
In practice, however, deviations from the
ideal conditions are common, so that
additional stipulations must be made
for the instrument accuracies. It is also
important to establish if the accuracy is
based on the measured value (% of rate)
or on the range end value (% of max).
P rocess media
In selecting a particular temperature
meter for installation, the phase media
compatibility with the device must be
taken into consideration, otherwise,
corrosion, wear, etc is likely will occur.
Corrosion of parts will eventually cause
10
PECM Issue 32