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外文翻译：
Thermocouple Temperatur
sensor
Introduction to Thermocouples
The thermocouple is one of the simplest
of all sensors. It consists of two wires of
dissimilar
metals
joined
near
the
measurement
point.
The
output
is
a
small
voltage
measured between the two wires.
While
appealingly
simple
in
concept,
the
theory
behind
the
thermocouple
is
subtle,
the
basics
of
which
need
to
be
understood
for
the
most
effective
use
of
the
sensor.
Thermocouple theory
A
thermocouple circuit has at least two junctions:
the measurement junction and
a
reference junction. Typically, the reference
junction is created where the two wires
connect to the measuring device. This
second junction it is really two junctions: one
for each of the two wires, but because
they are assumed to be at the same temperature
(isothermal) they are considered as one
(thermal) junction.
It is the point
where the
metals
change
-
from
the
thermocouple
metals
to
what
ever
metals
are
used
in
the
measuring
device - typically copper.
The
output
voltage
is
related
to
the
temperature
difference
between
the
measurement and the reference
junctions. This is phenomena is known as the
Seebeck
effect.
(See
the
Thermocouple
Calculator
to
get
a
feel
for
the
magnitude
of
the
Seebeck voltage). The
Seebeck effect generates a small voltage along the
length of a
wire, and is greatest where
the temperature gradient is greatest. If the
circuit is of wire
of identical
material, then they will generate identical but
opposite Seebeck voltages
which will
cancel. However, if the wire metals are different
the Seebeck voltages will
be different
and will not cancel.
In
practice
the
Seebeck
voltage
is
made
up
of
two
components:
the
Peltier
voltage generated at
the junctions, plus the Thomson voltage generated
in the wires by
the temperature
gradient.
The Peltier voltage is
proportional to the temperature of each junction
while the
Thomson voltage is
proportional to the square of the temperature
difference between
the two junctions.
It is the Thomson voltage that accounts for most
of the observed
voltage and non-
linearity in thermocouple response.
Each thermocouple type has its
characteristic Seebeck voltage curve. The curve
is dependent on the metals, their
purity, their homogeneity and their crystal
structure.
In the case of alloys, the
ratio of constituents and their distribution in
the wire is also
important. These
potential inhomogeneous characteristics of metal
are why thick wire
thermocouples
can
be
more
accurate
in
high
temperature
applications,
when
the
thermocouple metals and their
impurities become more mobile by diffusion.
The practical considerations of
thermocouples
The above theory of
thermocouple operation has important practical
implications
that are well worth
understanding:
1.
A
third
metal
may
be
introduced
into
a
thermocouple
circuit
and
have
no
impact,
provided
that
both
ends
are
at
the
same
temperature.
This
means
that
the
thermocouple
measurement
junction
may
be
soldered,
brazed
or
welded
without
affecting
the
thermocouple's
calibration,
as
long
as
there
is
no
net
temperature
gradient along
the third metal.
Further, if the
measuring circuit metal (usually copper) is
different to that of the
thermocouple,
then provided the temperature of
the
two connecting terminals is
the
same and known, the reading will not be
affected by the presence of copper.
2.
The thermocouple's output is generated by the
temperature gradient along the
wires
and not at the junctions as is commonly believed.
Therefore it is important that
the
quality of the wire be maintained where
temperature gradients exists. Wire quality
can
be
compromised
by
contamination
from
its
operating
environment
and
the
insulating material. For
temperatures below 400°
C, contamination
of insulated wires
is
generally not
a problem.
At temperatures above
1000°
C, the choice of insulation
and sheath materials, as well as the
wire thickness, become critical to the calibration
stability of the thermocouple.
The
fact
that
a
thermocouple's
output
is
not
generated
at
the
junction
should
redirect attention to other potential
problem areas.
3.
The
voltage
generated
by
a
thermocouple
is
a
function
of
the
temperature
difference
between
the
measurement
and
reference
junctions.
Traditionally
the
reference junction was held at
0°
C by an ice bath:
The ice
bath is now considered impractical and is replace
by a reference junction
compensation
arrangement.
This
can
be
accomplished
by
measuring
the
reference
junction
temperature
with
an
alternate
temperature
sensor
(typically
an
RTD
or
thermistor) and applying
a correcting voltage to the measured thermocouple
voltage
before scaling to temperature.
The
correction
can
be
done
electrically
in
hardware
or
mathematically
in
software. The software method is
preferred as it is universal to all thermocouple
types
(provided the characteristics are
known) and it allows for the correction of the
small
non-linearity over the reference
temperature range.
4. The low-level
output from thermocouples (typically 50mV full
scale) requires
that
care
be
taken
to
avoid
electrical
interference
from
motors,
power
cable,
transformers and
radio signal pickup. Twisting the thermocouple
wire pair (say 1 twist
per 10 cm) can
greatly reduce magnetic field pickup. Using
shielded cable or running
wires in
metal conduit can reduce electric field pickup.
The measuring device should
provide
signal filtering, either in hardware or by
software, with strong rejection of the
line frequency (50/60 Hz) and its
harmonics.
5.
The
operating
environment
of
the
thermocouple
needs
to
be
considered.
Exposure to oxidizing or reducing
atmospheres at high temperature can significantly
degrade some thermocouples.
Thermocouples containing rhodium (B,R and S types)
are not suitable under neutron
radiation.
The advantages and
disadvantages of thermocouples
Because
of their physical characteristics, thermocouples
are the preferred method
of
temperature
measurement
in
many
applications.
They
can
be
very
rugged,
are
immune to shock and vibration, are
useful over a wide temperature range, are simple
to manufactured, require no excitation
power, there is no self heating and they can be
made very small. No other temperature
sensor provides this degree of versatility.
Thermocouples
are
wonderful
sensors
to
experiment
with
because
of
their
robustness, wide
temperature range and unique properties.
On the down side, the thermocouple
produces a relative low output signal that is
non-linear. These characteristics
require a sensitive and stable measuring device
that
is able provide reference junction
compensation and linearization.
Also
the
low
signal
level
demands
that
a
higher
level
of
care
be
taken
when
installing to minimise potential noise
sources.
The measuring hardware
requires good noise rejection capability. Ground
loops
can
be
a
problem
with
non-
isolated
systems,
unless
the
common
mode
range
and
rejection is adequate.
Types
of thermocouple
About
13
'standard'
thermocouple
types
are
commonly
used.
Eight
have
been
given an internationally recognised
letter type designators. The letter type
designator
refers to the emf table, not
the composition of the metals - so any
thermocouple that
matches
the
emf
table
within
the
defined
tolerances
may
receive
that
table's
letter
designator.
Some
of
the
non-
recognised
thermocouples
may
excel
in
particular
niche
applications and have gained a degree
of acceptance for this reason, as well as due to
effective marketing by the alloy
manufacturer. Some of these have been given letter
type designators by their manufacturers
that have been partially accepted by industry.
Each thermocouple type has
characteristics that can be matched to
applications.
Industry
generally
prefers
K
and
N
types
because
of
their
suitability
to
high
temperatures, while others often prefer
the T type due to its sensitivity, low cost and
ease of use.
A table of
standard thermocouple types is presented below.
The table also shows
the temperature
range for extension grade wire in brackets.
Accuracy of thermocouples
Thermocouples will function over a wide
temperature range - from near absolute
zero to their melting point, however
they are normally only characterized over their
stable range. Thermocouple accuracy is
a difficult subject due to a range of factors. In
principal
and
in
practice
a
thermocouple
can
achieve
excellent
results
(that
is,
significantly better than
the above table indicates) if
calibrated, used well below its
nominal
upper temperature limit and if protected from
harsh atmospheres. At higher
temperatures
it
is
often
better
to
use
a
heavier
gauge
of
wire
in
order
to
maintain
stability (Wire Gauge below).
As mentioned previously, the
temperature and voltage scales were redefined in
1990.
The
eight
main
thermocouple
types
-
B,
E,
J,
K,
N,
R,
S
and
T
-
were
re-characterised in
1993 to reflect the scale changes. (See: NIST
Monograph 175 for
details). The
remaining types: C, D, G
, L, M, P and U
appear to have been informally
re-
characterised.
Try the thermocouple
calculator. It allows you the determine the
temperature by
knowing the measured
voltage and the reference junction temperature.
Thermocouple wire grades
There
are
different
grades
of
thermocouple
wire.
The
principal
divisions
are
between measurement
grades and extension grades. The measurement grade
has the
highest purity and should be
used where the temperature gradient is
significant. The
standard measurement
grade (Class 2) is most commonly used. Special
measurement
grades
(Class
1)
are
available
with
accuracy
about
twice
the
standard
measurement
grades.
The
extension
thermocouple
wire
grades
are
designed
for
connecting
the
thermocouple to the measuring device.
The extension wire may be of different metals
to the measurement grade, but are
chosen to have a matching response over a much
reduced temperature range - typically
-40°
C to 120°
C. The reason
for using extension
wire is reduced
cost - they can be 20% to 30% of the cost of
equivalent measurement
grades. Further
cost savings are possible by using thinnergauge
extension wire and a
lower temperature
rated insulation.
Note: When
temperatures within the extension wire's rating
are being measured,
it is OK to use the
extension wire for the entire circuit. This is
frequently done with T
type extension
wire, which is accurate over the -60 to
100°
C range.
Thermocouple
wire gauge
At
high
temperatures,
thermocouple
wire
can
under
go
irreversible
changes
in
the
form
of modified
crystal
structure, selective migration of
alloy
components
and
chemical
changes
originating
from
the
surface
metal
reacting
to
the
surrounding
environment.
With
some
types,
mechanical
stress
and
cycling
can
also
induce
changes.
Increasing the diameter of the wire
where it is exposed to the high temperatures
can reduce the impact of these effects.
The following table can be
used as a very approximate guide to wire gauge:
At
these
higher
temperatures,
the
thermocouple
wire
should
be
protected
as
much
as possible from hostile gases. Reducing or
oxidizing gases can corrode some
thermocouple wire very
quickly. Remember, the purity of the
thermocouple wire is
most important
where the temperature gradients are greatest. It
is with this part of the
thermocouple
wiring where the most care must be taken.
Other
sources
of
wire
contamination
include
the
mineral
packing
material
and
the protective metal
sheath.
Metallic vapour
diffusion can be significant
problem at
high temperatures.
Platinum
wires
should only
be used
inside a nonmetallic sheath,
such as
high-purity alumna.
Neutron
radiation (as in a nuclear reactor) can have
significant permanent impact
on
the
thermocouple
calibration.
This
is
due
to
the
transformation
of
metals
to
different elements.
High temperature measurement is very
difficult in some situations. In preference,
use
non-contact
methods.
However
this
is
not
always
possible,
as
the
site
of
temperature measurement
is not always visible to these types of sensors.
Colour coding of
thermocouple wire
The colour coding of
thermocouple wire is something of a nightmare!
There are
at least seven different
standards. There are some inconsistencies between
standards,
which seem to have been
designed to confuse. For example the colour red in
the USA
standard is always used for the
negative lead, while in German and Japanese
standards
it is
always the
positive lead. The
British, French and
International
standards
avoid
the use of red entirely!
Thermocouple mounting
There
are
four
common
ways
in
which
thermocouples
are
mounted
with
in
a
stainless steel or Inconel
sheath and electrically insulated with mineral
oxides. Each
of the methods has its
advantages and disadvantages.
Sealed
and
Isolated
from
Sheath
:
Good
relatively
trouble-free
arrangement.
The principal
reason for not using this arrangement for all
applications is its sluggish
response
time - the typical time constant is 75 seconds
Sealed
and
Grounded
to
Sheath
:
Can
cause
ground
loops
and
other
noise
injection, but
provides a reasonable time constant (40 seconds)
and a sealed enclosure.
Exposed Bead
: Faster
response time constant (typically 15 seconds), but
lacks
mechanical
and
chemical
protection,
and
electrical
isolation
from
material
being
measured. The porous insulating mineral
oxides must be sealed
Exposed Fast
Response: Fastest response time constant,
typically 2 seconds but
with fine gauge
of junction wire the time constant can be 10-100
ms. In addition to
problems of the
exposed bead type, the protruding and light
construction makes the
thermocouple
more prone to physical damage.
Thermocouple compensation and
linearization
As mentioned above, it is
possible to provide reference junction
compensation in
hardware or in
software. The principal is the same in both cases:
adding a correction
voltage
to
the
thermocouple
output
voltage,
proportional
to
the
reference
junction
temperature.
To
this
end,
the
connection
point
of
the
thermocouple
wires
to
the
measuring
device (i.e. where the thermocouple materials
change to the copper of the