What
is a thermocouple A thermocouples is a
temperature sensors that measure temperature by generating a small voltage signal
proportional to the temperature difference between the junctions of two dissimilar
metals. One junction (the hot junction) is typically encased in a metal tube at
the point of measurement; the other junction (the cold junction) is connected
to the measuring instrument. The instrument measures the voltage signal and the
cold junction temperature then computes the temperature. Precautions
and considerations for using thermocouples
Most measurement problems and
errors with thermocouples are due to a lack of understanding of how thermocouples
work. Listed below are some of the common problems. Connection
problems: Many measurement errors are caused by unintentional thermocouple
junctions. Remember that any junction of two different metals will cause a junction.
If you need to increase the length of wires from your thermocouple, you must use
the correct type of thermocouple extension wire. Using the wrong wire will introduce
a thermocouple junction. Any connectors used must be made from the correct thermocouple
material and correct polarity must be observed. Lead
Resistance: To minimize thermal shunting and improve response time, thermocouples
are made from thinner wire. Remember that thinner wire will cause high resistance
which can make it sensitive to noise and can cause errors due to the input impedance
of the measuring instrument. Using twisted or shielded thermocouple wires will
help to reduce the noise. Also, larger transition wire can be used. Thermocouple
Type: Select the correct type of calibration will help to extend the life
of any temperature sensors. Insulations
Type: Choose the correct insulations type for the correct working temperature
to suit that environment will extend the life of the sensors. Elements
Protecting Sheath
Remember when ordering thermocouple probes, that the
thicker the sheath or insulating material the slower the respond time.
If possible used grounded thermocouple for larger diameter
protection tubing. Thermocouple
Junctions Basically, they are 3
junction types, grounded, ungrounded and exposed. For grounded thermocouples it
has a metal sheath and the thermocouple wires are welded to the casing. Usually,
it is done with a tig welding machine, but it can be done by soldering the wires.
However, it may cause contamination to the thermocouple wires. Usually,
grounded thermocouples are not filled with MgO power unless it is intended for
higher temperature applications. Alternatively, to mineral insulated thermocouples,
the wires can be insulated with ceramic insulators for high temperature applications. Why
grounded thermocouples? Grounded thermocouple temperature sensors are widely used,
because it offer faster respond time, more accurate reading at short distant.
It is a preferred junction type for high temperature applications. Precaution
for ground loop at long distances and at low temperature usage. Why
ungrounded Thermocouples? Ungrounded thermocouples are used primarily for isolating
the control system from the sensor and to prevent ground loop. It is more inaccurate
and slow respond time. Exposed Junction, offer
the fastest respond time. It is not intended for pressurized, corrosive or wet
environments. Thermocouple
Types Type K --- Chromel ( Nickel-Chromium
Alloy ) / Alumel ( Nickel-Aluminium Alloy )
This
is the most commomly used "general purpose" thermocouples. They are
available in the ~ -200 °C to +1200 °C range. The characteristic of the
thermocouple undergoes a step change when a magnetic material reaches its Curie
point. This occurs for this thermocouple at ~ 352°C. Sensitivity is ~ 41 µV/°C.
Type K and N are the most oxidation resistant base metal thermocouples. Not recommended
for temperature of 1200C for long period.
Type
J --- Iron / Constantan
Limited range (~ -40
to +750 °C) makes type J less popular than type K. The main application is
with old equipment that cannot accept modern thermocouples. J types cannot be
used above 760 °C as an abrupt magnetic transformation causes permanent decalibration.
The magnetic properties also prevent use in some applications. Type J's have a
sensitivity of ~ 53µV/°C. Not recommended for temperature higher than
480C because of the higher oxidation rate of the iron element.
Type
T --- Copper / Constantan
Good for temperature
in the ~ -200 to 350 °C range. Non-magnetic, type T thermocouples are a popular
choice for strong magnetic fields and subzero environments. Type T thermocouples
have a sensitivity of ~ 43 µV/°C. Most accurate base-metal thermocouple.
Can be used in vacuum and in oxidizing, reducing, or inert atmospheres.
Type E --- Chromel / Constantan ( Copper-Nickel Alloy )
Type E has a high output ~ 68µV/°C which makes it well suited
to cryogenic use. It is also non-magnetic. Recommended for use up to 870C in oxidizing
or inert atmospheres.
Type
N --- Nicrosil (Nickel-Chromium - Silicon Alloy) / Nisil (Nicke l- Silicon Alloy)
High stability and resistance to high temperature
oxidation makes type N suitable for high temperature measurements without the
cost of platinum (B, R, S) types. They can withstand temperatures above ~1200
°C. Sensitivity ~ 38 µV/°C at 900°C.
Thermocouple
types R, S and B are all noble metal thermocouples
and exhibit similar characteristics. They are the most stable of all thermocouples,
but due to their low sensitivity ~ 10 µV/°C they are usually used for
high temperature measurement (>300 °C). Type
B --- Platinum 30% Rhodium / Platinum 6% Rhodium
Good
for high temperature measurements up to ~ 1800 °C. Type B thermocouples (due
to the shape of their temperature to voltage curve) give the same output at 0
°C and 42 °C. This makes them useless below 50°C. Type
C --- Tungsten 5% Rhenium / Tungsten 26% Rhenium
Good
for temperature ~ 35 to 4200°F. This thermocouple is well suited for vacuum
furnaces at extremely high temperatures and must never be used in the presence
of oxygen at temperatures above 500°F. Type
R --- Platinum 13% Rhodium / Platinum
Good for
high temperature up to ~1600 °C. Low sensitivity ~10 µV/°C and high
cost makes them unsuitable for general purpose use.
Type
S --- Platinum 10% Rhodium / Platinum
Good for
high temperature measurements up to 1600 °C. Low sensitivity ~10 µV/°C
and high cost makes them unsuitable for general purpose use. Due to its high stability,
type S is used as the standard of calibration for the melting point of gold (1064.43
°C).
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