WHAT IS A THERMOCOUPLE?
In 1821, Thomas Seebeck discovered if metals of two different materials were joined
at both ends and one end was at a different temperature than the other, a current was
created. This phenomenon is known as the Seebeck effect and is the basis for all
thermocouples.
T1 T2
A thermocouple is a type of temperature sensor, which is made by joining two
dissimilar metals at one end. The joined end is referred to as the HOT JUNCTION.
The other end of these dissimilar metals is referred to as the COLD END or COLD
JUNCTION. The cold junction is actually formed at the last point of thermocouple
material
Positive leg
Hot junction Cold Junction
(Joined End)
Negative leg
Certain combinations of metals must be used to make up the thermocouple pairs.
If there is a difference in temperature between the hot junction and cold junction, a
small voltage is created. This voltage is referred to as an EMF (electro-motive force)
and can be measured and in turn used to indicate temperature.
The voltage created by a thermocouple is extremely small and is measured in terms of
millivolts (one millivolt is equal to one thousandth of a volt). In fact, the human body
creates a larger millivolt signal than a thermocouple.
To establish a means to measure temperature with thermocouples, a standard scale of
millivolt outputs was established. This scale was established using 32 deg. F (0°C) as
the standard cold junction temperature (32 deg. F (0°C) = 0 millivolts output). THERMOCOUPLE THEORY
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COLD JUNCTION COMPENSATION
As we mentioned earlier, the last point of thermocouple material is known as the cold
junction. The amount of output the t/c produces is determined by the difference
between the hot junction and the cold junction temperatures. The cold junction
temperature must be known to accurately determine the temperature.
Lets look at the following examples;
If we had a thermocouple in a heat treat furnace and wanted to know what
temperature it was in that furnace, we could attach a voltmeter to the cold junction
and measure the voltage.
Let’s say that the furnace is operating at 1000 deg. F. and it is 100 deg. F at the
cool end of the T/C. Since we said that a T/C measures the difference between the
hot and cold junctions, our formula would be:
1000 (hot junction) - 100 (cold junction) = 900 deg. F.
There seems to be a problem since we said that the furnace was at 1000 deg. F.
This brings us to COLD JUNCTION COMPENSATION.
COLD JUNCTION COMPENSATION is usually done automatically by the measuring
instrument. The instrument measures the temperature at the cold junction and adds it
back to the equation.
1000 (hot junction) - 100 (cold junction) = 900 deg. F + 100 deg. F
(cold junction temp) = 1000 deg F
This way the instrument indicates the actual temperature of the hot junction.
This COLD JUNCTION compensator is usually located at the terminals on the back
of the indicating instrument and you must maintain T/C material all the way to this
point.
For a thermocouple to function properly, there must be no other metals used between
the hot junction and the cold junction. If wire is needed to connect the T/C to the
indicating instrument, the leadwire must be made of the same material as the T/C.
It is acceptable to use terminal blocks and lugs made of plain copper in a
thermocouple circuit as long as the positive and negative terminals are at the same
temperature. (Example: terminal blocks in heads or spade lugs on wire)
If you were to use plain copper wire instead of T/C extension wire to run to the
instrument, your cold junction would be formed at the junction between the copper
and the T/C wire. This junction would most likely not be at the same temperature as
the back of the instrument where the compensator is located. This would then create
an error in the indicated temperature. THERMOCOUPLE THEORY
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If a customer were to use the wrong T/C extension wire, the same problem could
appear. This is why we must use the correct T/C extension wire on our assemblies.
It is also acceptable to have a third metal in the hot junction as long as that metal is at
the same temperature as the thermocouple material.
REFERENCE TABLES
There are printed tables that show the temperature vs. millivolt output figures. These
reference tables are all based on the cold junction being at the freezing temperature of
water (32 deg F or 0 deg C).
We use these tables in our Certification Lab along with ice baths to make our cold
junctions at 32 deg. F.
Based on ASTM E-230
THERMOCOUPLE TYPES
All thermocouples have a corresponding color code per ASTM E-230 (replaces ANSI
MC96.1)
Consult the Pyromation catalog page GEN-6 for a complete list of American color
codes
BASE METAL THERMOCOUPLES
Base metal thermocouples are known as Types E, J, K, T and N and comprise the
most commonly used category of Thermocouple. The conductor materials in base
metal thermocouples are made of common and inexpensive metals such as Nickel,
Copper and Iron.
Type E: The Type E thermocouple has a Chromel (Nickel-10% Chromium)
positive leg and a Constantan (Nickel- 45% Copper) negative leg. Type E has a
temperature range of -330 to 1600F, has the highest EMF Vs temperature values of
all the commonly used thermocouples, and can be used at sub-zero temperatures.
Type E thermocouples can be used in oxidizing or inert atmospheres, and should not
be used in sulfurous atmospheres, in a vacuum or in low oxygen environments where
selective oxidation will occur. The color code for TYPE E wire is purple and red.
Type J: The Type J thermocouple has an Iron positive leg and a Constantan
negative leg. Type J thermocouples can be used in vacuum, oxidizing, reducing and
inert atmospheres. Due to the oxidation (rusting) problems associated with the iron
leg, care must be used when using this thermocouple type in oxidizing environments
above 1000F. The temperature range for Type J is 32 to 1400F and it has a wire
color code of white and red. THERMOCOUPLE THEORY
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Type K: The Type K thermocouple has a Chromel positive leg and an Alumel
(Nickel- 5% Aluminum and Silicon) negative leg. Type K is recommended for use
in oxidizing and completely inert environments. Because it’s oxidation resistance is
better than Types E, J, and T they find widest use at temperatures above 1000F.
Type K, like Type E should not be used in sulfurous atmospheres, in a vacuum or in
low oxygen environments where selective oxidation will occur. The temperature
range for Type K is -330 to 2300F and it’s wire color code is yellow and red.
Type N: The Type N thermocouple has a Nicrosil (Nickel-14% Chromium- 1.5%
Silicon) positive leg and a Nisil (Nickel- 4.5% Silicon- .1% Magnesium) negative
leg. Type N is very similar to TYPE K but is less susceptible to selective oxidation
effects. Type N should not be used in a vacuum or in reducing atmospheres in an
unsheathed condition. The temperature range is 32-2300 deg F and its wire color code
is orange and red.
Type T: The Type T thermocouple has a Copper positive leg and a Constantan
negative leg. Type T thermocouples can be used in oxidizing, reducing or inert
atmospheres, except the copper leg restricts their use in air or oxidizing environments
to 700F or below. The temperature range for Type T is -330 to 700F and it’s wire
color code is blue and red.
NOBLE METAL THERMOCOUPLE TYPES
Noble Metal Thermocouples are another category of thermocouples and are made of
the expensive precious metals Platinum and Rhodium. There are three types of noble
metal thermocouples:
Type B (Platinum/Platinum-30% Rhodium)
Type R (Platinum/Platinum-13% Rhodium)
Type S (Platinum/Platinum-10% Rhodium)
Types R and S have temperature ranges of 1000 to 2700F and Type B thermocouples
have a temperature range of 32 to 3100F.
As can be seen above, the difference between these three thermocouples is the amount
of Rhodium contained in the negative leg. Types R and S can exhibit excessive grain
growth in the platinum when exposed to the higher end of its temperature range. The
increased amount of Rhodium in the Type B thermocouple helps to reduce the grain
growth problem allowing for a slightly increased temperature range.
Noble metal thermocouples are intended for use in oxidizing or inert atmospheres.
They must not be used in reducing atmospheres or in applications containing metallic
or nonmetallic vapors. Noble metal thermocouples are soft and prone to being
damaged if mishandled. These thermocouple assemblies are usually assembled in
ceramic insulators and supplied with ceramic protection tubes. Noble metals should
never be supplied in metal protection tubes only. The color code for Types R and S is
black and red, and the color code for Type B is gray and red. THERMOCOUPLE THEORY
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Refractory Metal Thermocouples
Refractory Metal Thermocouples are the last category of thermocouple that
Pyromation manufactures. These thermocouples are made of the exotic metals
Tungsten and Rhenium, which are expensive, difficult to manufacture, brittle, and
must be handled carefully. There are three types of refractory metal thermocouples:
Type G (Tungsten/Tungsten 26% Rhenium)
Type D (Tungsten 3% Rhenium/Tungsten 26% Rhenium)
Type C (Tungsten 5% Rhenium/Tungsten 26% Rhenium)
(Note: The Type designations listed for the refractory thermocouples are industry
standard designations, not standardized thermocouple types)
All of these types have a temperature range of 32-4200 deg F. Refractory metal
thermocouples are normally used in vacuum furnaces beyond the temperature
capabilities of platinum. They are seldom used below approximately 2500F since
there are other thermocouple types more suited for the lower temperatures. Refractory
metal thermocouples and their associated refractory metal protection tubes must not
be used in the presence of oxygen at temperatures above 500°F.
The color code for refractory thermocouples are:
Type G: White with blue tracer and red
Type D: White with yellow tracer and red
Type C: White with red tracer and red
LIMITS OF ERROR
Tolerances now covered by ASTM E-230
T/C wire is manufactured, then tested and sorted by accuracy.
Refer to Pyromation catalog page GEN-5
THERMOCOUPLE RULES OF THE ROAD
The most important rule to remember when manufacturing thermocouples is that red
is always negative! (For domestic manufacturers)
The second most important rule to remember is that red is always negative!
Regardless of the T/C type, the red leg is the negative leg. This is different than your
car battery or a DC circuit.
Each thermocouple type has a different color code, which is shown on the handout.
This will allow you to make sure that you are using the right calibration plug or jack
with the right T/C.
THERMOCOUPLE THEORY
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It is very important to memorize the color codes for types K, J, T, E, and N.
Thermocouple types J and K have one leg, which is magnetic. This is one of the
easiest ways to determine polarity if there is no color-coding to help (as with MgO
T/C’s). If you were to refer to the THERMOCOUPLE TYPE COLOR CODE catalog
handout, it has a column that indicates the magnetic leg.
On the type J thermocouple, the POSITIVE leg is strongly magnetic. According to the
handout, the white leg would be the magnetic leg.
With the type K T/C, the RED OR NEGATIVE leg is the magnetic leg. It is not as
strongly magnetic and can be hard to detect on smaller diameter wire.
These are the only two types of thermocouples that have a magnetic leg so if a magnet
sticks to one of the wires, you know it is type K or J.
Color codes and magnetic legs are just tools to help assure that our connections are
made properly.
THERMOCOUPLE MYTHS
There should never be a third metal in the hot junction
To create a thermocouple junction, all that is needed is to electrically short the ends
together. Butting the wire ends against a metal surface will create a junction.
Remember, that the thermocouple signal is generated over the entire length of wire.
You must use special limits of error extension wire if your thermocouple is special
limits
This is not necessary if the extension wire is outside the temperature gradient area.
Although the signal is generated over the entire length of wire, the important area is
the gradient between the hot and ambient areas.
Non-thermocouple materials cannot be used in the thermocouple circuit.
It is permissible to use non-thermocouple materials as terminal blocks or splices as
long as there is no temperature gradient across these devises.
I can get an average temperature just by wiring my thermocouples in parallel.
To get a true average, all thermocouples in a parallel circuit must be the same length
or have the same resistance.
The largest possible extension wire should be used to connect a thermocouple.
This phrase used to be true 30 years ago before there was solid-state electronics. The
old instruments were Voltage based circuits and resistance was critical. The newer
solid-state electronics are current based so extension wire resistance is not important.
