CONSTRUCTION AND CALIBRATION OF A THERMOCOUPLE BY N. N. Garba,* N.Rabi’u,* A. M. Yusuf,* Yusuf,* A. Isma’il,* A. K. Abubakar,* I. Abdullahi** * Department of Physics, Ahmadu Bello University, Zaria **School of Basics and Remedial Studies (A.B.U Zaria), Funtua. Corresponding author’s address
[email protected] [email protected]..+2348034298444
ABSTRACT
Temperature, being one of the factors that affect man and his environment, requires a sensitive device for its measurement over a wide range with a relatively high degree of accuracy. accuracy. In an effort to determine temperature value using various laboratory thermometers, one hardly gets accurate results, or wide range of values, therefore, there is the need to come up with an instrument to solve this problem. In this work a type k thermocouple was constructed and calibrated and the values obtained are in good agreement with the standard of type k thermocouple, the values va lues are E = 10.20mV, 10.20mV, a = 0.055mV/°C and b = 0.001mV/°C both at t0 = 275°C which, within the experimental limit, will take c are of most of the aforementioned problems. A thermocouple is an instrument whose principle of operation is based on the theory of thermoelectricity. Furthermore, the components used in the construction are cheap and available in our market today. INTRODUCTION
potential difference difference into electric potential
It turns out nowadays that, in electronics
difference(2). They are relatively cheap (3) in our
and electrical engineering, thermocouples are a widely widely used type for temperature sensing/measurement
(1)
markets and interchangeable have standard
and can also be
connectors and can measure a wide range
used as a means to convert thermal
of temperature. The main limitation is accuracy; a research by Kieran Thomas 1
degree Celsius (1oc) can be difficult to
shows that system errors of less than one
achieve.
Thermocouple used principle of
the complete the circuit creates a circuit
thermoelectricity in its operation
in which the two ends legs generate
thermoelectricity is the electricity
different voltages, leaving a small
generated by application of heat to the
difference in voltage available for
junction of two dissimilar metals arranges
measurement that difference incrassates
so that two junctions are formed (4)
with temperature, and can typically be
In 1821, the German –Estonian physicist
between 1 and 70 (µV/oc) for the modern
Thomas Johann see beck discovered that
range of available metal combinations
when any conductor (such as a metal) is
A thermocouple can produce
subjected to a thermal any conductor, it
current, which means it can be used to
will generate a voltage. This is known as
drive some processes directly without the
thermoelectric effect or sees beck effect.
need for extra circuiting and power
Any attempt to measure this voltage
sources
necessarily involves connecting another
For example, the power from a
conductor to the “hot” ends. This
thermocouple can active a value when a
additional conductor will then also
temperature difference difference arises. The electric
experience the temperature gradient, and
power generated by a thermocouple is a
develop a voltage of its own which will
conversion of the heat energy that one
oppose the original. Fortunately , the
must couple to maintain the electric
magnitude of the effect depends on the
cause the current flowing flowing through the
metal in use, using a dissimilar metal on 2
thermocouple tends to heat up this is
touches the probes2. As both conductors
known as Peltier effect(5)
are non-magnetic type T thermocouples are a popular choice for application such as a electrical generators which contain
TYPES OF THERMOCOUPLE
A variety of thermocouples are
strong magnetic fields-its sensitivity is
available, suitable for different
about 43µV/0C (tungsten 5% rhenium –
measurements. They are usually selected
tungsten 26% rhenium) thermocouple are
based on the temperature range and
suited for measurement in the range O0C
sensitivity needed. Thermocouple with
to 23200c. This thermocouple is well-
low sensitivities (B, R and S types) criteria
suited for vacuum furnace at extremely
include the inertness of the thermocouple
high temperatures and must never be used
material, and whether or not is magnetic.
in the presence of oxygen at temp .above
In this research we focused on
2600c6.
constructing and calibrating type T
Materials and method
thermocouple because its construction
The materials needed for this construction
does not require any hardship, the
are
construction is simple and direct. It gives
-
larger E.M.F for a small temperature
insulated copper wire
difference which is not so to other type1.
-
Type Type T thermocouple is made of
2 pieces of 32SWG. 1m length
32 SWG, IM length insulated
constantan wire
copper – constantan and this is suitable for
-
6cm Tinmans solder
temperature ranges between (-200 – 350)
-
100w, 240V soldering iron with
0
soldering bit
C often used as a differential
measurement since only copper wire
Method
3
The construction was done as
TESTING
follows.
After the construction, the thermocouple
-
the the insu insula lati tion on was was caref careful ully ly remo remove ved d at
was tested and a highly impressive result
one end of the copper wire and also at
was obtain
both ends of the constantan wire
procedure: First the insulation insulation of the
One One end end of of the the con const stan anta tan n was was then then
ends of the copper wires of the
soldered carefully with bare end of the
thermocouple were removed and the bare
copper wires. The other end of the
ends were connected to a galvanometer
constantan wire was again soldered
(G) one copper-constantan junction was
carefully with the bare end of the other
maintained at room temperature and the
piece of copper wire and the
other junction was taken of a furnace as:
-
thermocouple was then formed.
Cu
Cu
Constantan
Furnace
Fig(1) Testing of Thermocouple
The furnace was then switched on and the
the furnace reaches 300C. The
galvanometer deflects when the temp of
galvanometer keeps deflecting with 4
increasing temperature of the furnace.
repeated but the reference junction inside a
Maximum deflection was obtained at a
melting the in a beaker of the
temperature around 2400C beyond which
galvanometer happened with maximum
the deflection reduces until it finally
deflection of the galvanometer now at
became zero at temperature around 6850C
temp around 3000c and the reversal reversal takes
and the deflection increases in the reverse
place at about 7100C6.
direction. The same procedure was Test Result Reference junction
Furnace
Galvanometer
Temperature ( 0c ) 25 25 25 25 25 25 25 25 25 0 0 0 0 0 0 0 0 0
Temperature ( 0c ) 25 30 40 100 200 250 300 600 690 25 30 40 100 200 300 400 500 700
Deflection No deflection Very small deflection Deflection increase Significant increase Continue increases Steady deflection Fall in deflection Significant fall Almost zero def Start deflection Increase to deflection Continue to deflect Some significant increases Significant deflection Steady deflection Fall in deflection Significance fall Almost zero deflection
Thermal EMF measurement
5
After test result was obtained then the
melting ice in a beaker was joined to
EMF (thermal) was found by connecting
A, to which the positive terminal of X
the thermocouple to a potentiometer
is connected. The other junction in was
whose resistances was measured by an
warmed in fingers and a balance point
ohmmeter.
was obtained closed to A, this hot
Apparatus
junction switched on
-
2v accumulator X
-
Varia ariabl blee resi resist stor or (Re (Resi sist stanc ancee box box R)
balance points were then located until
-
Pote Potent ntio iome mete terr of of len lengt gth h AB
the balance point L reached a
-
Sens Sensit itiv ivee gal galva vano nome mete terr G wit with h
maximum near temperature of about
protective resists P=13Ω
3000c. this temp was then fairly
Jock Jockey ey,, 2 blo block ck of of ice ice and and a 500m 500mll
maintained steady and the value of R
beaker
from the resistance box final value of
-
Furnac nace an and ohm ohmm meter
R was noted and not been altered
-
Voltmeter
again, the furnace was heated up to
Method
about 5000c and then allowed to cool
The potentiometer was connected to
slowly to room temp and carefully carefully the
the thermocouple as can be seen in fig
balance point was taken at 500c
2.0 with R set temporarily at about 500
interval after the protective resistive
Ω. It should be noted that the cold
was removed.
-
As temperature rises, approximate
junction C, to which surrounded by
6
R x
L
C B
o c o o o - o o Furnace
Fig (2) Measurement of thermal EMF
The values measured are as follows: Resistance of wire AB (S)
= 3.6Ω
EMF of the accumulator X
= 2.0V
Resistance R
= 682Ω
Cold junction temperature.
= 00C.
7
Hot Junction temp.( oC) 0 50 100 150 200 250 300 350 400 450 500
Balance length(cm) 0 19 39 65.7 84.7 96.2 96.2 78.1 57.1 39 22.9
E.M.F of the couple(mV) 0.00 2.00 4.10 6.90 8.90 10.10 10.10 8.20 6.00 4.10 2.40
The P.d P.d across the wire AB was was found by the relation P.d. = 1000E (R+S)
×S
= 3.6 x 1000 × 2 682 +3.6
= 10.5 mV
Then the thermal E.M.F was found using the relation
Ethermal = L x P.d across AB 100 = L x 10.5 100
8
The EMF thermal was found at all other balance points and the result was tabulated as follows. T(0C) 0 50 100 150 200 250 300 350 400 450 500
L(cm) 0 .0 19.0 39.0 65.7 84.7 96.2 96.2 78.1 57.1 39.0 22.9
EMF thermal (mV) 0.00 2.00 4.10 7.00 9.00 10.10 10.10 8.20 6.00 4.10 2.40
origin which is given by all values chosen from the graph i.e. a =
Calibration
Calibrating a thermocouple means
0.055mV/0C Also as can be seen seen from
finding the numerical values of a and
the graph temp. At which the
b in the equation E = at1 +bt22
maximum EMF was obtained is known as neutral temp. Which is 2750C from
(1) in which one of the temp is is known
graph denoted by to with a =
and the corresponding E thermal is
0.55mV/0c equal thus become
known as well the constants can be
E = 0.055t1 +bt22
from the calibration growth plotted
But rate of change of E with temp
(fig 3.0) which is the graph of E
is zero at stationing point
thermal against temp for this this couple
A +2bt = 0
the calibration graph was plotted and
Gy (2) becomes
the constants of the graph at the
dE = 0.055 +2bt 9
(2)
dt
2b (275) = -0.055 0.055+2bt = 0 2bt = - 0.055
(3)
550b = 0.055/550 b = -0.0001 ؞
E = 0.55t – 0.0001t2
(4)
But stationing point the graph was By the above equationn a given 0
neutral temp t = 275 c temp will lead to the determination of Eqn. 3 becomes E and vice versa.
Graph of thermal EMF against Temperature
12
E M 10 F ( 8 m V ) 6 4 2 0 0
200
400
600
Temperature ( 0C)
Figure (3) calibration graph of constructed type T thermocouple
At the end of the research we
CONCLUSION
were able to find out the 10
maximum EMF to be 10.20mV,
thermoelectric effects: Department
neutral temperature t o = 2750C, a
of physics and mathematics Zwitch
=0.055mV/0C and,
University of applied sciences 8401
b = - 0.001mV/ 0C which is in
Winterthur Switzerland.
good agreement with the
4. M. NELKON, PARKER.
standard value of type T
(1984); Advance level physics:
thermocouple.
Heinemann educational books London. 5. A. KEY (1990); Fundamental
REFERENCES
physics laboratory pp34:
1.(http://www.temperatures.co 1.(http://www.temperatures.co m/tcs.html); m/tcs.html ); Thermocouple
Macmillan publishing company.
temperature sensors: Retrieved on
6. RICHARD PARK (2000);
11/04/2007.
Thermocouple gradient theory:
2.(http://www.omega.com/thermoc 2.(http://www.omega.com/thermoc
Thermocouple works marlin
ouples.html); ouples.html ); Thermocouple – an
manufacturing company.
introduction: Omega engineering
retrieved 11/04/2007 3. HANS U.F. (2002); a simple
continuum model leading to the reciprocity relation for 11
