A PROJECT REPORT ON TEMPERATURE

MEASUREMENT: THERMOCOUPLE

CONTENTS

1 Temperature measurement

2 Thermocouples

3 Principle of operation

4 Practical use

5 Types of thermocouples

6 Thermocouple comparisons

7 Applications of thermocouple

8 Thermocouples used in alumina plant

9 Installation

10 Industrial assemblies

11 Selection factor

12 Advantages

13 Disadvantages

14 Conclusion

15 References

TEMPERATURE MEASUREMENT

Temperature is an expression denoting a physical condition of matter just as mass, dimension and time. Yet the idea of temperature is a relative one arrived at by number of conflicting theories. The classical theory depicts heat as a form of energy associated with activities of molecules of the substances. These minute particles of all matter are assumed to be in continuous motion which is sensed at heat temperature and is a measure of this heat.

To standardize on the temperature of heat object under varying condition several scale have been devised. The Fahrenheit scale arbitrarily assigns the number 32 to the freezing point of water and number 212 to the boiling point of water and divides the interval into 180 equal intervals. The centigrade scale shows the freezing point of water 0 and boiling point water 100.

In line with the classical theory some relation to the point where molecules motion is of minimum had to be to established and the Kelvin scale in terms of absolute zero. Zero Kelvin was determining to be -273.19⁰ C.

There are several ways to determine temperature thermodynamically including gas thermometers,

paramagnetically (low temperature) and by plank radiation method (high temperature). To establish some degree of continuity in the determination of international practical temperature scale (IPTS) was adopted in 1927.This enables temperature measurement to take place with the order of physical condition.

We cannot build a temperature divider as we can a voltage divider, nor can we add temperatures as we

would add lengths to measure distance.We must rely upon temperatures established by physical phenomena which are easily observed and consistent in nature. The International Practical Temperature Scale (IPTS) is based on such phenomena. Revised in 1968, it establishes eleven reference temperatures.

Since we have only these fixed temperatures to use as a reference, we must use instruments to interpolate between them. But accurately interpolating between these temperatures can require some fairly exotic transducers, many of which are too complicated or expensive to use in a practical situation. We shall limit our discussion to the four most common temperature transducers: thermocouples, resistance-temperature detector’s (RTD’s), thermistors, and integrated circuit sensors.

IPTS-68 REFERENCE TEMPERATURES

EQUILIBRIUM POINT K ⁰C

Triple Point of Hydrogen 13.81 -259.34

Liquid/Vapor Phase of Hydrogen 17.042 -256.108

at 25/76 Std. Atmosphere

Boiling Point of Hydrogen 20.28 -252.87

Boiling Point of Neon 27.102 -246.048

Triple Point of Oxygen 54.361 -218.789

Boiling Point of Oxygen 90.188 -182.962

Triple Point of Water 273.16 0.01

Boiling Point of Water 373.15 100

Freezing Point of Zinc 692.73 419.58

Freezing Point of Silver 1235.08 961.93

Freezing Point of Gold 1337.58 1064.43

THERMOCOUPLES

A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert heat gradient into electricity. They are inexpensive[2] and interchangeable, are supplied fitted with standard connectors, and can

measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one degree Celsius (C) can be difficult to achieve.

Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

PRINCIPLE OF OPERATION

Seebeck effect

In 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement. That difference increases with temperature, and is between 1 and 70 microvolt per degree Celsius (µV/°C) for standard metal combinations.

The voltage is not generated at the junction of the two metals of the thermocouple but rather along that portion of the length of the two dissimilar metals that is subjected to a temperature

gradient. Because both lengths of dissimilar metals experience the same temperature gradient, the end result is a measurement of the temperature at the thermocouple junction.

PRACTICLE USE

A thermocouple is a device made by two different wires joined at one end, called junction end or measuring end. The two wires are called thermoelements or legs of the thermocouple: the two thermoelements are distnguished as positive and negative ones. The other end of the thermocouple is called tail end or reference end (Figure1). The junction end is immersed in the enviroment whose temperature T2 has to be measured, which can be for instance the temperature of a furnace at about 500°C, while the tail end is held at a different temperature T1, e.g. at ambient temperature.

Figure1:Schematic drawing of a thermocouple

Because of the temperature difference between junction end and tail end a voltage difference can be measured between the two thermoelements at the tail end: so the thermocouple is a temperature-voltage transducer.

The temperature vs voltage relationship is given by:

Equation1

where Emf is the Electro-Motive Force or Voltage produced by the thermocople at the tail end, T1 and T2 are the temperatures of reference and measuring end respectively, S12 is called Seebeck coefficient of the thermocouple and S1 and S2 are the Seebeck coefficient of the two

thermoelements; the Seebeck coefficient depends on the material the thermoelement is made of. Looking at Equation1 it can be noticed that:

1. a null voltage is measured if the two thermoelements are made of the same materials: different materials are needed to make a temperature sensing device,

2. a null voltage is measured if no temperature difference exists between the tail end and the junction end: a temperature difference is needed to operate the thermocouple,

3. the Seebeck coefficient is temperature dependent.

In order to clarify the first point let us consider the following example (Figure2): when a temperature difference is applied between the two ends of a single Ni wire a voltage drop is developed across the wire itself. The end of the wire at the highest temperature, T2, is called hot end, while the end at the lowest temperature, T1, is called cold end.

Figure2: Emf produced by a single wire

When a voltmeter, with Cu connection wires, is used to measure the voltage drop across the Ni wire, two junctions need to be made at the hot and cold ends between the Cu wire and the Ni wire; assuming that the voltmeter is at room temperature T1, one of the Cu wires of the voltmeter will experience along it the same temperature drop from T2 to T1 the Ni wire is experiencing. In the attempt to measure the voltage drop on the Ni wire a Ni-Cu thermocouple has been made and so the measured voltage is in reality the voltage drop along the Ni wire plus the voltage drop along the Cu wire.

The Emf along a single thermoelement cannot be measured: the Emf measured at the tail end in Figure1 is the sum of the voltage drop along each of the thermoelements. As two thermoelements are needed, the temperature measurement with thermocuoples is a differential measurement.

Note: if the wire in Figure2 was a Cu wire a null voltage would have been measured at the voltmeter.

The temperature measurement with thermocouples is also a differential measurement because two different temperatures, T1 and T2, are involved. The desired temperature is the one at the junction end, T2. In order to have a useful transducer for measurement, a monotonic Emf versus junction end temperature T2 relationship is needed, so that for each temperature at the junction end a unique voltage is produced at the tail end.

However, from the integral in Equation1 it can be understood that the Emf depends on both T1

and T2: as T1 and T2 can change indipendently, a monotonic Emf vs T2 relationship cannot be defined if the tail end temperature is not constant. For this reason the tail end is mantained in an ice bath made by crushed ice and water in a Dewar flask: this produces a reference temperature of 0°C. All the voltage versus temperature relationships for thermocouples are referenced to 0°C.

The resulting measuring system required for a thermocople is shown in Figure3.

Voltage–temperature relationship

For typical metals used in thermocouples, the output voltage increases almost linearly with the temperature difference (ΔT) over a bounded range of temperatures. For precise measurements or measurements outside of the linear temperature range, non-linearity must be corrected. The nonlinear relationship between the temperature difference (ΔT) and the output voltage (mV) of a thermocouple can be approximated by a polynomial:

The coefficients an are given for n from 0 to between 5 and 13 depending upon the metals. In some cases better accuracy is obtained with additional non-polynomial terms. A database of voltage as a function of temperature, and coefficients for computation of temperature from voltage and vice-versa for many types of thermocouple is available online.

In modern equipment the equation is usually implemented in a digital controller or stored in a look-up table; older devices use analog circuits.

Piece-wise linear approximations are an alternative to polynomial corrections.

Polynomial Coefficients 0-500 °C

n an (for Type K)

1 25.08355

2 7.860106x10−2

3 -2.503131x10−1

4 8.315270x10−2

5 -1.228034x10−2

6 9.804036x10−4

7 -4.413030x10−5

8 1.057734x10−6

9 -1.052755x10−8

Cold junction compensation

Thermocouples measure the temperature difference between two points, not absolute temperature. To measure a single temperature one of the junctions—normally the cold junction—is maintained at a known reference temperature, and the other junction is at the temperature to be sensed.

Having a junction of known temperature, while useful for laboratory calibration, is not convenient for most measurement and control applications. Instead, they incorporate an artificial cold junction using a thermally sensitive device such as a thermistor or diode to measure the temperature of the input connections at the instrument, with special care being taken to minimize any temperature gradient between terminals. Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation. Some integrated circuits such as the LT1025 are designed to output a compensated voltage based on thermocouple type and cold junction temperature.

 

Figure4: An example of Cold Junction Compensation

TYPES OF THERMOCOUPLE

Certain combinations of alloys have become popular as industry standards. Selection of the combination is driven by cost, availability, convenience, melting point, chemical properties, stability, and output. Different types are best suited for different applications. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether it is magnetic or not. Standard thermocouple types are listed below with the positive electrode first, followed by the negative electrode.

KType K (chromel{90 percent nickel and 10 percent chromium}–alumel)(Alumel consisting of 95% nickel, 2% manganese, 2% aluminium and 1% silicon) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel.[7] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C / -328 °F to +2462 °F range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics may vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they may undergo a step change in output when the magnetic material reaches its Curie point (around 354 °C for type K thermocouples). (However, reference data for Type K thermocouples at http://srdata.nist.gov/its90/type_k/300to600.html shows no such step change near 354C.)

E

Type E (chromel–constantan)[5] has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic.

J

Type J (iron–constantan) has a more restricted range than type K (−40 to +750 °C), but higher sensitivity of about 55 µV/°C.[2] The Curie point of the iron (770 °C)[8] causes an abrupt change in the characteristic, which determines the upper temperature limit.

N

Type N (Nicrosil–Nisil) (Nickel-Chromium-Silicon/Nickel-Silicon) thermocouples are suitable for use at high temperatures, exceeding 1200 °C, due to their stability and ability to resist high temperature oxidation. Sensitivity is about 39 µV/°C at 900 °C, slightly lower than type K. Designed to be an improved type K due to increased stability at higher temperatures, it is becoming more popular, though the differences may or may not be substantial enough to warrant a change.

Platinum types B, R, and S

Types B, R, and S thermocouples use platinum or a platinum–rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only for high temperature measurements due to their high cost and low sensitivity.

B

Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C.

R

Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C.

S

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

T

Type T (copper–constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.

C

Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures. It must never be used in the presence of oxygen at temperatures above 260 °C.

MType M thermocouples use a nickel alloy for each wire. The positive wire (20 Alloy) contains 18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. These thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. It is less commonly used than other types.

Chromel-gold/iron

In chromel-gold/iron thermocouples, the positive wire is chromel and the negative wire is gold with a small fraction (0.03–0.15 atom percent) of iron. It can be used for cryogenic applications (1.2–300 K and even up to 600 K). Both the sensitivity and the temperature range depends on the iron concentration. The sensitivity is typically around 15 µV/K at low temperatures and the lowest usable temperature varies between 1.2 and 4.2 K.

Thermocouple comparison

The table below describes properties of several different thermocouple types. Within the tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. For example, a thermocouple with a tolerance of ±0.0025×T would have a tolerance of ±2.5 °C at 1000 °C.

TypeTemperature range °C (continuous)

Temperature range °C (short term)

Tolerance class one (°C)

Tolerance class two (°C)

IEC Color code

BS Color code

ANSI Color code

K 0 to +1100−180 to +1300

±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 1000 °C

±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 1200 °C

J 0 to +750 −180 to +800

±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 750 °C

±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 750 °C

N 0 to +1100−270 to +1300

±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 1000 °C

±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 1200 °C

R 0 to +1600 −50 to +1700

±1.0 between 0 °C and 1100 °C±[1 + 0.003×(T − 1100)] between 1100 °C and 1600 °C

±1.5 between 0 °C and 600 °C±0.0025×T between 600 °C and 1600 °C

Not defined.

S 0 to 1600 −50 to +1750

±1.0 between 0 °C and 1100 °C±[1 + 0.003×(T − 1100)] between 1100 °C and 1600 °C

±1.5 between 0 °C and 600 °C±0.0025×T between 600 °C and 1600 °C

Not defined.

B +200 to 0 to +1820 Not ±0.0025×T No No Not

+1700 Availablebetween 600 °C and 1700 °C

standard use copper wire

standard use copper wire

defined.

T −185 to +300 −250 to +400

±0.5 between −40 °C and 125 °C±0.004×T between 125 °C and 350 °C

±1.0 between −40 °C and 133 °C±0.0075×T between 133 °C and 350 °C

E 0 to +800 −40 to +900

±1.5 between −40 °C and 375 °C±0.004×T between 375 °C and 800 °C

±2.5 between −40 °C and 333 °C±0.0075×T between 333 °C and 900 °C

Chromel/AuFe −272 to +300 n/a

Reproducibility 0.2% of the voltage; each sensor needs individual calibration.

APPLICATIONS

Thermocouples are suitable for measuring over a large temperature range, up to 2300 °C. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy, for example the range 0–100 °C with 0.1 °C accuracy. For such applications thermistors, silicon bandgap temperature sensors and resistance temperature detectors are more suitable. Applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes.

Steel industry

Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process. Disposable, immersible, type S thermocouples are regularly used in the electric arc furnace process to accurately measure the temperature of steel before tapping. The cooling curve of a small steel sample can be analyzed and used to estimate the carbon content of molten steel.

Heating appliance safety

Many gas-fed heating appliances such as ovens and water heaters make use of a pilot flame to ignite the main gas burner when required. If it goes out gas may be released, which is a fire risk and a health hazard. To prevent this some appliances use a thermocouple in a fail-safe circuit to sense when the pilot light is burning. The tip of the thermocouple is placed in the pilot flame, generating a voltage which operates the supply valve which feeds gas to the pilot. So long as the pilot flame remains lit, the thermocouple remains hot, and the pilot gas valve is held open. If the pilot light goes out, the thermocouple temperature falls, causing the voltage across the thermocouple to drop and the valve to close. Some combined main burner and pilot gas valves (mainly by honeywell) reduce the power demand to within the range of a single universal thermocouple heated by a pilot (25mV open circuit falling by half with the coil connected to 10~12mV @ 0.2~0.25A typically) by sizing the coil to be able to hold the valve open against a light spring, only after the initial turning on force is provided by a the user pressing and holding a knob to compress the spring during first lighting. These systems are identifiable by the 'press and hold for x minutes' in the pilot lighting instructions. (The holding current requirement of such a valve is much less than a bigger solenoid designed for pulling the valve in from closed would require.) Special test sets are made to confirm the valve let-go and holding currents as an ordinary milliameter cannot be used as it introduces more resistance than the gas valve coil. Apart from testing the open circuit voltage of the thermocouple, and the near short-circuit DC continuity through the thermocouple gas valve coil, the easiest non-specialist test is substitution of a known good gas valve.

Some systems, known as millivolt control systems, extend the thermocouple concept to both open and close the main gas valve as well. Not only does the voltage created by the pilot thermocouple activate the pilot gas valve, it is also routed through a thermostat to power the main gas valve as well. Here, a larger voltage is needed than in a pilot flame safety system described above, and a thermopile is used rather than a single thermocouple. Such a system requires no external source of electricity for its operation and so can operate during a power failure, provided all the related system components allow for this. Note that this excludes common forced air furnaces because external power is required to operate the blower motor, but this feature is especially useful for un-powered convection heaters. A similar gas shut-off safety mechanism using a thermocouple is sometimes employed to ensure that the main burner ignites within a certain time period, shutting off the main burner gas supply valve should that not happen.

Out of concern for energy wasted by the standing pilot, designers of many newer appliances have switched to an electronically controlled pilot-less ignition, also called intermittent ignition. With no standing pilot flame, there is no risk of gas buildup should the flame go out, so these appliances do not need thermocouple-based pilot safety switches. As these designs lose the benefit of operation without a continuous source of electricity, standing pilots are still used in some appliances. The exception is later model instantaneous water heaters that use the flow of water to generate the current required to ignite the gas burner, in conjunction with a thermocouple as a safety cut-off device in the event the gas fails to ignite, or the flame is extinguished.

Thermopile radiation sensors

Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink. It is possible to measure radiative intensities of only a few μW/cm2 with commercially available thermopile sensors. For example, some laser power meters are based on such sensors.

Manufacturing

Thermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. For example, switchgear under test for its current carrying capacity may have thermocouples installed and monitored during a heat run test, to confirm that the temperature rise at rated current does not exceed designed limits.

Radioisotope thermoelectric generators

Thermopiles can also be applied to generate electricity in radioisotope thermoelectric generators.

Process plants

Chemical production and petroleum refineries will usually employ computers for logging and limit testing the many temperatures associated with a process, typically numbering in the hundreds. For such cases a number of thermocouple leads will be brought to a common reference block (a large block of copper) containing the second thermocouple of each circuit. The temperature of the block is in turn measured by a thermistor. Simple computations are used to determine the temperature at each measured location.

THERMOCOUPLES USED IN ALUNINA PLANT

Thermocouples are used for measurement of high temperature. In alumina plant thermocouples are used in the calciner to measure the high temperature. In alumina plant R, J and H type of thermocouples are used for measurement of high temperature. In the process of calcinations the alumina is heated to a temperature up to 1100⁰ C to make it free from moisture. So these thermocouples are capable for measurement of this temperature.

INSTALLATION:

Its normal to be apprehensive about fixing anything to do with a furnace. Natural gas and flame together can be a hazardous combination to say the least. While there are precautions to follow, when you install a thermocouple you are only establishing the electrical current that allows your pilot light to run safely. You are not working directly on any gas line. The following steps will show you how to safely install a thermocouple and soon you'll be on your way to becoming a plumber for the day.

First thing to do is to manually shut off the gas supply valve to the furnace. Remove the furnace access panel. You will need to get full visualization of the thermocouple/pilot light junction so it may be necessary to remove an additional access panel that covers up the burners. Once you've located the thermocouple begin to loosen the nut located directly underneath the thermocouple. This will have a copper wire coming out of it. In some furnaces the pilot light is located further away from the panels and you may have to remove the bracket that attaches

the thermocouple/pilot light component to the burner. After you unscrew the bracket from the burner you can bring the component closer to you so it is easier to loosen the nut keeping the thermocouple in place.

Now you can slide the thermocouple out of its steel sleeve. Follow the copper wire back from the thermocouple and you will see that the other end connects to the gas valve (usually with a red knob for the pilot light on/off control). Loosen this nut and fully remove the thermocouple component from the furnace.

The hard part is over. You are now ready to install the new thermocouple. Slide the new thermocouple up inside the steel sleeve near the pilot light. Tighten the nut to the bracket. If you removed the bracket from the burner earlier, screw the bracket back in with the new thermocouple already tight. Now you can tighten the nut connecting the wire to the gas valve. The copper wire is pliable so just move it so it is out of the way of other wires or furnace components that might damage it. Check once more to ensure the nuts have been properly tightened then replace the panel covering the burner if you've removed it. Now you can test out the thermocouple. Turn on the gas valve feeding the furnace back to the open position. Turn the red knob on the gas valve to the pilot setting. Push and hold the button in. Ignite the pilot light with a long tipped butane lighter. The pilot light should stay lit after you let go of the red button.

Additional tips

Vertical installation is preferred in very high temperatures to avoid protection tube or element sagging.Install thermocouples away from AC power lines to prevent electrical noise.

Do not run thermocouple wires in the same conduit with electrical wires.

Do not run a single thermocouple to two different instruments. This will result in instrument imbalance.

A dual thermocouple should be used instead.

Industrial Assemblies

In most process applications the temperature sensor is inserted into a thermowell or protection tube. This protects the sensor from its environment and facilitates easy removal and replacement. These assemblies generally consist of a head, nipple-union-nipple and thermo well. Smart industrial thermocouples and RTDs are available in virtually any calibration and resistance temperature coefficient.

ASSEMBLY TYPES:

Threaded Wells

o Step Down

o Straight

o Tapered

Flanged Wells

o Step Down

o Straight

o Tapered

Weld in Well

Socket Wells

o Step Down

o Straight

o Tapered

Sanitary Well

Pipe Well

o Flanged

o Threaded

High Temperature Protection TubAssemblies

SELECTION FACTOR

Why choose one T/C over another

The usual goals in picking a thermocouple type are to provide an adequate measurement over

the longest possible life, and at the lowest cost. It is prudent, for such comparisons, to consider

the total cost over some suitable time period. It is easy to overlook such hidden costs as

maintenance, testing, and replacement, or loss of production due to down time or as a result of

inaccurate readings. Other factors in making the best choice might be the availability of

instrumentation, and sometimes a need to standardize on the thermocouple type or types to

be used at a given site.

Service life

Useful thermocouple life is a very difficult prediction to make, even when most of the details of an application are known. And unfortunately, such information is often very hard to determine. The very best test for any application is to actually install, use, and evaluate the in-use performance a design that is thought likely to succeed. The recommendations, and non-recommendations, listed under the thermocouple type descriptions are a good starting place to pick a type to try.

ADVANTAGES

1 Small size.

2 Convenient to mounting. It can be mounted on wall.3 Low cost – expandable.4 Rugged - can take off use.5 Wide range – from near absolute zero to over 5000⁰F.6 Fairly accurate, calibration easily performed.7 Signal can be used by recording instrument.8 Long transmission distances are feasible.

DISADVANTAGE

1 Stray pickup a factor.2 Calibration must match temperature emf relationship.3 Must avoid temperature gradients.4 Not as simple as direct reading thermometers. 70⁰f nominal minimum span.

CONCLUSION

A key to the successful use of thermocouples is the understanding of how and why they

operate. Once the basic principle namely distributed generation of the thermocouple’s emf,

driven by the temperature gradients, or differences, through which the wires pass is known

and understood, applying thermocouples to most applications becomes straightforward and

logical.

Commercially available thermocouples are standardized by letter-designated type and by

tolerance levels of conformance to published tables or ‘curves’ of emf versus temperature.

Custom calibration of thermocouple materials is available to answer needs more critical than

can be covered by the usual tolerance grades.

An important fact to be remembered is that laboratory calibration of thermo elements is

predicated upon good uniformity or homogeneity of the thermo element being tested. New

materials will possess this property, but used materials may not, so it is not normally possible to

recalibrate used thermocouples. This is particularly true for base metal types after use at high

temperatures.

Thermocouples are available in an almost endless variety of constructions and configurations. It

is possible and practical to connect them in special ways to sense either temperature

differences or temperature averages over a number of sites. It is even possible to ‘gang’ these

devices together to boost the amount of electrical signal arising from small temperature

differences. But applications like these are specialized. The major use for thermocouples is to

make reliable and direct measurements of temperature in many diverse applications.\

Thermocouples are fundamentally simple devices. They are extremely versatile and rugged,

and are capable of operating over a very wide range of temperatures. Thermocouples can be

made to very tiny dimensions and into many different forms for standard or special purposes. In

addition, they are low in cost and are readily interchanged or replaced. But they do need to be

understood, so that they will measure the quantity that is desired with the precision that is

required.

REFERENCES

1. ̂ "Thermocouple temperature sensors". Temperatures.com. http://www.temperatures.com/tcs.html. Retrieved 2007-11-04.

2. ^ a b Ramsden, Ed (September 1, 2000). "Temperature measurement". Sensors. http://www.sensorsmag.com/sensors/temperature/temperature-measurement-1030. Retrieved 2010-02-19.

3. ̂ "Technical Notes: Thermocouple Accuracy". IEC 584-2(1982)+A1(1989). http://www.microlink.co.uk/tctable.html. Retrieved 2010-04-28.

4. ^ a b c "NIST ITS-90 Thermocouple Database". http://srdata.nist.gov/its90/main/. 5. ^ a b Baker, Bonnie C. (September 1, 2000). "Designing the embedded temperature

circuit to meet the system's requirements". Sensors. http://www.sensorsmag.com/sensors/temperature/designing-embedded-temperature-circuit-meet-system039s-requi-1089?print=1. Retrieved 2010-04-26.

6. ̂ " Thermocouple Calibration , Microstar Laboratories" 7. ̂ Manual on the Use of Thermocouples in Temperature Measurements. ASTM, 1974 8. ̂ Buschow, K. H. J.Encyclopedia of materials : science and technology, Elsevier, 2001

ISBN 0-08-043152-6 page 5021 table 1

CONTENTS

1 Bauxite unloading and crushing

2 Ball mill

3 Critical speed

4 Desilication

5 Digestion technology

6 Clarification

7 Precipitation

8 Calciner