TEMPEARTURE MEASUREMENT DEFINATION A TEMPERATURE IS A COMPARATIVE

TEMPEARTURE MEASUREMENT DEFINATION: A TEMPERATURE IS A COMPARATIVE OBJECTIVE MEASURE OF HOT AND COLD. IT IS MEASURED, TYPICALLY BY A THERMOMETER, THROUGH THE BULK BEHAVIOR OF A THERMOMETRIC MATERIAL, DETECTION OF HEAT RADIATION, OR BY PARTICLE VELOCITY OR KINETIC ENERGY. IT MAY BE CALIBRATED IN ANY OF VARIOUS TEMPERATURE SCALES, CELSIUS, FAHRENHEIT, KELVIN, ETC. MEASUREMENTS WITH A SMALL THERMOMETER, OR BY DETECTION OF HEAT RADIATION, CAN SHOW THAT THE TEMPERATURE OF A BODY OF MATERIAL CAN VARY FROM TIME TO TIME AND FROM PLACE TO PLACE WITHIN IT.

TEMPERATURE SCALES : q CELSIUS q FAHRENHEIT q KELVIN q RANKINE ETC.

CELSIUS SCALE: THE CELSIUS, OR CENTIGRADE, SCALE RECEIVED ITS NAME FROM ASTRONOMER ANDREW CELSIUS. IT IS BASED ON A WATER FREEZING POINT OF 0 C AND A WATER BOILING POINT OF 100 C. THE 100 -DEGREE DIFFERENCE BETWEEN THOSE VALUES EXPLAINS THE ALTERNATE NAME OF CENTIGRADE. THE CELSIUS VALUE FOR ABSOLUTE ZERO IS - 273. 15. CONVERSION AS 0 C = (0 F – 32)*5/9 OR 0 F = 0 C *9/5 +32

FAHRENHEIT SCALE THE FAHRENHEIT SCALE, NAMED AFTER PHYSICIST DANIEL GABRIEL FAHRENHEIT, WAS USED IN MOST ENGLISH-SPEAKING COUNTRIES UNTIL THE 1970 S, WHEN MOST OF THOSE COUNTRIES SWITCHED TO THE CELSIUS SCALE. THIS SCALE FEATURES A WATER BOILING POINT OF 212 F AND A WATER FREEZING POINT OF 32 F. ABSOLUTE ZERO HAS A VALUE OF MINUS 459. 67 F.

KELVIN SCALE: THE KELVIN SCALE WAS NAMED FOR THE PHYSICIST WILLIAM THOMSON, BARON KELVIN. THE SCALE HAS DEGREES EQUIVALENT IN SIZE TO THE CELSIUS SCALE, BUT THE KELVIN SCALE HAS AN ABSOLUTE ZERO OF 0 COMPARED TO CELSIUS' MINUS 273. 15. 0 C + 273. 15 = 0 K WATER BOILS AT 373. 15 K AND FREEZES AT 273. 15.

RANKINE SCALE LIKE KELVIN, RANKINE IS A THERMODYNAMIC SCALE, MEANING THE ABSOLUTE ZERO EQUALS ZERO. RANKINE'S DEGREES, HOWEVER, ARE EQUAL IN SIZE TO THOSE OF THE FAHRENHEIT SCALE. PRIMARILY USED IN ENGINEERING. THE SCALE WAS NAMED FOR ENGINEER AND PHYSICIST WILLIAM JOHN MACQUORN RANKINE. THE SCALE HAS A WATER BOILING POINT OF 671. 67 R AND A WATER FREEZING POINT OF 491. 67 R. 0 R = 0 F + 459. 67

Temperature Scale Formulae And Equations From To Fahrenheit Celsius C = ( F - 32) / 1. 8 Fahrenheit kelvin K = ( F + 459. 67) / 1. 8 Fahrenheit Rankine Ra = F + 459. 67 Celsius Fahrenheit F = C × 1. 8 + 32 Celsius kelvin K = C + 273. 15 Celsius Rankine Ra = C × 1. 8 + 32 + 459. 67 kelvin Celsius C = K - 273. 15 kelvin Fahrenheit kelvin Rankine Ra = K × 1. 8 Rankine Celsius C = ( Ra - 32 - 459. 67) / 1. 8 Rankine Fahrenheit Rankine kelvin Formulae Conversion Factors 9/5 = 1. 8 9/4 = 2. 25 10/8 = 1. 25 Formulae F = K × 1. 8 - 459. 67 F = Ra - 459. 67 K = Ra / 1. 8

1. CLASSIFICATION OF TEMP. MEASURING METERS GLASS THERMOMETER WITH MERCURY (-38 TO 760 0 F) ALCOHOL PENTANE AND OTHER ORGANIC LIQUID. 2. PRESSURE GAUGE THERMOMETER ( -38 TO 1000 0 F) *LIQUID FILLED : FILLED WITH MERCURY, ETHYL ALCOHOL * VAPOUR FILLED : METHYL CHLORIDE , SULPHUR DIOXIDE * GAS FILLED : NITROGEN, HAVING FAST RESPONSE AND UNIFORM SCALE. 3. BIMETALLIC THERMOMETER : (-100 TO 1000 0 F) DIFFERENTIAL EXPANSION OF TWO DISSIMILAR METALS IS ISED. 4. ELECTRICAL RESISTANCE THERMOMETER (-400 TO 1800 0 F) TEMP. IS DETERMINED BY MEASURING THE RESISTANCE OF A CALIBRATED WIRE. 5. THERMOCOUPLE (-300 TO 2000 0 F) 6. PYROMETER * OPTICAL PYROMETER ( MORE THAN 1400 0 F) * RADIATION PYROMETER ( MORE THAN 1000 0 F)

1. LIQUID-IN-GLASS THERMOMETERS A WORKING LIQUID WHICH IS GENERALLY EITHER MERCURY OR ALCOHOL.

IT MAINLY COMPRISES: Ø A BULB WHICH ACTS AS A CONTAINER FOR THE FUNCTIONING LIQUID WHERE IT CAN EASILY EXPAND OR CONTRACT IN CAPACITY. Ø A STEM, “A GLASS TUBE CONTAINING A TINY CAPILLARY CONNECTED TO THE BULB AND ENLARGED AT THE BOTTOM INTO A BULB THAT IS PARTIALLY FILLED WITH A WORKING LIQUID”. Ø A TEMPERATURE SCALE WHICH IS BASICALLY PRESET OR IMPRINTED ON THE STEM FOR DISPLAYING TEMPERATURE READINGS. ØPOINT OF REFERENCE I. E. A CALIBRATION POINT WHICH IS MOST COMMONLY THE ICE POINT. Ø A WORKING LIQUID WHICH IS GENERALLY EITHER MERCURY OR ALCOHOL. AN INERT GAS, MAINLY ARGON OR NITROGEN WHICH IS FILLED INSIDE THERMOMETER ABOVE MERCURY TO TRIM DOWN ITS VOLATILIZATION.

üTHE RESPONSE TIME OF A LIQUID-IN-GLASS THERMOMETER VARIES ACCORDING TO THE KIND OF THERMOMETER, ITS BULB VOLUME, THICKNESS AND OVERALL WEIGHT. ü FOR GETTING QUICK RESPONSE, THE BULB OF THERMOMETER SHOULD BE DESIGNED IN SUCH A WAY THAT IT RESULTS IN SMALL AND THE BULB WALL THIN. ü THEIR SENSITIVITY IS BASED UPON THE REVERSIBLE THERMAL EXPANSION CHARACTERISTICS OF THE LIQUID IN COMPARISON TO THE GLASS. THE MORE THERMAL EXPANSION OF THE LIQUID, THE HIGHLY SENSITIVE THERMOMETER IS.

APPLICATIONS LIQUID-IN-GLASS THERMOMETERS ARE MAINLY EMPLOYED IN NAVY AND MARINE CORPS IN DIFFERENT CONFIGURATIONS. THEY ARE ALSO APPLIED IN METEOROLOGICAL AND OCEANOGRAPHIC APPLICATIONS, WHERE THEY ARE GENERALLY CALIBRATED BY GRADUATIONS ETERNALLY ENGRAVED INTO THE GLASS.

ADVANTAGES v THEY ARE COMPARATIVELY CHEAPER THAN OTHER TEMPERATURE MEASUREMENT DEVICES. v THEY ARE HANDY AND CONVENIENT TO USE. v UNLIKE ELECTRICAL THERMOMETERS, THEY DO NOT NECESSITATE POWER SUPPLY OR BATTERIES FOR CHARGING. v THEY CAN BE FREQUENTLY APPLIED IN AREAS WHERE THERE IS PROBLEM OF ELECTRICITY. v THEY PROVIDE VERY GOOD REPEATABILITY AND THEIR CALIBRATION REMAINS UNAFFECTED.

LIMITATIONS • THEY ARE CONSIDERED INAPT FOR APPLICATIONS INVOLVING EXTREMELY HIGH OR LOW TEMPERATURES. • THEY CAN NOT BE APPLIED IN REGIONS WHERE HIGHLY ACCURATE RESULTS ARE DESIRABLE. • AS COMPARED TO ELECTRICAL THERMOMETERS, THEY ARE VERY WEAK AND DELICATE. THEREFORE, THEY MUST BE HANDLED WITH EXTRA CARE BECAUSE THEY ARE LIKELY TO BREAK. • BESIDES, THEY CAN NOT PROVIDE DIGITAL AND AUTOMATED RESULTS. HENCE, THEIR USE IS LIMITED TO AREAS WHERE ONLY MANUAL READING IS ADEQUATE, FOR EXAMPLE, A HOUSEHOLD THERMOMETER. • “TEMPERATURE READINGS SHOULD BE NOTED IMMEDIATELY AFTER REMOVAL BECAUSE A GLASS THERMOMETER CAN BE AFFECTED BY THE ENVIRONMENTAL TEMPERATURE, HEAT PRODUCED BY THE HAND HOLDING IT, CLEANING, ETC. THIS TEMPERATURE SHOULD BE RECORDED BECAUSE A GLASS THERMOMETER DOES NOT OFFER A RECALL OF THE MEASURED TEMPERATURE. ”

• READING TEMPERATURE VIA LIQUID-IN-GLASS THERMOMETERS CALL FOR BRILLIANT EYESIGHT. • LIQUID ELEMENT CONTAINED IN A GLASS THERMOMETER MAY BE PERILOUS OR RISKY TO HEALTH OWING TO THEIR POTENTIAL CHEMICAL SPILLS. • THESE THERMOMETERS DISPLAY TEMPERATURE EITHER IN CELSIUS OR FAHRENHEIT SCALES. THUS, TEMPERATURE CONVERSION WOULD BE NEEDED IF THE TEMPERATURE READING IS WANTED IN SOME OTHER SCALE.

2. BIMETALLIC THERMOMETERS A CANTILEVER STRIP: THE BIMETALLIC STRIPS ARE BONDED TOGETHER IN A CANTILEVER. THE DEFLECTION IS USED TO INDICATE TEMPERATURE. BIMETAL : SUCH AS STEEL OR STAINLESS STEEL, COUPLED WITH A LOW THERMAL EXPANSION ALLOY SUCH AS INVAR.

BIMETALLIC THERMOMETERS ARE MADE UP OF BIMETALLIC STRIPS FORMED BY JOINING TWO DIFFERENT METALS HAVING DIFFERENT THERMAL EXPANSION COEFFICIENTS. BASICALLY, BIMETALLIC STRIP IS A MECHANICAL ELEMENT WHICH CAN SENSE TEMPERATURE AND TRANSFORM IT INTO A MECHANICAL DISPLACEMENT. THIS MECHANICAL ACTION FROM THE BIMETALLIC STRIP CAN BE USED TO ACTIVATE A SWITCHING MECHANISM FOR GETTING ELECTRONIC OUTPUT. ALSO IT CAN BE ATTACHED TO THE POINTER OF A MEASURING INSTRUMENT OR A POSITION INDICATOR. VARIOUS TECHNIQUES SUCH AS RIVETING, BOLTING, FASTENING CAN BE USED TO BOND TWO LAYERS OF DIVERSE METALS IN A BIMETALLIC STRIP. HOWEVER THE MOST COMMONLY USED METHOD IS WELDING.

WORKING THE WORKING OF A BIMETALLIC STRIP THERMOMETER IS BASED UPON THE FACT THAT TWO DISSIMILAR METALS BEHAVE IN A DIFFERENT MANNER WHEN EXPOSED TO TEMPERATURE VARIATIONS OWING TO THEIR DIFFERENT THERMAL EXPANSION RATES. ONE LAYER OF METAL EXPANDS OR CONTRACTS MORE THAN THE OTHER LAYER OF METAL IN A BIMETALLIC STRIP ARRANGEMENT WHICH RESULTS IN BENDING OR CURVATURE CHANGE OF THE STRIP. “ONE END OF A STRAIGHT BIMETALLIC STRIP IS FIXED IN PLACE. AS THE STRIP IS HEATED, THE OTHER END TENDS TO CURVE AWAY FROM THE SIDE THAT HAS THE GREATER COEFFICIENT OF LINEAR EXPANSION. ”

MAIN FEATURES THESE TYPES OF THERMOMETERS WORK BEST AT HIGHER TEMPERATURES, SINCE THEIR ACCURACY AND SENSITIVITY TENDS TO REDUCE AT LOW TEMPERATURES. BIMETALLIC STRIP THERMOMETERS ARE MANUFACTURED IN VARIOUS DESIGNS. ONE OF THE MOST POPULAR DESIGN I. E. FLAT SPIRAL IS SHOWN IN THE FIGURE ABOVE. THEY CAN ALSO BE WOUND INTO A SINGLE HELIX OR MULTIPLE HELIX FORM. A SPIRAL STRIP: THE BIMETALLIC STRIP IS COILED INTO A SPIRAL ATTACHED TO A DIAL THAT INDICATES TEMPERATURE

BIMETALLIC THERMOMETERS CAN BE CUSTOMIZED TO WORK AS RECORDING THERMOMETERS TOO BY AFFIXING A PEN TO THE POINTER. THE PEN IS LOCATED IN SUCH A WAY THAT IT CAN MAKE RECORDINGS ON A CIRCLING CHART. BIMETALLIC STRIPS OFTEN COME IN VERY LONG SIZES. HENCE, THEY ARE USUALLY COILED INTO SPIRALS WHICH MAKE THEM COMPACT AND SMALL IN SIZE. THIS ALSO IMPROVES THE SENSITIVITY OF BIMETALLIC STRIPS TOWARDS LITTLE TEMPERATURE VARIATIONS. “THE BIMETALLIC STRIP CAN BE SCALED UP OR DOWN. ON A LARGE SCALE, IT CAN PROVIDE LITERALLY TONES OF FORCE FOR MECHANICAL CONTROL OR OTHER PURPOSES. ON A SMALLER SCALE, IT CAN PROVIDE THE FORCE AND MOVEMENT FOR MICRO MACHINE INTEGRATED CIRCUITS (MMIS). ”

APPLICATIONS • BIMETALLIC STRIPS ARE ONE OF THE OLDEST TECHNIQUES TO MEASURE TEMPERATURE. • THEY CAN BE DESIGNED TO WORK AT QUITE HIGH TEMPERATURES I. E. UPTO 500°F OR 260°C. • MAJOR APPLICATION AREAS OF A BIMETALLIC STRIP THERMOMETER INCLUDE: o FOR VARIOUS HOUSEHOLD APPLIANCES SUCH AS OVENS ETC. o THERMOSTAT SWITCHES o WALL THERMOMETERS o GRILLS o CIRCUIT BREAKERS FOR ELECTRICAL HEATING DEVICES

BIMETALLIC STRIP ADVANTAGES AND DISADVANTAGES • POWER SOURCE NOT REQUIRED • ROBUST, EASY TO USE AND CHEAP BUT NOT VERY ACCURATE. CAN BE USED TO 500 °C • LIMITED TO APPLICATIONS WHERE MANUAL READING IS ACCEPTABLE, E. G. A HOUSEHOLD THERMOMETER • NOT SUITABLE FOR VERY LOW TEMPERATURES BECAUSE THE EXPANSION OF METALS TEND TO BE TOO SIMILAR, SO THE DEVICE BECOMES A RATHER INSENSITIVE THERMOMETER

3. PRESSURE THERMOMETER • • LIQUID PRESSURE THERMOMETER VAPOUR PRESSURE THERMOMETER INDICATOR LIQUID PRESSURE THERMOMETER PRESSURE SENSER SUCH AS BOURDON TUBE BULB CAPILLARY

THE INSIDES OF THERMO-SENSOR CYLINDER, CAPILLARY, AND BOURDON TUBE ARE FILLED WITH LIQUID WHICH CHANGES VOLUME WITH VARYING TEMPERATURE. THE LIQUID IN THERMO-SENSOR CYLINDER IS EXPANDED AND CONTRACTED AS THE TEMPERATURE CHANGES, AND THE PRESSURE CHANGE IN THE BOURDON TUBE THAT IS TRANSMITTED THROUGH THE CAPILLARY IS INDICATED AS A TEMPERATURE. SINCE THE DISPLACEMENT OF THE BOURDON TUBE DUE TO AN INTERNAL PRESSURE CHANGE IS DIRECTLY INDICATED ON THE SCALE DIVISION PLATE WITHOUT PASSING THROUGH THE TRANSMISSION MECHANISM AND EXPANSION DEVICE, THIS PRODUCT IS EXCELLENT IN VIBRATION RESISTANCE.

VAPOR PRESSURE THERMOMETERS PRINCIPLE THE VAPOR PRESSURE OF A LIQUID IS THE PRESSURE UNDER WHICH A LIQUID IS IN EQUILIBRIUM WITH ITS VAPOR PHASE. THE VAPOR PRESSURE IS A FUNCTION OF THE TEMPERATURE OF THE LIQUID AT A TEMPERATURE CORRESPONDING PRESSURE. AT THE TEMPERATURE INCREASE IS AN INCREASE IN THE TRANSITION FROM LIQUID TO GAS, A STATE OF EQUILIBRIUM IS CREATED BETWEEN LIQUID AND VAPOR, AS WELL AS THE PRESSURE HAS INCREASED. THESE THERMOMETERS ARE VERY SENSITIVE BUT THE SCALE IS NOT LINEAR. THE MEASUREMENT ACCURACY IS 1%. THE TYPICAL TEMPERATURE RANGE IS FROM - 20° C TO 280° C DEPENDING ON THE NATURE OF THE GAS (BUTANE PROPANE, AMMONIA. . . ).

APPLICATIONS : o INDUSTRIAL EQUIPMENT, o PIPING, o BOILERS, o PIPELINE, o HEATING, o COOLING, o VENTILATION. . . THESE THERMOMETERS ARE OFTEN MOUNTED ON PIPES ONLINE.

4. RTD - RESISTANCE TEMPERATURE DETECTOR • A PLATINUM RESISTANCE TEMPERATURE DETECTOR (RTD) IS A DEVICE WITH A TYPICAL RESISTANCE OF 100 Ω AT 0 °C. • IT CONSISTS OF A THIN FILM OF PLATINUM ON A PLASTIC FILM. ITS RESISTANCE VARIES WITH TEMPERATURE AND IT CAN TYPICALLY MEASURE TEMPERATURES UP TO 850 °C. • PASSING CURRENT THROUGH AN RTD GENERATES A VOLTAGE ACROSS THE RTD. • BY MEASURING THIS VOLTAGE, YOU CAN DETERMINE ITS RESISTANCE AND, THUS, ITS TEMPERATURE. • THE RELATIONSHIP BETWEEN RESISTANCE AND TEMPERATURE IS RELATIVELY LINEAR.

PHYSICAL ARCHITECTURE OF AN RTD : RTDS OPERATE ON THE PRINCIPLE OF CHANGES IN ELECTRICAL RESISTANCE OF PURE METALS AND ARE CHARACTERIZED BY A LINEAR POSITIVE CHANGE IN RESISTANCE WITH TEMPERATURE. TYPICAL ELEMENTS USED FOR RTDS INCLUDE NICKEL (NI) AND COPPER (CU), BUT PLATINUM (PT) IS BY FAR THE MOST COMMON BECAUSE OF ITS WIDE TEMPERATURE RANGE, ACCURACY, AND STABILITY.

RTDS ARE COMMONLY CATEGORIZED BY THEIR NOMINAL RESISTANCE AT 0 °C. TYPICAL NOMINAL RESISTANCE VALUES FOR PLATINUM THIN-FILM RTDS INCLUDE 100 AND 1000 Ω. THE RELATIONSHIP BETWEEN RESISTANCE AND TEMPERATURE IS NEARLY LINEAR AND FOLLOWS THIS EQUATION: FOR <0 °C RT = R 0 [ 1 + AT + BT 2 + CT 3 (T - 100) ] (EQUATION 1) FOR >0 °C RT = R 0 [ 1 + AT + BT 2 ] WHERE , RT = RESISTANCE AT TEMPERATURE T R 0 = NOMINAL RESISTANCE A, B, AND C = CONSTANTS USED TO SCALE THE RTD

THE RESISTANCE/TEMPERATURE CURVE FOR A 100 Ω PLATINUM RTD, COMMONLY REFERRED TO AS PT 100, IS SHOWN IN FIGURE 2. Figure 2. Resistance-Temperature Curve for a 100 Ω Platinum RTD, a = 0. 00385 THIS RELATIONSHIP APPEARS RELATIVELY LINEAR, BUT CURVE FITTING IS OFTEN THE MOST ACCURATE WAY TO MAKE AN ACCURATE RTD MEASUREMENT.

ADVANTAGES DISADVANTAGES

HOW TO MEASURE TEMP. BY RTD G 2 -Wire Configuration RTD CONNECTED TO ONE ARM OF WHEATSTONE BRIDGE, AND WITH PYROMETER IT IS KEPT IN THE FURNACE TO MEASURE TEMP.

• TO MEASURE CHANGE IN RESISTANCE WHEATSTONES BRIDGE IS USED • THE RSISTANCE THERMOMETER IS CONNECTED TO ONE ARM OF BRIDGE • WHEN IT IS SUBJECTED TO CHANGE IN RESISTANCE THE BRIDGEGETS UNBALANCED • THE GALVANOMETER DEFLECTION CAN BE DIRECTLY CALIBRATED TO GIVE TEMP. • THE RESISTANCE R 3 IS ADJESTED UNTIL THE METER G SHOWS NO CURRENT OR NULL DEFLECTION OF GALVANOMETER R 1/R 2 = R 3/T

THE RESISTANCE OF WIRE AT t 0 C IS GIVEN BY Rt = R 0 (1+ α 0 t) Where, Rt – resistance at t 0 C R 0 - resistance at 0 0 C α 0 - resistance temp. coefficient t - change in temp. THE UNKNOWN TEMP. t IS GIVEN BY t = (Rt - R 0) / (R 100 - R 0) *100 WHERE , R 100 - resistance OF WIRE at 100 0 C

APPLICATIONS: q IN A NUCLEAR STATION q IN A REACTOR AREA TEMP. MEASUREMENT q FUEL CHANNEL COOLENT TEMP. MEASUREMENT

EXPT 6. TEMPERATURE MEASUREMENT BY USING ANYONE THERMOCOUPLES, THERMISTORS. EXPT 7. TEMPERATURE MEASUREMENT USING RADIATION / OPTICAL PYROMETER.

EXPT 6. TEMPERATURE MEASUREMENT BY USING ANYONE THERMOCOUPLES, THERMISTORS. q. THERMOCOUPLES, q. THERMISTORS.

THERMOCOUPLE TWO METALS JOINED TOGETHER emf or voltage can be measured.

THERMOCOUPLE WHAT IS A THERMOCOUPLE & HOW DOES IT WORK? A THERMOCOUPLE IS A DEVICE USED EXTENSIVELY FOR MEASURING TEMPERATURE. 0 to 1200 deg c

THERMOCOUPLES ARE AVAILABLE EITHER AS BARE WIRE ‘BEAD’ THERMOCOUPLES WHICH OFFER LOW COST AND FAST RESPONSE TIMES, OR BUILT INTO PROBES. A WIDE VARIETY OF PROBES ARE AVAILABLE, SUITABLE FOR DIFFERENT MEASURING APPLICATIONS (INDUSTRIAL, SCIENTIFIC, FOOD TEMPERATURE, MEDICAL RESEARCH ETC).

How Thermocouple works - A simple example The Seebeck effect is present whenever two dissimilar metals -of any material- performs a junction. Nevertheless, there are some metal pairs that have a predictable voltage according to temperature, and have also larger temperature gradients. These pairs are named with a letter, as for example the type-E thermocouple.

The most popular type of thermocouple is the type-K. This type is made of Nickel. Chromium versus Nickel-Aluminum wires and has a very wide range of temperature gradients (from -200 o. C up to 1300 o. C) where the voltage changes almost linear.

The following drawing demonstrated a thermocouple. The left junction is heated. The right junction is kept at room temperature. Thus, a current flows within the wires:

As someone would expect, if the right junction does not exists, a voltage difference shall be created across the wire endings: The three Thermo-Electric Laws 1. The Law of Homogeneous Circuits An electric current cannot be sustained in a circuit of a single homogeneous metal

Look at the two following images. The two circuits are identical and the temperature in the junction is equal. Both will generate a voltage difference. On the right circuit, a second heater is placed under one of the two thermocouple wires and heat it up. The first law states that, because the second heater heats ONLY one wire and NOT the junction, the output voltage of both circuit will be the same. The output voltage is only affected by the junction temperature and not the temperature of the wires. That is because, any temperature change to a homogeneous wire will create no voltage. Voltage is the function of junction temp. Voltage is not the function of temp. of individual wire.

2. The Law of Intermediate Metals If two dissimilar metals performs a thermocouple, and a third dissimilar metal is introduced to the circuit, as long as the temperature along the entire length of the third metal is kept uniform, the output voltage will NOT be affected

Look at the following circuit. The blue wire of thermocouple is joined with a third dissimilar metal. If the temperature of this wire across it's entire length is the same, then the output voltage will NOT be affected by this insert. The Law of Intermediate Metals

3. The Law of Intermediate Temperatures If a thermocouple with 2 junctions with temperatures T 1 and T 2 produces a voltage difference V 1, and voltage difference of V 2 in temperatures T 2 and T 3, then voltage generated when the temperatures are T 1 and T 3 will be V 1+V 2.

v 2 v 1 V 1+v 2 For the left circuit, suppose that T 1 = 0 oc (first reference temperature). According to the measured voltage, we can directly convert to temperature for T 2. In our example, for a K -type thermocouple, the measured voltage would be 4. 095 m. V, that corresponds to 100 o. C. But a measuring instrument is rarely placed in 0 o. C.

On the second circuit (right side), the measuring instrument is placed in a room with 20 o. C. Although the temperature T 2 is still 100 o. C, the measured voltage will be now 3. 297 m. V! Why? Because the junctions where the voltmeter is connected is NOT in 0 o. C, thus they introduce another opposite current to the circuit!

To calculate the T 2, we need to compensate T 3! According to the 3 rd law, we need to add to the measured voltage, the voltage that would be generated if T 2 was T 3 and T 3 was 0 o. C. In this case, the voltage can be directly found from the table (as we are talking about 0 o. C reference temperature) and would be 0. 798 m. V. So, we add this value to the measured voltage and the total voltage is 0. 798+3. 297 = 4. 095 m. V, and this corresponds to 100 o. C!

A THERMOCOUPLE IS COMPRISED OF AT LEAST TWO METALS JOINED TOGETHER TO FORM TWO JUNCTIONS. ONE IS CONNECTED TO THE BODY WHOSE TEMPERATURE IS TO BE MEASURED; THIS IS THE HOT OR MEASURING JUNCTION. THE OTHER JUNCTION IS CONNECTED TO A BODY OF KNOWN TEMPERATURE; THIS IS THE COLD OR REFERENCE JUNCTION. THEREFORE THERMOCOUPLE MEASURES UNKNOWN TEMPERATURE OF THE BODY WITH REFERENCE TO THE KNOWN TEMPERATURE OF THE OTHER BODY.

WORKING PRINCIPLE THE WORKING PRINCIPLE OF THERMOCOUPLE IS BASED ON THREE EFFECTS, DISCOVERED BY SEEBECK, PELTIER AND THOMSON. THEY ARE AS FOLLOWS: 1) SEEBECK EFFECT: THE SEEBECK EFFECT STATES THAT WHEN TWO DIFFERENT OR UNLIKE METALS ARE JOINED TOGETHER AT TWO JUNCTIONS, AN ELECTROMOTIVE FORCE (EMF) IS GENERATED AT THE TWO JUNCTIONS. THE AMOUNT OF EMF GENERATED IS DIFFERENT FOR DIFFERENT COMBINATIONS OF THE METALS.

2) PELTIER EFFECT: AS PER THE PELTIER EFFECT, WHEN TWO DISSIMILAR METALS ARE JOINED TOGETHER TO FORM TWO JUNCTIONS, EMF IS GENERATED WITHIN THE CIRCUIT DUE TO THE DIFFERENT TEMPERATURES OF THE TWO JUNCTIONS OF THE CIRCUIT.

3) THOMSON EFFECT: AS PER THE THOMSON EFFECT, WHEN TWO UNLIKE METALS ARE JOINED TOGETHER FORMING TWO JUNCTIONS, THE POTENTIAL EXISTS WITHIN THE CIRCUIT DUE TO TEMPERATURE GRADIENT ALONG THE ENTIRE LENGTH OF THE CONDUCTORS WITHIN THE CIRCUIT. IN MOST OF THE CASES THE EMF SUGGESTED BY THE THOMSON EFFECT IS VERY SMALL AND IT CAN BE NEGLECTED BY MAKING PROPER SELECTION OF THE METALS. THE PELTIER EFFECT PLAYS A PROMINENT ROLE IN THE WORKING PRINCIPLE OF THERMOCOUPLE.

THERMOCOUPLE TEMP. TO BE MEASURED p q

THE GENERAL CIRCUIT FOR THE WORKING OF THERMOCOUPLE IS SHOWN IN THE FIGURE ABOVE. IT COMPRISES OF TWO DISSIMILAR METALS, A AND B. THESE ARE JOINED TOGETHER TO FORM TWO JUNCTIONS, P AND Q, WHICH ARE MAINTAINED AT THE TEMPERATURES T 1 AND T 2 RESPECTIVELY. REMEMBER THAT THERMOCOUPLE CANNOT BE FORMED IF THERE ARE NOT TWO JUNCTIONS.

THE TWO JUNCTIONS ARE MAINTAINED AT DIFFERENT TEMPERATURES THE PELTIER EMF IS GENERATED WITHIN THE CIRCUIT AND IT IS THE FUNCTION OF THE TEMPERATURES OF TWO JUNCTIONS. IF THE TEMPERATURE OF BOTH THE JUNCTIONS IS SAME, EQUAL AND OPPOSITE EMF WILL BE GENERATED AT BOTH JUNCTIONS AND THE NET CURRENT FLOWING THROUGH THE JUNCTION IS ZERO. IF THE JUNCTIONS ARE MAINTAINED AT DIFFERENT TEMPERATURES, THE EMF’S WILL NOT BECOME ZERO AND THERE WILL BE A NET CURRENT FLOWING THROUGH THE CIRCUIT. THE TOTAL EMF FLOWING THROUGH THIS CIRCUIT DEPENDS ON THE METALS USED WITHIN THE CIRCUIT AS WELL AS THE TEMPERATURE OF THE TWO JUNCTIONS. THE TOTAL EMF OR THE CURRENT FLOWING THROUGH THE CIRCUIT CAN BE MEASURED EASILY BY THE SUITABLE DEVICE.

NOW, THE TEMPERATURE OF THE REFERENCE JUNCTIONS IS ALREADY KNOWN, WHILE THE TEMPERATURE OF MEASURING JUNCTION IS UNKNOWN. THE OUTPUT OBTAINED FROM THERMOCOUPLE CIRCUIT IS CALIBRATED DIRECTLY AGAINST THE UNKNOWN TEMPERATURE. THUS THE VOLTAGE OR CURRENT OUTPUT OBTAINED FROM THERMOCOUPLE CIRCUIT GIVES THE VALUE OF UNKNOWN TEMPERATURE DIRECTLY.

DEVICES USED FOR MEASURING EMF THE AMOUNT OF EMF DEVELOPED WITHIN THERMOCOUPLE CIRCUIT IS VERY SMALL, USUALLY IN MILLIVOLTS, THEREFORE HIGHLY SENSITIVE INSTRUMENTS SHOULD BE USED FOR MEASURING THE EMF GENERATED IN THERMOCOUPLE CIRCUIT. TWO DEVICES USED COMMONLY ARE THE ORDINARY GALVANOMETER AND VOLTAGE BALANCING POTENTIOMETER. OF THOSE TWO, A MANUALLY OR AUTOMATICALLY BALANCING POTENTIOMETER IS USED MOST OFTEN.

MATERIALS USED FOR THERMOCOUPLES FOR THE FORMATION OF THERMOCOUPLE AT LEAST TWO METALS SHOULD BE JOINED TOGETHER TO FORM TWO JUNCTIONS. SOME OF THE ELEMENTS USED COMMONLY FOR THERMOCOUPLE ARE: 1) COPPER AS INDEPENDENT ELEMENT 2) IRON AS INDEPENDENT ELEMENT 3) PLATINUM AS INDEPENDENT ELEMENT 4) RHODIUM AS AN INDEPENDENT ELEMENT 5) IRIDIUM AS AN INDEPENDENT ELEMENT

6) CONSTANTAN: COMBINATION OF 60% COPPER AND 40% NICKEL 7) CHROMEL: COMBINATION OF 10% CHROMIUM, 90% NICKEL 8) ALUMEL: COMBINATION OF 2% ALUMINUM, 90% NICKEL AND REMAINDER SILICON AND MANGANESE

PROPERTIES OF SOME COMBINATIONS OF MATERIALS USED FOR THERMOCOUPLES COMMONLY USED COMBINATIONS OF THE ELEMENTS FOR THERMOCOUPLES. 1) COPPER - CONSTANTAN: USED FOR -300 TO 650 F. INEXPENSIVE, HIGH EMF OUTPUT 2) CHROMEL - CONSTANTAN: (TYPE E) USED FOR 0 TO 1000 F. HIGHEST EMF OUTPUT, GOOD STABILITY TYPE E HAS A HIGH OUTPUT (68 µV/°C) WHICH MAKES IT WELL SUITED TO LOW TEMPERATURE (CRYOGENIC) USE. ANOTHER PROPERTY IS THAT IT IS NON–MAGNETIC.

3) IRON - CONSTANTAN: (TYPE J ) USED FOR 0 TO 1500 F. INEXPENSIVE, HIGH EMF OUTPUT, IRON OXIDIZES AFTER 1500 F (IRON / CONSTANTAN) LIMITED RANGE (-40 TO +750 °C) MAKES TYPE J LESS POPULAR THAN TYPE K. THE MAIN APPLICATION IS WITH OLD EQUIPMENT THAT CAN NOT ACCEPT ‘MODERN’ THERMOCOUPLES. J TYPES SHOULD NOT BE USED ABOVE 760 °C AS AN ABRUPT MAGNETIC TRANSFORMATION WILL CAUSE PERMANENT DECALIBRATION. 4) CHROMEL - ALUMEL: (type k) USED FOR 600 TO 2000 F. RESISTANT TO THE TEMPERATURE WITHIN THE SPECIFIED RANGE TYPE K IS THE ‘GENERAL PURPOSE’ THERMOCOUPLE. IT IS LOW COST AND, OWING TO ITS POPULARITY, IT IS AVAILABLE IN A WIDE VARIETY OF PROBES. THERMOCOUPLES ARE AVAILABLE IN THE 200 °C TO +1200 °C RANGE. SENSITIVITY IS APPROX 41 µV/°C.

5) PLATINUM - 10% RHODIUM: (TYPE S) USED FOR 1300 TO 2850 F. EXPENSIVE AND GIVES LOW OUTPUT, RESISTANT TO OXIDATION, STABLE, USED ONLY FOR HIGH TEMP. SUITED 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).

Thermocouple To emf measuring device Temp to be measured

Thermocouple type Overall range °C 0. 1°C resolution 0. 025°C resolution B 20 to 1820 150 to 1820 600 to 1820 E -270 to 910 -260 to 910 J -210 to 1200 K -270 to 1370 -250 to 1370 N -270 to 1300 -260 to 1300 -230 to 1300 R -50 to 1760 20 to 1760 S -50 to 1760 20 to 1760 T -270 to 400 -250 to 400

THERMISTERS THERMISTORS ARE AVAILABLE IN VARIOUS SHAPES LIKE DISC, ROD, WASHER, BEAD ETC. THEY ARE OF SMALL SIZE AND THEY ALL CAN BE FITTED EASILY TO THE BODY WHOSE TEMPERATURE HAS TO BE MEASURED AND ALSO CAN BE CONNECTED TO THE CIRCUIT EASILY. MOST OF THERMISTORS ARE QUITE CHEAP.

WHAT ARE THERMISTORS? THERMISTORS ARE ONE OF THE MOST COMMONLY USED DEVICES FOR THE MEASUREMENT OF TEMPERATURE. THERMISTORS ARE RESISTORS WHOSE RESISTANCE CHANGES WITH THE TEMPERATURE. WHILE FOR MOST OF THE METALS THE RESISTANCE INCREASES WITH TEMPERATURE, THERMISTORS RESPOND NEGATIVELY TO THE TEMPERATURE AND THEIR RESISTANCE DECREASES WITH THE INCREASE IN TEMPERATURE. SINCE THE RESISTANCE OF THERMISTORS IS DEPENDENT ON THE TEMPERATURE, THEY CAN BE CONNECTED IN THE ELECTRICAL CIRCUIT TO MEASURE THE TEMPERATURE OF THE BODY.

MATERIALS USED FOR THERMISTORS AND THEIR FORMS THERMISTORS ARE MADE UP OF CERAMIC LIKE SEMICONDUCTING MATERIALS. THEY ARE MOSTLY COMPOSED OF OXIDES OF MANGANESE, NICKEL AND COBALT HAVING THE RESISTIVITIES IF ABOUT 100 TO 450, 000 OHM-CM. SINCE THE RESISTIVITY OF THERMISTORS IS VERY HIGH THE RESISTANCE OF THE CIRCUIT IN WHICH THEY ARE CONNECTED FOR MEASUREMENT OF TEMPERATURE CAN BE MEASURED EASILY. THIS RESISTANCE IS CALIBRATED AGAINST, THE INPUT QUANTITY, WHICH IS THE TEMPERATURE, AND ITS VALUE CAN BE OBTAINED EASILY.

DIFFERENT FORMS OF THERMISTORS

ROD TYPE THERMISTOR SYMBOL

THE THERMISTOR IS ANOTHER TYPE OF TEMPERATURE SENSOR, WHOSE NAME IS A COMBINATION OF THE WORDS THERM-ALLY SENSITIVE RES-ISTOR. A THERMISTOR IS A SPECIAL TYPE OF RESISTOR WHICH CHANGES ITS PHYSICAL RESISTANCE WHEN EXPOSED TO CHANGES IN TEMPERATURE. THERMISTOR MATERIALS THERMISTORS ARE GENERALLY MADE FROM CERAMIC MATERIALS SUCH AS OXIDES OF NICKEL, MANGANESE OR COBALT COATED IN GLASS WHICH MAKES THEM EASILY DAMAGED. THEIR MAIN ADVANTAGE OVER SNAP-ACTION TYPES IS THEIR SPEED OF RESPONSE TO ANY CHANGES IN TEMPERATURE, ACCURACY AND REPEATABILITY. NTC-Thermistor Negative Temperature Coefficient of resistance

MOST TYPES OF THERMISTOR’S HAVE A NEGATIVE TEMPERATURE COEFFICIENT OF RESISTANCE OR(NTC), THAT IS THEIR RESISTANCE VALUE GOES DOWN WITH AN INCREASE IN THE TEMPERATURE, AND OF COURSE THERE ARE SOME WHICH HAVE A POSITIVE TEMPERATURE COEFFICIENT, (PTC), IN THAT THEIR RESISTANCE VALUE GOES UP WITH AN INCREASE IN TEMPERATURE. THERMISTORS ARE CONSTRUCTED FROM A CERAMIC TYPE SEMICONDUCTOR MATERIAL USING METAL OXIDE TECHNOLOGY SUCH AS MANGANESE, COBALT AND NICKEL, ETC. THE SEMICONDUCTOR MATERIAL IS GENERALLY FORMED INTO SMALL PRESSED DISCS OR BALLS WHICH ARE HERMETICALLY SEALED TO GIVE A RELATIVELY FAST RESPONSE TO ANY CHANGES IN TEMPERATURE.

THERMISTORS ARE RATED BY THEIR RESISTIVE VALUE AT ROOM TEMPERATURE (USUALLY AT 25 OC), THEIR TIME CONSTANT (THE TIME TO REACT TO THE TEMPERATURE CHANGE) AND THEIR POWER RATING WITH RESPECT TO THE CURRENT FLOWING THROUGH THEM. LIKE RESISTORS, THERMISTORS ARE AVAILABLE WITH RESISTANCE VALUES AT ROOM TEMPERATURE FROM 10’S OF MΩ DOWN TO JUST A FEW OHMS, BUT FOR SENSING PURPOSES THOSE TYPES WITH VALUES IN THE KILO-OHMS ARE GENERALLY USED. THERMISTORS ARE PASSIVE RESISTIVE DEVICES WHICH MEANS WE NEED TO PASS A CURRENT THROUGH IT TO PRODUCE A MEASURABLE VOLTAGE OUTPUT. THEN THERMISTORS ARE GENERALLY CONNECTED IN SERIES WITH A SUITABLE BIASING RESISTOR TO FORM A POTENTIAL DIVIDER NETWORK AND THE CHOICE OF RESISTOR GIVES A VOLTAGE OUTPUT AT SOME PREDETERMINED TEMPERATURE POINT OR VALUE FOR EXAMPLE:

Temperature Sensors Example No 1 THE FOLLOWING THERMISTOR HAS A RESISTANCE VALUE OF 10 KΩ AT 25 OC AND A RESISTANCE VALUE OF 100Ω AT 100 OC. CALCULATE THE VOLTAGE DROP ACROSS THERMISTOR AND HENCE ITS OUTPUT VOLTAGE (VOUT) FOR BOTH TEMPERATURES WHEN CONNECTED IN SERIES WITH A 1 KΩ RESISTOR ACROSS A 12 V POWER SUPPLY. At 25 o. C At 100 o. C THERMISTER

PRINCIPLE OF WORKING OF THERMISTORS AS MENTIONED EARLIER THE RESISTANCE OF THERMISTORS DECREASES WITH THE INCREASE ITS TEMPERATURE. THE RESISTANCE OF THERMISTOR IS GIVEN BY: R = RO EK AND K = Β(1/T – 1/TO) WHERE, R IS THE RESISTANCE OF THERMISTOR AT ANY TEMPERATURE T IN OK (DEGREE KELVIN). RO IS THE RESISTANCE OF THERMISTORS AT PARTICULAR REFERENCE TEMPERATURE TOIN OK E IS THE BASE OF THE NAPERIAN LOGARITHMS Β IS A CONSTANT WHOSE VALUE RANGES FROM 3400 TO 3900 DEPENDING ON THE MATERIAL USED FOR THERMISTORS AND ITS COMPOSITION.

UNLIKE OTHER SENSORS, THERMISTORS ARE NONLINEAR, MEANING THE POINTS ON A GRAPH REPRESENTING THE RELATIONSHIP BETWEEN RESISTANCE AND TEMPERATURE WILL NOT FORM A STRAIGHT LINE. THE LOCATION OF THE LINE AND HOW MUCH IT CHANGES IS DETERMINED BY THE CONSTRUCTION OF THERMISTOR. A TYPICAL THERMISTOR GRAPH LOOKS LIKE THIS:

WHAT ARE PTC THERMISTORS? PTC STANDS FOR „POSITIVE TEMPERATURE COEFFICIENT“. PTC THERMISTORS ARE RESISTORS WITH A POSITIVE TEMPERATURE COEFFICIENT, WHICH MEANS THAT THE RESISTANCE INCREASES WITH INCREASING TEMPERATURE. PTC THERMISTORS ARE DIVIDED INTO TWO GROUPS, BASED ON THE MATERIALS USED, THEIR STRUCTURE AND THE MANUFACTURING PROCESS. THE FIRST GROUP OF PTC THERMISTORS IS COMPRISED OF SILISTORS, WHICH USE SILICON AS THE SEMICONDUCTIVE MATERIAL. THEY ARE USED AS PTC TEMPERATURE SENSORS FOR THEIR LINEAR CHARACTERISTIC. PTC thermistors Positive Temperature Coefficient

THEY ARE USED AS PTC TEMPERATURE SENSORS FOR THEIR LINEAR CHARACTERISTIC. THE SECOND GROUP IS THE SWITCHING TYPE PTC THERMISTOR. THIS TYPE OF PTC THERMISTORS IS WIDELY USED IN PTC HEATERS, SENSORS ETC. POLYMER PTC THERMISTORS, MADE OF A SPECIAL PLASTIC, ARE ALSO IN THIS SECOND GROUP, OFTEN USED AS RESETTABLE FUSES. THE SWITCHING TYPE PTC THERMISTOR HAS A HIGHLY NONLINEAR RESISTANCE-TEMPERATURE CURVE. WHEN THE SWITCHING TYPE PTC THERMISTOR IS HEATED, THE RESISTANCE STARTS TO DECREASE AT FIRST, UNTIL A CERTAIN CRITICAL TEMPERATURE IS REACHED. AS THE TEMPERATURE IS FURTHER INCREASED ABOVE THAT CRITICAL VALUE, THE RESISTANCE INCREASES DRAMATICALLY. THIS ARTICLE WILL FOCUS ON THE SWITCHING TYPE PTC THERMISTORS.

CHARACTERISTICS OF PTC THERMISTORS SWITCHING PTC THERMISTORS ARE USUALLY MADE OF POLYCRYSTALLINE CERAMIC MATERIALS THAT ARE HIGHLY RESISTIVE IN THEIR ORIGINAL STATE AND ARE MADE SEMI-CONDUCTIVE BY THE ADDITION OF DOPANTS. THEY ARE MOSTLY USED AS PTC SELF-REGULATING HEATERS. THE TRANSITION TEMPERATURE OF MOST SWITCHED PTC THERMISTORS IS BETWEEN 60°C AND 120°C. HOWEVER, THERE ARE SPECIAL APPLICATION DEVICES MANUFACTURED THAT CAN SWITCH AS LOW AS 0°C OR AS HIGH AS 200°C. SILISTORS HAVE A LINEAR RESISTANCE-TEMPERATURE CHARACTERISTIC, WITH A SLOPE THAT IS RELATIVELY SMALL THROUGH MOST OF THEIR OPERATIONAL RANGE. THEY MAY EXHIBIT A NEGATIVE TEMPERATURE COEFFICIENT AT TEMPERATURES ABOVE 150 °C. SILISTORS HAVE TEMPERATURE COEFfiCIENTS OF RESISTANCE OF ABOUT 0. 7 TO 0. 8% °C.

CHARACTERISTICS OF PTC THERMISTORS

IN THE DIAGRAM BELOW, ILLUSTRATING AN EXAMPLE SYSTEM, THERE ARE THREE MAIN COMPONENTS USED TO REGULATE THE TEMPERATURE OF A DEVICE: Ø THE TEMPERATURE SENSOR, Ø THE TEMPERATURE CONTROLLER, Ø THE PELTIER DEVICE (LABELED HERE AS A TEC, OR THERMOELECTRIC COOLER).

THE SENSOR HEAD IS ATTACHED TO THE COOLING PLATE THAT NEEDS TO MAINTAIN A SPECIFIC TEMPERATURE TO COOL THE DEVICE, AND THE WIRES ARE ATTACHED TO THE TEMPERATURE CONTROLLER IS ALSO ELECTRONICALLY CONNECTED TO THE PELTIER DEVICE, WHICH HEATS AND COOLS THE TARGET DEVICE. THE HEATSINK IS ATTACHED TO THE PELTIER DEVICE TO HELP WITH HEAT DISSIPATION.

THE JOB OF THE TEMPERATURE SENSOR IS TO SEND THE TEMPERATURE FEEDBACK TO THE TEMPERATURE CONTROLLER. THE SENSOR HAS A SMALL AMOUNT OF CURRENT RUNNING THROUGH IT, CALLED BIAS CURRENT, WHICH IS SENT BY THE TEMPERATURE CONTROLLER. THE CONTROLLER CAN’T READ RESISTANCE, SO IT MUST CONVERT RESISTANCE CHANGES TO VOLTAGE CHANGES BY USING A CURRENT SOURCE TO APPLY A BIAS CURRENT ACROSS THERMISTOR TO PRODUCE A CONTROL VOLTAGE. THE TEMPERATURE CONTROLLER IS THE BRAINS OF THIS OPERATION. IT TAKES THE SENSOR INFORMATION, COMPARES IT TO WHAT THE UNIT TO BE COOLED NEEDS (CALLED THE SETPOINT), AND ADJUSTS THE CURRENT THROUGH THE PELTIER DEVICE TO CHANGE THE TEMPERATURE TO MATCH THE SETPOINT.

THE LOCATION OF THERMISTOR IN THE SYSTEM AFFECTS BOTH THE STABILITY AND THE ACCURACY OF THE CONTROL SYSTEM. FOR BEST STABILITY, THERMISTOR NEEDS TO BE PLACED AS CLOSE TO THERMOELECTRIC OR RESISTIVE HEATER AS POSSIBLE. FOR BEST ACCURACY, THERMISTOR NEEDS TO BE LOCATED CLOSE TO THE DEVICE REQUIRING TEMPERATURE CONTROL. IDEALLY, THERMISTOR IS EMBEDDED IN THE DEVICE, BUT IT CAN ALSO BE ATTACHED USING THERMALLY CONDUCTIVE PASTE OR GLUE. EVEN IF A DEVICE IS EMBEDDED, AIR GAPS SHOULD BE ELIMINATED USING THERMAL PASTE OR GLUE. THE FIGURE BELOW SHOWS TWO THERMISTORS, ONE ATTACHED DIRECTLY TO THE DEVICE AND ONE REMOTE, OR DISTANT FROM THE DEVICE. IF THE SENSOR IS TOO FAR AWAY FROM THE DEVICE, THERMAL LAG TIME SIGNIFICANTLY REDUCES THE ACCURACY OF THE TEMPERATURE MEASUREMENT, WHILE PLACING THERMISTOR TOO FAR FROM THE PELTIER DEVICE REDUCES THE STABILITY.

THERMISTOR MOUNTING SHOWN IN THE FOLLOWING DIAGRAM

APPLICATIONS • NTC THERMISTORS ARE USED FOR TEMPERATURE MEASUREMENTS (USUALLY IN A NARROW SPAN AND LOW TEMPERATURE RANGES). • THE DEVICE CAN BE USED TO LIMIT THE SUDDEN OVER CURRENT THAT FLOWS IN SUPPLY CIRCUITS. THE DEVICE IS KNOWN TO HAVE A VERY HIGH VALUE OF RESISTANCE IN THE BEGINNING. THE RESISTANCE GRADUALLY DECREASES BY THE HEATING UP OF THE DEVICE. AS THE RESISTANCE DECREASES, THE USUAL OPERATION OF THE CIRCUIT IS RESTORED AND THE HIGH CURRENT FLOWS THROUGH IT WITHOUT DAMAGING OTHER PARTS OF THE CIRCUIT. • THIS DEVICE IS USED TO MEASURE THE TEMPERATURE OF INCUBATORS. NTC THERMISTORS ARE USED TO MEASURE AND MONITOR BATTERIES WHILE THEY ARE KEPT FOR CHARGING. • THEY ARE USED TO KNOW THE TEMPERATURE OF OIL AND COOLANT USED INSIDE AUTOMOTIVE ENGINES. THIS INFORMATION IS SENT BACK TO THE DRIVER THROUGH INDIRECT WAYS.

APPLICATIONS THE DEVICE IS FAMOUS FOR ITS APPLICATION AS A CIRCUIT PROTECTING DEVICE, SUCH AS A FUSE. THE FLOW OF CURRENT THROUGH THE DEVICE CAUSES A HEAT TO BUILD UP DUE TO ITS RESISTIVE PROPERTY. THUS, IF EXCESSIVE CURRENT FLOWS THROUGH THE DEVICE, THE DEVICE BEGINS TO HEAT UP ACCORDINGLY AND THUS INCREASES ITS RESISTANCE. THIS INCREASE IN RESISTANCE AGAIN BUILDS UP MORE HEAT. THIS CREATES SUCH AN EFFECT THAT DEVELOPS MORE RESISTANCE IN THE DEVICE, AND LIMITS THE AMOUNT OF VOLTAGE AND CURRENT IN THE DEVICE.

ANOTHER MAJOR APPLICATION IS AS A TIMER IN DEGAUSSING COIL CIRCUIT OF CRT MONITORS. WHEN A CRT MONITOR IS TURNED ON, AN INITIAL CURRENT REACHES THE PTC THERMISTOR AND DEGAUSSING COIL. THE PTC THERMISTOR WILL BE OF LARGE SIZE AND THUS, THE RESISTANCE OF THE DEVICE INCREASES AS THE CURRENT FLOWS IN. THIS CAUSES THE HEAT TO BUILD UP AND THUS THE DEGAUSSING COIL SHUTS OFF VERY FAST. THE DEGAUSSING COIL IS NECESSARY TO DECREASE THE CONTINUOUS MAGNETIC FIELD IN A SMOOTH MANNER. THIS HELP CAN BE PROVIDED ONLY BY THE PTC THERMISTOR.

ADVANTAGES OF THERMISTORS 1) WHEN THE RESISTORS ARE CONNECTED IN THE ELECTRICAL CIRCUIT, HEAT IS DISSIPATED IN THE CIRCUIT DUE TO FLOW OF CURRENT. THIS HEAT TENDS TO INCREASE THE TEMPERATURE OF THE RESISTOR DUE TO WHICH THEIR RESISTANCE CHANGES. FOR THERMISTOR THE DEFINITE VALUE OF THE RESISTANCE IS REACHED AT THE GIVEN AMBIENT CONDITIONS DUE TO WHICH THE EFFECT OF THIS HEAT IS REDUCED.

2) IN CERTAIN CASES EVEN THE AMBIENT CONDITIONS KEEP ON CHANGING, THIS IS COMPENSATED BY THE NEGATIVE TEMPERATURE CHARACTERISTICS OF THERMISTOR. THIS IS QUITE CONVENIENT AGAINST THE MATERIALS THAT HAVE POSITIVE RESISTANCE CHARACTERISTICS FOR THE TEMPERATURE. 3) THERMISTORS ARE USED NOT ONLY FOR THE MEASUREMENT OF TEMPERATURE, BUT ALSO FOR THE MEASUREMENT OF PRESSURE, LIQUID LEVEL, POWER ETC. 4) THEY ARE ALSO USED AS THE CONTROLS, OVERLOAD PROTECTORS, GIVING WARNINGS ETC.

DISADVANTAGES OF THERMISTERS: • IT REQUIRES EXTERNAL ENERGY SOURCE. • UNSUITABLE FOR WIDE RANGE. IT HAS NON LIEAR CHARACTERISTICS. • NEED OF SHIELDED POWER LINES.

EXPT 7. TEMPERATURE MEASUREMENT USING RADIATION / OPTICAL PYROMETER. q OPTICAL PYROMETER. q RADIATION PYROMETER.

PYROMETER TEMP. MEASURING RANGE 700 - 3000 DEG C USED TO MEASURE FURNACE TEMP.

Optical Pyrometer PYROMETER A PYROMETER IS A DEVICE THAT IS USED FOR THE TEMPERATURE MEASUREMENT OF AN OBJECT. THE DEVICE ACTUALLY TRACKS AND MEASURES THE AMOUNT OF HEAT THAT IS RADIATED FROM AN OBJECT. THERMAL HEAT RADIATES FROM THE OBJECT TO THE OPTICAL SYSTEM PRESENT INSIDE THE PYROMETER.

THE OPTICAL SYSTEM MAKES THERMAL RADIATION INTO A BETTER FOCUS AND PASSES IT TO THE DETECTOR. THE OUTPUT OF THE DETECTOR WILL BE RELATED TO THE INPUT THERMAL RADIATION. THE BIGGEST ADVANTAGE OF THIS DEVICE IS THAT, UNLIKE A RESISTANCE TEMPERATURE DETECTOR (RTD) AND THERMOCOUPLE, THERE IS NO DIRECT CONTACT BETWEEN THE PYROMETER AND THE OBJECT WHOSE TEMPERATURE IS TO BE FOUND OUT.

OPTICAL PYROMETER IN AN OPTICAL PYROMETER, A BRIGHTNESS COMPARISON IS MADE TO MEASURE THE TEMPERATURE. AS A MEASURE OF THE REFERENCE TEMPERATURE, A COLOR CHANGE WITH THE GROWTH IN TEMPERATURE IS TAKEN. THE DEVICE COMPARES THE BRIGHTNESS PRODUCED BY THE RADIATION OF THE OBJECT WHOSE TEMPERATURE IS TO BE MEASURED, WITH THAT OF A REFERENCE TEMPERATURE.

THE REFERENCE TEMPERATURE IS PRODUCED BY A LAMP WHOSE BRIGHTNESS CAN BE ADJUSTED TILL ITS INTENSITY BECOMES EQUAL TO THE BRIGHTNESS OF THE SOURCE OBJECT. FOR AN OBJECT, ITS LIGHT INTENSITY ALWAYS DEPENDS ON THE TEMPERATURE OF THE OBJECT, WHATEVER MAY BE ITS WAVELENGTH.

AFTER ADJUSTING THE TEMPERATURE, THE CURRENT PASSING THROUGH IT IS MEASURED USING A MULTIMETER, AS ITS VALUE WILL BE PROPORTIONAL TO THE TEMPERATURE OF THE SOURCE WHEN CALIBRATED. THE WORKING OF AN OPTICAL PYROMETER IS SHOWN IN THE FIGURE BELOW.

OPTICAL PYROMETER – WORKING AS SHOWN IN THE FIGURE ABOVE, AN OPTICAL PYROMETER HAS THE FOLLOWING COMPONENTS. Ø AN EYE PIECE AT THE LEFT SIDE AND AN OPTICAL LENS ON THE RIGHT. Ø A REFERENCE LAMP, WHICH IS POWERED WITH THE HELP OF A BATTERY. Ø A RHEOSTAT TO CHANGE THE CURRENT AND HENCE THE BRIGHTNESS INTENSITY.

SO AS TO INCREASE THE TEMPERATURE RANGE WHICH IS TO BE MEASURED, AN ABSORPTION SCREEN IS FITTED BETWEEN THE OPTICAL LENS AND THE REFERENCE BULB. A RED FILTER PLACED BETWEEN THE EYE PIECE AND THE REFERENCE BULB HELPS IN NARROWING THE BAND OF WAVELENGTH.

WORKING THE RADIATION FROM THE SOURCE IS EMITTED AND THE OPTICAL OBJECTIVE LENS CAPTURES IT. THE LENS HELPS IN FOCUSING THERMAL RADIATION ON TO THE REFERENCE BULB. THE OBSERVER WATCHES THE PROCESS THROUGH THE EYE PIECE AND CORRECTS IT IN SUCH A MANNER THAT THE REFERENCE LAMP FILAMENT HAS A SHARP FOCUS AND THE FILAMENT IS SUPER-IMPOSED ON THE TEMPERATURE SOURCE IMAGE.

THE OBSERVER STARTS CHANGING THE RHEOSTAT VALUES AND THE CURRENT IN THE REFERENCE LAMP CHANGES. THIS IN TURN, CHANGES ITS INTENSITY. THIS CHANGE IN CURRENT CAN BE OBSERVED IN THREE DIFFERENT WAYS.

1. THE FILAMENT IS DARK. THAT IS, COOLER THAN THE TEMPERATURE SOURCE. 2. FILAMNET IS BRIGHT. THAT IS, HOTTER THAN THE TEMPERATURE SOURCE. 3. FILAMENT DISAPPEARS. THUS, THERE IS EQUAL BRIGHTNESS BETWEEN THE FILAMENT AND TEMPERATURE SOURCE. AT THIS TIME, THE CURRENT THAT FLOWS IN THE REFERENCE LAMP IS MEASURED, AS ITS VALUE IS A MEASURE OF THE TEMPERATURE OF THE RADIATED LIGHT IN THE TEMPERATURE SOURCE, WHEN CALIBRATED.

ADVANTAGES • SIMPLE ASSEMBLING OF THE DEVICE ENABLES EASY USE OF IT. • PROVIDES A VERY HIGH ACCURACY WITH +/-5 DEGREE CELSIUS. • THERE IS NO NEED OF ANY DIRECT BODY CONTACT BETWEEN THE OPTICAL PYROMETER AND THE OBJECT. THUS, IT CAN BE USED IN A WIDE VARIETY OF APPLICATIONS.

• AS LONG AS THE SIZE OF THE OBJECT, WHOSE TEMPERATURE IS TO MEASURED FITS WITH THE SIZE OF THE OPTICAL PYROMETER, THE DISTANCE BETWEEN BOTH OF THEM IS NOT AT ALL A PROBLEM. THUS, THE DEVICE CAN BE USED FOR REMOTE SENSING.

• THIS DEVICE CAN NOT ONLY BE USED TO MEASURE THE TEMPERATURE, BUT CAN ALSO BE USED TO SEE THE HEAT PRODUCED BY THE OBJECT/SOURCE. • THUS, OPTICAL PYROMETERS CAN BE USED TO MEASURE AND VIEW WAVELENGTHS LESS THAN OR EQUAL TO 0. 65 MICRONS. • BUT, A RADIATION PYROMETER CAN BE USED FOR HIGH HEAT APPLICATIONS AND CAN MEASURE WAVELENGTHS BETWEEN 0. 70 MICRONS TO 20 MICRONS.

DISADVANTAGES 1. AS THE MEASUREMENT IS BASED ON THE LIGHT INTENSITY, THE DEVICE CAN BE USED ONLY IN APPLICATIONS WITH A MINIMUM TEMPERATURE OF 700 DEGREE CELSIUS. 2. THE DEVICE IS NOT USEFUL FOR OBTAINING CONTINUOUS VALUES OF TEMPERATURES AT SMALL INTERVALS.

APPLICATIONS ü USED TO MEASURE TEMPERATURES OF LIQUID METALS OR HIGHLY HEATED MATERIALS. ü CAN BE USED TO MEASURE FURNACE TEMPERATURES.

RADIATION PYROMETER

RADIATION PYROMETER THE WAVELENGTHS MEASURED BY THE DEVICE ARE KNOWN TO BE PURE RADIATION WAVELENGTHS, THAT IS, THE COMMON RANGE FOR RADIOACTIVE HEAT. THIS DEVICE IS USED IN PLACES WHERE PHYSICAL CONTACT TEMPERATURE SENSORS LIKE THERMOCOUPLE, RTD, (Resistance Temperature Detectors ) AND THERMISTORS WOULD FAIL BECAUSE OF THE HIGH TEMPERATURE OF THE SOURCE.

THE MAIN THEORY BEHIND A RADIATION PYROMETER IS THAT THE TEMPERATURE IS MEASURED THROUGH THE NATURALLY EMITTEDHEAT RADIATION BY THE BODY. THIS HEAT IS KNOWN TO BE A FUNCTION OF ITS TEMPERATURE. ACCORDING TO THE APPLICATION OF THE DEVICE, THE WAY IN WHICH THE HEAT IS MEASURED CAN BE SUMMARIZED INTO TWO:

1. TOTAL RADIATION PYROMETER – IN THIS METHOD, THE TOTAL HEAT EMITTED FROM THE HOT SOURCE IS MEASURED AT ALL WAVELENGTHS. 2. SELECTIVE RADIATION PYROMETER – IN THIS METHOD, THE HEAT RADIATED FROM THE HOT SOURCE IS MEASURED AT A GIVEN WAVELENGTH.

AS SHOWN IN THE FIGURE BELOW, THE RADIATION PYROMETER HAS AN OPTICAL SYSTEM, INCLUDING A LENS, A MIRROR AND AN ADJUSTABLE EYE PIECE. THE HEAT ENERGY EMITTED FROM THE HOT BODY IS PASSED ON TO THE OPTICAL LENS, WHICH COLLECTS IT AND IS FOCUSED ON TO THE DETECTOR WITH THE HELP OF THE MIRROR AND EYE PIECE ARRANGEMENT.

THE DETECTOR MAY EITHER BE A THERMISTOR OR PHOTOMULTIPLIER TUBES. THOUGH THE LATTER IS KNOWN FOR FASTER DETECTION OF FAST MOVING OBJECTS, THE FORMER MAY BE USED FOR SMALL SCALE APPLICATIONS. THUS, THE HEAT ENERGY IS CONVERTED TO ITS CORRESPONDING ELECTRICAL SIGNAL BY THE DETECTOR AND IS SENT TO THE OUTPUT TEMPERATURE DISPLAY DEVICE.

ADVANTAGES v. THE DEVICE CAN BE USED TO MEASURE VERY HIGH TEMPERATURES WITHOUT DIRECT CONTACT WITH THE HOT SOURCE (MOLTEN METAL). v. THE BIGGEST ADVANTAGE IS THAT THE OPTICAL LENS CAN BE ADJUSTED TO MEASURE TEMPERATURE OF OBJECTS THAT ARE EVEN 1/15 INCH IN DIAMETER AND THAT TOO KEPT AT A LONG S=DISTANCE FROM THE MEASURING DEVICE. v. THE SIGHT PATH OF THE DEVICE IS MAINTAINED BY THE CONSTRUCTION OF THE INSTRUMENT COMPONENTS, SUCH AS THE LENS AND CURVED MIRRORS.

THANKS !!!