Instrumentasi Sistem Pengaturan -...

Instrumentasi Sistem Pengaturan - 04 1

Instrumentasi Sistem Pengaturan

Pengukuran Temperatur

Ir. Josaphat Pramudijanto, M.Eng.Jurusan Teknik Elektro FTI ITS

Telp. 5947302 Fax.5931237Email: [email protected]


PengantarObjektif:

Macam sensor temperaturMetode pengukuran dengan sensor temperatur

Definisi:Suatu sensor yang mengkonversikan perubahan energi termal suatu objek menjadi energi listrikEnergi termal per molekul dari material dinyatakan dalam derajat temperatur tertentu


Latar Belakang (1)Galileo (1592). Ilmuwan yang pertama kali menemukan thermometer sebagai alat pengukur temperatur, tetapi termometer yang ia temukan adalah termometer yang tidak mempunyai skala tetap.Gabriel Fahrenheit (1700). Ilmuwan Belanda yang menemukan temperatur yang mempunyai akurasi dan repeatability bagus, termometer ini terbuat dari merkuri. Penentuan nilai terendah dia menggunakan campuran air es dan garam (amoniak klorida).


Latar Belakang (2)Anders Celcius (1742). Ilmuwan ini mengusulkan bahwa nilai yang ada pada es ataupun air mendidih bisa digunakan sebagai nilai titik lebur dan titik didih. Sehingga tahun 1948, disepakati bahwa 0° sebagai titik lebur dan 100° sebagai titik didih, yang kemudian lebih dikenal dengan skala celcius.Lord William Thompson Kelvin (1800). Ilmuwan ini mengembangkan teori termodinamik dan menciptakan konsep absolut zero.


Skala TemperaturSkala - skala temperatur berbeda dalam dua hal : lokasi temperatur nol besar dari satu unit ukuran; yaitu energi termal rata-rata per molekul

dinyatakan oleh satu unit dari skala tersebutSkala temperatur absolut : skala yang menetapkan temperatur nol suatu material yang tidak

mempunyai energi termal (tidak ada getaran molekuler)Skala yang biasa dipakai: skala Farenheit ( oF ) skala Celsius ( oC ) skala Kelvin ( oK ) skala Rankine ( oR )


Konversi Skala

Skala Temperatur Relatif: pergeseran sumbu nol


Pengukuran TemperaturPerubahan dalam sifat-sifat elektris

a) RTD (Resistance Temperature Detector)b) Thermistorc) Thermocoupled) Integrated Circuitry (IC) sensor

Perubahan dalam dimensi phisika) Bimetallic thermometerb) Filled-bulb dan Glass-stem thermometer

Perubahan dalam emisi (pancaran) radiasi thermalInfrared pyrometer


Perbandingan Daerah Pengukuran


Tabel Perbandingan Sensor Temperatur


Sensor Perubahan ResistansiThe electrical resistance of various materials changes in a reproducible way with temperature variations. Both RTD’s and Thermistors operate on this basis.

RTD (resistance temperature device or detector) is a precision temperature sensing device that utilizes metal conductors (typically a fine platinum wire winding or thin metallic layer applied to a substrate) and has a positive coefficient of resistance. That is, as T increases, resistance increases almost linearly. Positive temperature coefficient (PTC).

Thermistors are made from semiconductor materials which have a large negative coefficient of resistance. That is, as temperature increases then the resistance of the thermistor decreases. These are called negative temperature coefficient (NTC) thermistors.


Perbandingan Kurva Resistansi

Sensitivity=dR/dT

RTD

Thermistors

−

= oT1

T1β

0T eRR


RTD (Resistance Temperature Detector)Bekerja berdasarkan perubahan resistansi logam karena perubahan temperaturBerbagai logam yang sering digunakan untuk RTD

platina (linier, sangat mahal, umum dipakai)Nikel (range temperatur lebih rendah, lebih murah, nonlinier) Nickle alloys (range temperatur lebih rendah, lebih murah)Tembaga (range temperatur lebih rendah)


Resistansi dan Temperatur


Keunggulan dan Kelemahan

Resistansi rendah 100Ω (terbanyak) sampai 1000 ΩRange operasi lebar (-200 0C sampai 850 0C)Akurasi Tinggi (±0,0006 0C sampai 0,1 0C)Repeatability dan stabilitas tinggi

o Waktu respons lebih lambat (0,5 sampai 5 detik)o Sensitive terhadap shock dan vibrasio Disupport dengan keramik, glass tubeo Internal/self – heating (joule heating = I2R)o Lead wire can be significant


Konstruksi RTDpenghantar yang digulungkan pada suatu form (biasanya koil) dilindungi sheath atau tabung pelindung (menambah waktu respon)hubungan temperatur dan disipasi:

∆T = kenaikan temperatur karena self heating dalam oCP = disipasi daya pada RTD dalam W

PD = konstanta disipasi RTD dalam W/oC

DPPΔT =


Pengukuran dengan RTD

Most sensitive method is by using the Wheatstone bridge

R3

R2

R1

R4 = RRTD


Kompensasi untuk Jarak Jauh

The problem:Lead resistance can be 10% to 50% (or more) of the total resistance!

Solutions:

Constant Current 4-wire circuit


THERMISTORSensor temperatur berdasarkan perubahan resistansi semikonduktor terhadap temperaturPenambahan energi termal yang sama dengan atau melebihi energi gap ΔWgmenyebabkan elektron - elektron dalam pita valensi masuk ke pita konduksi dan bebas menghantarkan arus melalui bagian terbesar material tersebutEnergi gap dari material semikonduktor 0,01V - 4V, sebagai contoh silikon yang memiliki energi gap ΔWg = 1,07 Watt

-20 0 20 60 80 10040

5

10

15

20

25

30

35

40

0

Resis

tance

(kOhm

)

Temperature(C)


Karakteristik ThermistorResistansi tinggi 1kΩ sampai 100 kΩUkuran fisik (disk, manik-manik, batang) kecil Manik kecil (small bead diameternya 0,005 inchi)respon waktu cepat, untuk thermistor manik ½ detik.Lebih murah daripada RTDSensitivitas sangat tinggi (1000 kali lebih sensitif daripada RTD)Perubahan resistansi 10% per 0C. Misal resistansi nominal 10 kΩ maka resistansi akan berubah 1kΩ untuk setiap perubahan temperatur 1 0C.Tidak sensitif terhadap shock dan vibrasiThermistor dilindungi capsul (plastik, teflon/material lembam)memperlambat waktu respon karena kontak termal kurang baik


Kalibrasi Thermistor

−

= oT1

T1β

0T eRR

constant material = βre temperatumeasured = T

Tat measured resistance reference = Rwhere

o0


RTD dan Thermistor

Unlike RTDs, they have negative temperature dependence

Much higher sensitivity than RTDs

Highly nonlinear devices


What are thermocouples?Thermocouples operate under the principle that a circuit made by connecting two dissimilar metals produces a measurable voltage (emf-electromotive force) when the two ends of the thermocouple circuit are at different temperatures.

They are inexpensive, small in size, rugged and remarkably accurate when used with an understanding of their peculiarities.


Penemu Prinsip Kerja ThermocoupleIn, 1821 T. J. Seebeck observed the existence of and electromotive force (EMF) at the junction formed between two dissimilar metals (Seebeck effect).Seebeck effect is actually the combined result of two other phenomena, Thomson and Peltier effects.

Thomson observed the existence of an EMF due to the contact of two dissimilar metals at the junction temperature.Peltier discovered that temperature gradients along conductors in a circuit generate an EMF.The Thomson effect is normally much smaller than the Peltier effect.


Material EMF versus Temperature

With reference tothe characteristicsof pure Platinum


Tabel Temperatur Thermocouple tipe E (Chromel Constantan)


Polinomial untuk Akusisi Data


Pengukuran dengan Referensi

Ice BathsAccurate and inexpensive


Kode Warna Thermocouple


Thermocouple Material Vs EMFTypes T, J, and K are most commonly used thermocouples.


Bahan Thermocouple tipe T dan JCopper-Constantan (T Curve)

The Copper-Constantan thermocouple, with a positive copper wire and a negative Constantan wire is recommended for use in mildly oxidizing and reducing atmospheres up to 400ºC. They are suitable for applications where moisture is present. This alloy is recommended for low temperature work since the homogeneity of the component wires can be maintained better than other base metal wires. Therefore, errors due to the non-homogeneity of wires in zones of temperature gradients are greatly reduced.

Iron-Constantan (J Curve)The Iron-Constantan thermocouple with a positive iron wire and a negative Constantan wire is recommended for reducing atmospheres. The operating range for this alloy combination is 870ºC for the largest wire sizes. Smaller wire sizes should operate in correspondingly lower temperatures.


Bahan Thermocouple tipe K, E, dan JChromel-Alumel (K Curve)

The Chromel-Alumel thermocouple, with a positive Chromel wire and a negative Alumel wire, is recommended for use in clean oxidizing atmospheres. The operating range for this alloy is 1260ºC for the largest wire sizes.

Chromel-Constantan (E Curve)The Chromel-Constantan thermocouple may be used for temperatures up to 870ºC in a vacuum or inert, mildly oxidizing or reducing atmosphere. At sub-zero temperatures, the thermocouple is not subject to corrosion. This thermocouple has the highest emf output of any standard metallic thermocouple.

Tungsten-Rhenium (C Curve)– Three types of tungsten-rhenium thermocouples are in common use for

measuring temperatures up to 2760ºC. These alloys have inherently poor oxidation resistance and should be used in vacuum, hydrogen or inherent atmospheres.


Bahan Thermocouple tipe S dan R• Platinum-Rhodium (S and R Curve)– Three types of noble-metal thermocouples are in common use. They are:

1. The S curve shows a positive wire of 90% platinum and 10% rhodium used with a negative wire of pure platinum,

2. The R curve indicates a positive wire of 87% platinum and 13% rhodium used with a negative wire of pure platinum, and

3. (not shown) a positive wire of 70% platinum and 30% rhodium used with a negative wire of 94% platinum and 6% rhodium.

– They have a high resistance to oxidation and corrosion. However, hydrogen, carbon and many metal vapors can contaminate a platinum-rhodium thermocouple.

– The recommended operating range for the platinum-rhodium alloys is 1540ºC, although temperatures as high as 1780ºC can be measured with the Pt-30% Rh Vs Pt-6% Rh alloy combination.


Grades of TC wire

Grade is based on calibration accuracy of the wire.Precision grade (± 0.5 % reading or 1°C) , greater of the twoStandard grade (± 0.75 % reading or 2°C), greater of the twoExtension or Lead-wire grade (± 1 % reading or 4 °C)Based on cost of TC wire its hard to justify the purchase of any material other than precision grade.

Wire diameter0.001 inches and up is possibleStandard diameters are: 0.01”, 0.02”, 0.032”, 0.040”, 1/16”, 1/8”, 3/16”, and 1/4”Smaller the better but fragility of wire becomes an issue


Law of Intermediate Temperatures

If a thermocouple circuit develops a net emf1-2 for measuring junction temperatures T1 and T2, and a net emf2-3 for temperatures T2 and T3, then it will develop a net voltage of emf1-3 = emf1-2 + emf2-3 when the junctions are at temperatures T1 and T3.

emf1-2+ emf2-3= emf1-3

T2

T3 T1

T3 T2

T1


Rangkaian Thermocouple

Ambient Temp. = 24 °C

+-Voltmeter

4 ironconstantan TC's

? °C

49.312 mV

Isothermal Block at Ambient Temp.

Cu

Cu

Thermopile-Thermocouples connected in series between two temperature zones.

Good for determining small temperature differencesAmplification effect

Averaging Circuit- Thermocouple are connected in parallel between two temperature zones.

Ambient Temp. = 24 °C

+-Voltmeter 100 °C

Cu

Cu

Ice Bath

130 °C

110 °Ciron

constantan

?


Bimetal ThermometerDua metal berbeda dengan koefisien ekspansi termal (muai suhu) beda dilekatkan (bonded together). Perubahan temperatur yang tidak sama pada dua metal tersebut akan mengakibatkan bimetal strip melengkung

Seringkali digunakan sebagai control temperatur on-off

Range penggunaan –65 0C sampai 430 0CVariasi akurasi (± 0,5 sampai 12 0C)Advantages:

Low cost Negligible maintenance Stable operation over time


Liquid ThermometerPemuaian volume dari liquid dan padat digunakan untuk mengukur temperatur (biasanya beda muai antara liquid dan glass)Mercury filled thermometer (‘best’)

Range= -37 to 320 0C, Accuracy ± 0,3 0C

Alcohol filled thermometerRange= -75 to 120 0C, Accuracy ± 0,6 0C

Advantages/disadvantagesLow cost/no remote reading


Fluid Expansion ThermometerA fluid filled bulb is connected to a pressuremeasuring device via a capillary tube. As fluidis heated it expands thus pressure increases.

Pressure is linked to temperature.Accuracy and range depends on fluid.

Advantages/DisadvantagesLow costStable in operationWidely used in industrial applications.Remote readings are possible.Transient response is a function of bulb size andcapillary tube length.


Infrared ThermometersThermometer IR dapat menentukan temperatur objek tanpa menyentuhnya dengan pengukuran jumlah energi IR yang diemisikan dari objek tersebut.

IR portion of the spectrum spans wavelengths between 0,7 to 1000 microns. Based on the principle that all objects warmer than absolute zero ( 0 °K) emit energy somewhere in that range.

IR thermometer mengukur temperatur permukaan objek dalam titik pandangnya. Komponen-komponen IR

lensa untuk mengumpulkan energi radiasi dariobjekdetektor untuk mengubah energi thermal kesinyal listrik


Infrared CamerasModern infrared cameras are light weight, portable, and can accurately measure dynamic temperature changes in equipment and processes. They have the ability to measuring thermal variations of less than 0,1 °C and are non-invasive.

Attaching thermocouples, RTD’s, or thermistors is seldom an option in moving components. In addition, thermocouples cannot be attached at every location on the component.

The biggest advantage over conventional temperature measurement techniques is that a total picture of the component or system is possible.

Operating Temperature 5 to 122 °F (-15 to 50 °C)

Measuring Temperature -40 to 3630 °F (-40 to 2000 °C)

Accuracy ±2% of range or ±4 °F ( ±2 °C)

Sensitivity 0.2 °F (0.1 °C)

Image Storage Capacity 700 (14 bit) on 100 MB Card

Digital Voice Recorder for Active Documentation

yes, 30 sec per image

Camera Weight 5 lbs.


Contoh Hasil Kamera Inframerah

PipingHeat transfer coilsValve operation

Electronics

Engines/Compressors

Building/Structures


Sensor Temperatur InframerahNon-contact infrared thermometers uses:

Facility maintenanceUtilities and electrical inspectionMedical industryHVAC/R maintenance and inspectionFood safetyAutomotive and diesel maintenanceAsphalt, cement, and construction materials.

Infrared thermometers measure the surface temperature of objects within their field of view. Focal length of instrument is important consideration.


Thermocouple InframerahNew method of surface temperature measurement

UnpoweredLow costNon-invasiveThey can be installed on conventional thermocouple controllers.

How do they work?Receives heat energy from the object that it is aimed at and converts the heat to an electrical potential.Millivolt output signal is produced. This signal is scaled to the desired thermocouple characteristics.Adhere to the same laws as other infrared thermometers.


Keunggulan Themocouple Inframerah

IR thermometer dapat mengukur objek yang bergerak, berputar atau bergetarDapat mengukur temperatur > 1500 0CTidak merusak atau mengotori permukaan objek yang diukur Respon waktu dalam mili detik

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