Instrumentasi Sistem Pengaturan

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1 Instrumentasi Sistem Pengaturan Pengukuran Temperatur Ir. Josaphat Pramudijanto, M.Eng. Jurusan Teknik Elektro FTI ITS Telp Fax Instrumentasi Sistem Pengaturan

2 Pengantar Objektif: Macam sensor temperatur Metode pengukuran dengan sensor temperatur Definisi: Suatu sensor yang mengkonversikan perubahan energi termal suatu objek menjadi energi listrik Energi termal per molekul dari material dinyatakan dalam derajat temperatur tertentu Instrumentasi Sistem Pengaturan

3 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). Instrumentasi Sistem Pengaturan

4 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. Instrumentasi Sistem Pengaturan

5 Skala Temperatur Skala - 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 tersebut Skala temperatur absolut : skala yang menetapkan temperatur nol suatu material yang tidak mempunyai energi termal (tidak ada getaran molekuler) Skala yang biasa dipakai: skala Farenheit ( o F ) skala Celsius ( o C ) skala Kelvin ( o K ) skala Rankine ( o R ) Instrumentasi Sistem Pengaturan

6 Konversi Skala Skala Temperatur Relatif: pergeseran sumbu nol Instrumentasi Sistem Pengaturan

7 Pengukuran Temperatur Perubahan dalam sifat-sifat elektris a) RTD (Resistance Temperature Detector) b) Thermistor c) Thermocouple d) Integrated Circuitry (IC) sensor Perubahan dalam dimensi phisik a) Bimetallic thermometer b) Filled-bulb dan Glass-stem thermometer Perubahan dalam emisi (pancaran) radiasi thermal Infrared pyrometer Instrumentasi Sistem Pengaturan

8 Perbandingan Daerah Pengukuran Instrumentasi Sistem Pengaturan

9 Tabel Perbandingan Sensor Temperatur Instrumentasi Sistem Pengaturan

10 Sensor Perubahan Resistansi The 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. Instrumentasi Sistem Pengaturan

11 Perbandingan Kurva Resistansi Thermistors R T β 1 T 1 T o = R e 0 RTD Sensitivity=dR/dT Instrumentasi Sistem Pengaturan

12 RTD (Resistance Temperature Detector) Bekerja berdasarkan perubahan resistansi logam karena perubahan temperatur Berbagai 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) Instrumentasi Sistem Pengaturan

13 Resistansi dan Temperatur Instrumentasi Sistem Pengaturan

14 Keunggulan dan Kelemahan Resistansi rendah 100Ω (terbanyak) sampai 1000 Ω Range operasi lebar ( C sampai C) Akurasi Tinggi (±0, C sampai 0,1 0 C) Repeatability dan stabilitas tinggi o Waktu respons lebih lambat (0,5 sampai 5 detik) o Sensitive terhadap shock dan vibrasi o Disupport dengan keramik, glass tube o Internal/self heating (joule heating = I 2 R) o Lead wire can be significant Instrumentasi Sistem Pengaturan

15 Konstruksi RTD penghantar yang digulungkan pada suatu form (biasanya koil) dilindungi sheath atau tabung pelindung (menambah waktu respon) hubungan temperatur dan disipasi: ΔT = T = kenaikan temperatur karena self heating dalam o C P = disipasi daya pada RTD dalam W = konstanta disipasi RTD dalam W/ o C P D P P D Instrumentasi Sistem Pengaturan

16 Pengukuran dengan RTD Most sensitive method is by using the Wheatstone bridge R 1 R 3 R 2 R 4 = R RTD Instrumentasi Sistem Pengaturan

17 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 Instrumentasi Sistem Pengaturan

18 THERMISTOR Sensor temperatur berdasarkan perubahan resistansi semikonduktor terhadap temperatur Penambahan energi termal yang sama dengan atau melebihi energi gap ΔW g menyebabkan elektron - elektron dalam pita valensi masuk ke pita konduksi dan bebas menghantarkan arus melalui bagian terbesar material tersebut Energi gap dari material semikonduktor 0,01V - 4V, sebagai contoh silikon yang memiliki energi gap ΔW g = 1,07 Watt Resistance(kOhm) Temperature(C) Instrumentasi Sistem Pengaturan

19 Karakteristik Thermistor Resistansi tinggi 1kΩ sampai 100 kω Ukuran 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 RTD Sensitivitas sangat tinggi (1000 kali lebih sensitif daripada RTD) Perubahan resistansi 10% per 0 C. Misal resistansi nominal 10 kω maka resistansi akan berubah 1kΩ untuk setiap perubahan temperatur 1 0 C. Tidak sensitif terhadap shock dan vibrasi Thermistor dilindungi capsul (plastik, teflon/material lembam) memperlambat waktu respon karena kontak termal kurang baik Instrumentasi Sistem Pengaturan

20 Kalibrasi Thermistor R T β 1 T 1 T = o R 0 e where R 0 = reference resistance measured at T T = measured temperature β = material constant o Instrumentasi Sistem Pengaturan

21 RTD dan Thermistor Unlike RTDs, they have negative temperature dependence Much higher sensitivity than RTDs Highly nonlinear devices Instrumentasi Sistem Pengaturan

22 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. Instrumentasi Sistem Pengaturan

23 Penemu Prinsip Kerja Thermocouple In, 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. Instrumentasi Sistem Pengaturan

24 Material EMF versus Temperature With reference to the characteristics of pure Platinum Instrumentasi Sistem Pengaturan

25 Tabel Temperatur Thermocouple tipe E (Chromel Constantan) Instrumentasi Sistem Pengaturan

26 Polinomial untuk Akusisi Data Instrumentasi Sistem Pengaturan

27 Pengukuran dengan Referensi Ice Baths Accurate and inexpensive Instrumentasi Sistem Pengaturan

28 Kode Warna Thermocouple Instrumentasi Sistem Pengaturan

29 Thermocouple Material Vs EMF Types T, J, and K are most commonly used thermocouples. Instrumentasi Sistem Pengaturan

30 Bahan Thermocouple tipe T dan J Copper-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. Instrumentasi Sistem Pengaturan

31 Bahan Thermocouple tipe K, E, dan J Chromel-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. Instrumentasi Sistem Pengaturan

32 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 platinumrhodium 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. Instrumentasi Sistem Pengaturan

33 Grades of TC wire Grade is based on calibration accuracy of the wire. Precision grade (± 0.5 % reading or 1 C), greater of the two Standard grade (± 0.75 % reading or 2 C), greater of the two Extension 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 diameter inches and up is possible Standard 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 Instrumentasi Sistem Pengaturan

34 Law of Intermediate Temperatures T2 T3 T3 T1 T2 T1 If a thermocouple circuit develops a net emf 1-2 for measuring junction temperatures T 1 and T 2, and a net emf 2-3 for temperatures T 2 and T 3, then it will develop a net voltage of emf 1-3 = emf emf 2-3 when the junctions are at temperatures T 1 and T 3. emf emf 2-3 = emf 1-3 Instrumentasi Sistem Pengaturan

35 Rangkaian Thermocouple Thermopile-Thermocouples connected in series between two temperature zones. Good for determining small temperature differences Amplification effect Ambient Temp. = 24 C mv Voltmeter + - Cu Cu Isothermal Block at Ambient Temp. Averaging Circuit- Thermocouple are connected in parallel between two temperature zones. 4 iron constantan TC's? C Ambient Temp. = 24 C Ice Bath? iron 110 C Voltmeter + Cu C Cu 130 C constantan Instrumentasi Sistem Pengaturan

36 Bimetal Thermometer Dua 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 0 C sampai C Variasi akurasi (± 0,5 sampai 12 0 C) Advantages: Low cost Negligible maintenance Stable operation over time Instrumentasi Sistem Pengaturan

37 Liquid Thermometer Pemuaian volume dari liquid dan padat digunakan untuk mengukur temperatur (biasanya beda muai antara liquid dan glass) Mercury filled thermometer ( best ) Range= -37 to C, Accuracy ± 0,3 0 C Alcohol filled thermometer Range= -75 to C, Accuracy ± 0,6 0 C Advantages/disadvantages Low cost/no remote reading Instrumentasi Sistem Pengaturan

38 Fluid Expansion Thermometer A fluid filled bulb is connected to a pressure measuring device via a capillary tube. As fluid is heated it expands thus pressure increases. Pressure is linked to temperature. Accuracy and range depends on fluid. Advantages/Disadvantages Low cost Stable in operation Widely used in industrial applications. Remote readings are possible. Transient response is a function of bulb size and capillary tube length. Instrumentasi Sistem Pengaturan

39 Infrared Thermometers Thermometer 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 dari objek detektor untuk mengubah energi thermal ke sinyal listrik Instrumentasi Sistem Pengaturan

40 Infrared Cameras Operating Temperature Measuring Temperature Accuracy Sensitivity Image Storage Capacity Digital Voice Recorder for Active Documentation Camera Weight 5 to 122 F (-15 to 50 C) -40 to 3630 F (-40 to 2000 C) ±2% of range or ±4 F ( ±2 C) 0.2 F (0.1 C) 700 (14 bit) on 100 MB Card yes, 30 sec per image 5 lbs. Modern 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. Instrumentasi Sistem Pengaturan

41 Contoh Hasil Kamera Inframerah Piping Heat transfer coils Valve operation Electronics Engines/Compressors Building/Structures Instrumentasi Sistem Pengaturan

42 Sensor Temperatur Inframerah Non-contact infrared thermometers uses: Facility maintenance Utilities and electrical inspection Medical industry HVAC/R maintenance and inspection Food safety Automotive and diesel maintenance Asphalt, cement, and construction materials. Infrared thermometers measure the surface temperature of objects within their field of view. Focal length of instrument is important consideration. Instrumentasi Sistem Pengaturan

43 Thermocouple Inframerah New method of surface temperature measurement Unpowered Low cost Non-invasive They 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. Instrumentasi Sistem Pengaturan

44 Keunggulan Themocouple Inframerah IR thermometer dapat mengukur objek yang bergerak, berputar atau bergetar Dapat mengukur temperatur > C Tidak merusak atau mengotori permukaan objek yang diukur Respon waktu dalam mili detik Instrumentasi Sistem Pengaturan