Instrumentasi Sistem Pengaturan

Instrumentasi Sistem Pengaturan

Pengukuran Temperatur

Ir.

Josaphat

Pramudijanto, M.Eng.

Jurusan Teknik Elektro FTI ITS

Telp. 5947302 Fax.5931237

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

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 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 ( oF )  skala Celsius ( oC )  skala Kelvin ( oK )  skala Rankine ( oR )

Konversi Skala

Skala Temperatur Relatif: pergeseran sumbu

nol

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

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.

Perbandingan Kurva Resistansi

Sensitivity=dR/dT

RTD

Thermistors

      −

=

To 1 T 1 β 0 T

R

e

R

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)

Keunggulan dan Kelemahan

Resistansi rendah 100Ω (terbanyak) sampai 1000 Ω

Range operasi lebar (-200

0

_{C sampai 850}

0

_C)

Akurasi Tinggi (

±

0,0006

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)

Konstruksi RTD

penghantar 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

o

_C

P = disipasi daya pada RTD dalam W

P

_D

= konstanta disipasi RTD dalam W/

o

_C

D

P

P

Pengukuran dengan RTD

Most sensitive method is by using the

Wheatstone bridge

R

₃

R

₂

R

₁

Kompensasi untuk Jarak Jauh

The problem:

Lead resistance can be 10%

to 50% (or more) of the total

resistance!

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

-20 0 20 40 60 80 100 5 10 15 20 25 30 35 40 0 R esi st an ce (kO hm) Temperature(C)

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

Kalibrasi Thermistor













−

=

T

o

1

T

1

β

0

T

R

e

R

constant

material

=

β

re

temperatu

measured

=

T

T

at

measured

resistance

reference

=

R

where

o 0

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

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.

Material EMF versus Temperature

With reference to

the characteristics

of pure Platinum

Tabel Temperatur Thermocouple tipe E

Pengukuran dengan Referensi

Ice Baths

Thermocouple Material Vs EMF

Types T, J, and K are most commonly used thermocouples.

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.

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

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 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

0.001 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”

Law of Intermediate Temperatures

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

_1-2

+ emf

_2-3

when

the junctions are at

temperatures T

₁

and T

₃

.

emf

+ emf

= emf

T2

T3

T1

T3

T2

Rangkaian Thermocouple

Ambient Temp. = 24 °C + -Voltmeter 4 iron constantan 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 differences Amplification effect

Averaging Circuit- Thermocouple are connected in parallel

between two temperature zones.

Ambient Temp. = 24 °C Cu Ice Bath 110 °C iron ?

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 430

0

C

Variasi akurasi (

±

0,5 sampai 12

0

C)

Advantages:

Low cost

Negligible maintenance Stable operation over time

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 320 0_{C, Accuracy ± 0,3} 0_C

Alcohol filled thermometer

Range= -75 to 120 0_{C, Accuracy ± 0,6} 0_C

Advantages/disadvantages

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.

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

Infrared Cameras

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

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

Contoh Hasil Kamera Inframerah

Piping

Heat transfer coils

Valve operation

Electronics

Engines/Compressors

Building/Structures

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

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

Keunggulan Themocouple Inframerah

IR thermometer dapat mengukur objek yang

bergerak, berputar atau bergetar

Dapat mengukur temperatur > 1500

0

_C

Tidak merusak atau mengotori permukaan objek

yang diukur