4. SEPTEMBER 2012

TEMPERATURE SENSING

4. SEPTEMBER 2012

REFERENCE POINTS IN ITS-90

› Defining points› The table lists the

defining fixed points of ITS-90.

Substance and its state

Defining point in kelvin (range) Defining point in Celsius (range)

Vapor-pressure / temperature

relation of helium-3 (by equation)

(0.65 to 3.2) (−272.50 to −269.95)

Vapor-pressure / temperature

relation of helium-4 below its

lambda point (by equation)

(1.25 to 2.1768) (−271.90 to −270.9732)

Vapor-pressure / temperature

relation of helium-4 above its

lambda point (by equation)

(2.1768 to 5.0) (−270.9732 to −268.15)

Vapor-pressure / temperature

relation of helium (by equation)

(3 to 5) (−270.15 to −268.15)

Triple point of hydrogen 13.8033 −259.3467

Triple point of neon 24.5561 −248.5939

Triple point of oxygen 54.3584 −218.7916

Triple point of argon 83.8058 −189.3442

Triple point of mercury 234.3156 −38.8344

Triple point of water 273.16 0.01

Melting point1

of gallium 302.9146 29.7646

Freezing point1

of indium 429.7485 156.5985

Freezing point of tin 505.078 231.928

Freezing point of zinc 692.677 419.527

Freezing point of aluminum 933.473 660.323

Freezing point of silver 1234.93 961.78

Freezing point of gold 1337.33 1064.18

Freezing point of copper 1357.77 1084.62

4. SEPTEMBER 2012

DIFFERENCE IN REFERENCE POINTS FROM 1889 TO 1990

4. SEPTEMBER 2012

CALIBRATION WITH ICE

4

4. SEPTEMBER 2012

TRIPLE POINT

› The triple point is the temperature and pressure where a matter can exist in all three states (solid, liquid, gas)

› For water the triple point is at 0°C and

0.611 Kpa

› (std. Atmospheric pressure on Earth is set to 101.325 kPa,

› Average pressure on Mars is 0,636kPa and avg. Temperatureis -63°C)

4. SEPTEMBER 2012

TRIPLE POINT CALIBRATION

› For water the triple point is at 0°C

and 0.611 kPa

› Ethylene carbonate has a triple point temperature of 36.315 °C

4. SEPTEMBER 2012

THERMOELECTRIC EFFECT

Seebeck effectA loop made of two different materials will a current when a

temperature difference is applied to the junctions

Seebeck coefficient:› Is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across a material. The unit is Volts/Kelvin

› The voltage V developed can be derived from:

Where SA and SB are the Seeback coefficients for material

A and B, and T1 and T2 are the temperatures in the junctions

Unit: V/K

4. SEPTEMBER 2012

LIST OF SEEBECK COEFFICIENTS

Modern thermocouples are

made of semiconductor

materials which makes doping

possible and thereby losens

electrons

4. SEPTEMBER 2012

THERMOELECTRIC EFFECT

Peltier effect:

The ”opposite” of the Seebeck effect.

A solid state cooling (and heating) system often used for small devices

like car refrigators

4. SEPTEMBER 2012

10

4. SEPTEMBER 2012

THERMOELECTRIC EFFECT

Faraday effect:

Faraday found through experimentation that certain semiconductor materials

decrease their resistance as temperature increases. These materials are said to

have a negative temperature coefficient.

4. SEPTEMBER 2012

LAWS FOR THERMOCOUPLE CIRCUITS

1. A circuit made of one single material will not lead current because of

temperature differences alone

2. The algebraic sum of the thermoelectric Electromotive forces is zero if all

junctions are at the same temperature.

3. If two dissimilar homogeneous materials produce thermal EMF1 when the junctions are at T1 and T2 and produce thermal EMF2 when the junctions are at T2 and T3, the EMF generated when the junctions are at T1 and T3 will be EMF1 + EMF2, provided T1<T2<T3

4. SEPTEMBER 2012

THERMOCOUPLES

4. SEPTEMBER 2012

EXCERPTS OF THERMOCOUPLE

TABLES

The table use a reference

Junction temperatur på 0°C

4. SEPTEMBER 2012

WHY THERMOCOUPLE WORKS

› Let's start with a couple of experimental facts. Each metal has a certain density of conduction (essentially free to move) electrons in it, and each metal holds onto its electrons with a different amount of "eagerness". We can quantify the latter by firing photons at the metal and seeing that at a certain photon energy, electrons start being knocked loose (this is the photoelectric effect). The amount of energy required to knock and electron loose is called the workfunction of the metal. Let's say, for example, that we have a metal with a workfunction of 4.1eV and another with a workfunction of

4.7eV. If I pull an electron out of the 4.1eV material and drop it into the 4.7eV material, I release 0.6eV. Of course, when I do this, I also transfer an electric charge, making it harder to transfer the next electron. Now imagine butting the two metals together. Because energy is released by going from the 4.1eV workfunction metal to the 4.7eV

workfunctionmetal, electrons will spontaneously transfer that way... at first. As they do so, though, they separate charge and cause an electric field (in the direction to oppose further electrons from crossing the boundary) to build up. Now, just to make life difficult, Mom Nature also imposes a third effect. Since the density of electrons in the two materials is usually not the same, there is also a diffusion current driven by the concentration gradient of electrons. So we have three things going on: diffusion current, drift current (from the induced electric field) and the initial intrinsic workfunctiondifferences.

http://www.madsci.org/posts/archives/oct99/939333814.Ph.r.html

4. SEPTEMBER 2012

EXAMPLE

A chromel–alumel thermocouple (type K) with chromel–alumel extension wires is used to measure the

temperature of a fluid. In connecting up this measurement system, the instrumentation engineer responsible

has inadvertently interchanged the extension wires from the thermocouple. The ends of the extension wires

are held at a reference temperature of 0°C and the output e.m.f. measured is 14.1 mV. If the junction

between the thermocouple and extension wires is at a temperature of 40°C, what temperature of fluid is

indicated and what is the true fluid temperature?

Source: Butterworth-Heinemann,.Measurement and Instrumentation Principles, 3rd Ed.

4. SEPTEMBER 2012

EXAMPLE

A chromel–alumel thermocouple (type K) with chromel–alumel extension wires is used to measure the

temperature of a fluid. In connecting up this measurement system, the instrumentation engineer responsible

has inadvertently interchanged the extension wires from the thermocouple. The ends of the extension wires

are held at a reference temperature of 0°C and the output e.m.f. measured is 14.1 mV. If the junction

between the thermocouple and extension wires is at a temperature of 40°C, what temperature of fluid is

indicated and what is the true fluid temperature?

hense

From the thermocouple table we interpolate and see

that the true fluid temperature is 374,5°C

Source: Butterworth-Heinemann,.Measurement and Instrumentation Principles, 3rd Ed.

4. SEPTEMBER 2012

THERMOCOUPLES5 STANDARD TYPES:

E Chromel (+) – Constantan (-) ~80µV/°C

-200°C < t < 900°C Accuracy 0.5%

J Iron (+) – constantan (-) ~60µV/°C

-150°C < t < 1000°C Accuracy: 0,75%

K Chromel (+)alumel (-) ~45µV/°C

700°C < t < 1200°C Accuracy: 0,75%

N Nicrosil - Nisil ~40µV/°C

700°C < t < 1200°C

(stability during long-time use is

about three times better than ”k”)

Accuracy: 0,75%

T Copper (+) – Constantan (-) ~60µV/°C

-200°C < t < 350°C Accuracy: 0,75%

Chromel: 90% nickel and 10% chromium

Alumel:95% nickel, 2% manganese, 2% aluminium and 1% silicon

Constantan: 55% copper and 45% nickel

Nicrosil: 84,1% nicle,14.4% chromium, 1.4% silicon, and 0.1% magnesium

Nisil: 95,6% nickel, 4,4% silicon

NB: The temperature interval varies from literature to literature

4. SEPTEMBER 2012

RESISTANCE THERMOMETERS (RTDs)

Uses the principal of Faradays law.

the resistance of a metal varies with

temperature according to the relationship:

For some materials, notably platinum, copper and nickel, the higher order terms can be neglated:

Platinum has the most linear resistance–temperature characteristic. Its

resistance–temperature relationship is linear within ±0.4%

over the temperature range between -200°C and +40°C. Even at

+1000°C, the quoted inaccuracy figure is only ±1.2%

4. SEPTEMBER 2012

RESISTANCE THERMOMETERS (RTDs)

Source: Butterworth-Heinemann,.Measurement and Instrumentation Principles, 3rd Ed.

4. SEPTEMBER 2012Resistance table [link]

4. SEPTEMBER 2012

DIODES FOR TEMPERATURE SENSING

VCC

5V

R115kΩ

D1

BAV20

Sensor

Til buffer

The diode equation gives an expression for the current through a diode as a function of voltage.

The Diode Law, expressed as:

where:

I = the net current flowing through the diode;

I0

= "dark saturation current", the diode leakage current density in the absence of light;

V = applied voltage across the terminals of the diode;

q = absolute value of electron charge (1.60217646 × 10-19 Coulomns)

k = Boltzmann’s constant (1.3806503 × 10-23m

2 kg s-2 K-1)

T = absolute temperature (K).

n = ideality factor, a number between 1 and 2 which typically increases as the current decreases.

The "dark saturation current" (I0) is an extremely important parameter which differentiates one

diode from another. I0

is a measure of the recombination in a device. A diode with a larger

recombination will have a larger I0.

Note that I0increases as T increases; and I

0decreases as material quality increases.

At 300K, kT/q = 25.85 mV, the "thermal voltage".