Thermocouple Thermometer

What is a Thermocouple?

The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires, e.g., one chromel and one constantan, coupled at the probe tip (measurement junction) and extended to the reference (known temperature) junction.

The temperature difference between the probe tip and the reference junction is detected by measuring the change in voltage (electromotive force, EMF) at the reference junction.

The absolute temperature reading can then be obtained by combining the information of the known reference temperature and the difference of temperature between probe tip and the reference.

How does a thermocouple thermometer work?



If two junction are at different temperatures (e.g. one junction is placed in a hotter region than the other), a small voltage is produced. The greater the difference in temperature, the greater the voltage produced across the ends of the two junctions.If one junction is kept at a fixed temperature such as 0*C, then the other junction can be used as a tiny probe to measure the other temperature.

e.m.f produced is inversely proportional to the temperature difference between reference junction and the probe.

Advantages

- ability to measure a large temperature range
- very useful in a variety of circumstances in many different industries
- respond rapidly to changes in temperature and can be used at high temperatures at which other sensors would be destroyed.
- very versatile,robust,fairly accurate
- small mass
- made rugged, simply constructed and are immune to shock and vibration
- can be made very small, fitting into applications that other temperature sensors
- measure the temperature, as well as be coupled with certain circuits in order to discontinue the heating process once the wanted temperature is achieved.

(This can play an important part in safety measures, thus thermocouples can be considered important safety )

Common Temperature Scales

The Centigrade Scale


The centigrade temperature scale, more properly known as the Celsius temperature scale, is a scale for measuring temperatures which is based on the behavior of water at normal pressure. This scale is widely used in much of the world to express temperatures, with a few holdout nations using the Fahrenheit temperature scale. The Celsius scale is also used as the benchmark for the Kelvin scale, which is used in the scientific community.

Fixed Points




1. Ice Point(0*C) - Temperature of pure melting point at one atmosphere



Calibration at the Ice Point of Water


1. Fill a 600 mL Beaker three-quarters full of crushed Ice.

2. Add enough pre-cooled distilled Water to cover the Ice, but not so much Water such thatthe Ice floats.

3. Thoroughly stir the Ice-Water mixture.

4. Insert the thermometer into a rubber stopper with a slit hole. (Be careful doing this!!!)

5. Use a ring stand and clamp to suspend the thermometer in the Ice-Water bath up to theImmersion Mark. (If you have a Total Immersion thermometer, see you laboratory
instructor for directions.)

6. Allow the temperature shown by the thermometer to stabilize. After 3 minutes at the stable temperature, record the temperature to the correct precision.

7. The Ice Point of Water is remarkably stable at 0.00oC.

2. Steam Point - Temperature of steam from water boiling at one atmosphere



Calibration at the Boiling Point of Water


1. Set up a hot plate with a 500 mL Florence Flask resting on it. The flask should be
supported by a clamp from a ring stand.

2. Fill the flask about half full with distilled Water. Add a few boiling chips to promote smooth boiling.

3. Clamp the thermometer to the ring stand as before such that the immersion mark is in the neck of the flask.

4. Turn on the hot plate and allow the Water to come to its Boiling Point.

5. Allow the temperature shown by the thermometer to stabilize. After 3 minutes at the stable temperature, record the temperature to the correct precision.

6. The Boiling Point of Water is extremely sensitive to the atmospheric pressure. Use the barometer to measure the atmospheric pressure. Use the data in the Appendix to determine the correct Boiling Point of Water.

*For Centigrade scale, the interval between the ice point and steam point is divided into 100 equal parts.Each mark on the thermometer is a measure of 1 *C.


General Equation for Centigrade Scale


Let 'x' be the physical property of the thermometric substance which varies continuously with temperature.Quantity of 'X' can vary according to the various physical property.(e.g. If X refers to the length of Mercury column in a Mercury-i-glass thermometer, the quantity of X will be cm or m)

Y*C = [(X(y)-X(0*C))/(X(100*C)-X(0*C))]x 100%


The Kelvin Scale


The Kelvin temperature scale (K) was developed by Lord Kelvin in the mid 1800s.The unit for this scale is Kelvin(K). The zero point on this scale is base on the point at which the pressure of all dilute gases mathematically project to zero from the triple point of water.The triple point is the temperature at which liquid water, ice, and water vapor can coexist simultaneously.

When a material is cooled, it looses heat, and its temperature decreases, until a point is reached where it has no more heat left to loose. At this point it is not possible to lower the temperature any further. This low temperature is called absolute zero. Lord Kelvin suggested that this absolute zero temperature be the basis of a new scale which begins with the value zero at absolute zero. Just like the Celsius scale, there is a difference of 100 degrees between the freezing and boiling points of water.(1*C = 1K)

Kelvin = degrees Celsius + 273

Temperature

Relation between Temperature and Heat

Temperature - a physical property that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot.



The temperature of an ideal monatomic gas is related to the average kinetic energy of its atoms. In this animation, the size of helium atoms relative to their spacing is shown to scale under 1950 atmospheres of pressure. These atoms have a certain, average speed (slowed down here two trillion fold from room temperature).

Heat - energy transferred from one body or thermodynamic system to another due to thermal contact when the systems are at different temperatures. It is also often described as the process of transfer of energy between physical entities.



Heat generated from the infrared electromagnetic radiation from the Sun is one of the driving forces of life on Earth.

Physical properties that change with temperature:

1)Volume of a fixed mass of liquid
e.g. Mercury-in-glass thermometer, Alcohol-in-glass thermometer

2)Electrical voltage or electromotive force(e.m.f)
e.g. Thermocouple

3)Electrical Resistance of a piece of metal
e.g. Resistance thermometer

4)Pressure of a fixed mass of gas at constant volume
e.g. Constant-volume gas thermometer

What makes a good Thermometer?

1) An easy-to-read scale
2) Safe to use
3) Responsive to temperature changes
4) Sensitive to small temperature changes
5) Able to measure a wide range of temperatures