## What is Thermometry and Temperature Scale?

It is the branch of science which deals with the calculation of temperature. It is also known as the science of development and use of thermometers.

Temperature scale (Temp scale) is the measure of amount of heat lost by the body through radiation, convection, and evaporation by expressing temperature based on certain reference parameters.

Different Temp scales

Kelvin, Celsius and FS Temp scales showing freezing point of water, boiling point of water and normal body temperature

## Origin of Thermometry

Thermometry is mainly split into two subfields contact thermometry and non-contact thermometry. This part of science was developed to solve the problems related to measuring the temperature in 150 A.D. Initially there existed nearly 15 different scales to measure temperature with wine as the element of calculation. Later they were replaced with mercury for its better thermal expansion and other important factors.

When humans realized that they could measure the changes in temperatures with primitive glass devices filled with liquids they began searching for methods to increase the accuracy of them. Since liquids expand on heating, liquids like mercury and alcohol were mostly used to make thermometers. Later Scientist named Daniel Fahrenheit invented the modern thermometer in 1709 that had numerical with FS on it.

## Types of Thermometers

Different types of thermometers like digital thermometer, mercury thermometer

### Liquid in glass

This is the widely used device in modern era, it measures the temperature accurately between –200 to 600C. The temperature is measured using a thermally sensitive liquid contained in a glass envelope, although gages expand more than liquids they are avoided for reasons like visibility, condense, impure and others. It works on the principle of thermal expansion of liquid. Liquid elements in the thermometer have chances of being evaporated so inert gases like argon or nitrogen are filled above mercury.

### Thermocouple

This is made such that one spot is maintained as reference temperature and the other spot is used to measure the measure temperature of given temperature. It works by the voltage difference created and these are made using two non-similar conductors. It works on the See beck’s principle- which states that a voltage difference is created when two metal conductors are exposed to a temperature.

### Platinum resistance (RTD)

Also known as the Resistance temperature detectors which work baes on the change of the electrical resistance of a conductor. Platinum is the conductor used because of its wide temperature range, accuracy, linearity and stability. The accuracy is directly proportional to the pureness of the wire. The temperature is found with graphical method by finding about the resistance and voltage applied.

### Thermistors

These are made up of semiconductor material which resistance with change is temperature. Unlike the metal conductors which only show positive temperature coefficient these show negative temperature coefficient as well.

There are some positive temperature coefficient thermistors as well which are made from doped ceramics and polymers and used in electric current regulation.

### Integrated Circuit Temperature Sensors (IC sensors)

IC sensors are two transistors which are coupled in saturated state. They produce an output current proportional to temperature of the device on applying voltage in the range of 4-30V DC.

Although these devices are cheap and insensible to electromagnetic noises their operating range is only around –50C to 150C and these devices act as a high impedance constant current regulator.

## Development of Temperature Scales

Temp scales were necessary to measure quantifying the hotness or coldness of the body. For this purpose, there were many standard methods established by assuming certain reference parameters. With this there are many scales that came into existence but three among them are widely used in all fields. Those are Fahrenheit scale, Celsius and Kelvin scale.

## Types of Temperature Scales

### Fahrenheit temperature scale (F)

The two basic reference points are freezing point of water at 32 and boiling point of water at 212 and the interval between them is divided into 180 parts. While Fahrenheit was in use in most of English-speaking countries, the other parts of the world were using Celsius and Centigrade scale. The value -459.67^{o}Fis equal to 0 kelvin, that temp is known as absolute zero. The triple point of water is 32.018^{o}F.

### Celsius temperature scale (C)

The two basic reference point are freezing point of water at 0 degree Celsius and boiling point of water at 100 degree Celsius and the interval between is divided into 100 parts and this calculation was also called as centigrade for this reason. Celsius was mostly used in scientific work and in placed where metric units were accepted. The value -273^{o}C is equal to 0 kelvin, that temp is known as absolute zero. The triple point of water is 0.01^{o}C.

### Kelvin temperature scale (K)

This is one of the most important temperature calculation value in international system of measurements, this is also called as Kelvin scale. The basic reference points I.e., freezing point of water is at 273 and boiling point of water is at 373. The importance of this calculation is at that it has zero point at absolute zero. Absolute zero is the theoretical temperature where molecules of the matter are in its lowest possible energies. Most of the physical laws and formulas use Kelvin Temp calculation as standard scientific Temp scale. The triple point of water is 273.15K.

Comparison of different Temp scales

comparison between the important Temp scales like Celsius, kelvin, Fahrenheit scale etc.

## Formulas

Standard formula to convert from one scale to another scale is,

$\begin{array}{l}\frac{T-Lower\text{fixedpointonscale1}}{\text{Upperf}ixed\text{pointonscale1}-\text{Lowerfixedpointonscale1}}=\frac{T-Lower\text{fixedpointonscale2}}{Upper\text{fixedpointonscale2}-\text{Lowerfixedpointonscale2}}\\ \end{array}$*To convert from FS to Celsius scale:

Celsius scale: Lower Fixed Point = 0, Upper Fixed Point = 100.

FS Scale: Lower Fixed Point = 32, Upper Fixed Point = 212.

$\begin{array}{l}{\left(\frac{T-0}{100-0}\right)}_{in\text{Celsius}}={\left(\frac{T-32}{212-32}\right)}_{in\text{Fahrenheit}}\\ {T}_{c}=\frac{5}{9}\left({T}_{F}-32\right)\end{array}$*To convert from Kelvin to Celsius temperature:

Kelvin Scale: Lower Fixed Point = 273.15, Upper Fixed Point = 373.15

Celsius scale: Lower Fixed Point = 0, Upper Fixed Point = 100.

$$\begin{array}{l}{\left(\frac{T-273.15}{373.15-273.15}\right)}_{in\text{Kelvin}}={\left(\frac{T-0}{100-0}\right)}_{in\text{Celsius}}\\ {T}_{c}=\left({T}_{k}-273.15\right)\\ {T}_{k}=\left({T}_{c}+273.15\right)\end{array}$$To convert from kelvin to FS:

Kelvin Scale: Lower Fixed Point = 273.15, Upper Fixed Point = 373.15

FS Scale: Lower Fixed Point = 32, Upper Fixed Point = 212.

$\begin{array}{l}{\left(\frac{T-32}{212-32}\right)}_{in\text{Fahrenheit}}={\left(\frac{T-273.15}{373.15-273.15}\right)}_{in\text{Kelvin}}\\ {\left(\frac{T-273}{5}\right)}_{K}={\left(\frac{T-32}{9}\right)}_{F}\\ {T}_{K}=273+\frac{5}{9}\left(T-32\right)\end{array}$## Common Mistakes

- Different types of Temp scale
- Learn the temperature conversions from one measurement to another measurement they are highly important.
- Working principles of different thermometers – Remember all types of thermometers and their working principles.

## Context and Applications

Thermometers are used daily in many fields like household to measure temperatures of cooking vessels and grills to avoid burns, in automobile industry to avoid heating up of coils by friction, in medical fields to analyze persons condition by taking his body temperature and comparing it with normal body temperature they are used in restaurants as well to kill the germs at certain temperature and achieving perfection of the food being cooked.

## Related Concepts

- Types of Thermometers
- Thermometers and thermometric liquids
- Triple point
- Triple point of water
- Freezing point and boiling point of water
- Equilibrium

## Practice Problem

Q) Calculate the change in the temperature on Fahrenheit temperature scale and on Kelvin temperature scale, when an iron piece is heated from 30^{o}C to 90^{o}C?

ANSWER:

Temperature change on Celsius scale temperature is:

$\Delta {T}_{C}=90\xb0C-30\xb0C=60\xb0C$The equation to convert into the FS is:

$\Delta {T}_{F}=\left(\frac{9}{5}\right)\Delta {T}_{C}=\left(\frac{9}{5}\right)\left(60\xb0\right)=108\xb0F$To convert in to Kelvin scale:

$\begin{array}{l}\Delta T=\Delta {T}_{C}\\ =60K\end{array}$### Want more help with your physics homework?

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