Measurement of Temperature: Types of Thermometers & Comparison Between Different Temperature Scales

Temperature measurement is thermometry. It's a method to measure how hot or cold something is. That is possible with the use of special devices that we today know as thermometers.

So, the measurement of temperature becomes an important topic to learn when you want to learn the mechanism of thermometers. That includes a few things

• How they give correct readings
• What scales are there for measuring the temperature, and
• How to convert from one scale to another. 

We will discuss all below. 

When learning about temperature measurement, there are four types of scales. 

(a) The Centigrade or Celsius scale $(°C)$

(b) The Fahrenheit scale $(°F)$       

(c) The Reaumer scale $(°R)$

(d) Kelvin scale of temperature (K)

Below, we explain how temperature is measured using a general formula. 

So, when you are looking at this image below, there are different numbers for freezing and boiling points for each type of scale in temperature measurement. Suppose you want to convert from one scale to another. You will need a formula for that. 

The formula for the conversion between different temperature scales is:

$\frac{K-273}{100}$ = $\frac{C}{100}$ = $\frac{F-32}{180}$ = $\frac{R}{80}$

General formula for the conversion of temperature from one scale to another:

$\frac{Temp\text{on one scale}\left(S_1\right)\text{-Lower fixed point}\left(S_1\right)}{Upper\text{fixed point}\left(S_2\right)-Lower\text{fixed point}\left(S_1\right)}$            

= $\frac{Temp.\text{on other}\text{scale}\left(S_2\right)\text{-Lower fixed point}\left(S_2\right)}{Upper\text{fixed point}\left(S_2\right)-Lower\text{fixed point}\left(S_2\right)}$  

Thermometers are device that are used to measure temperatures. All thermometers are based on the principle that some physical property of a system changes as the system temperature changes.

Required properties of good thermometric substance.

(1) Non-sticky (absence of adhesive force)

(2) Low melting point (in comparison with room temperature)

(3) High boiling temperature

(4) Coefficient of volumetric expansion should be high (to increase accuracy in measurement).

(5) Heat capacity should be low.

(6) Conductivity should be high

Mercury (Hg) suitably exhibits above properties.

When learning about thermometry, it's ideal you learn about the types of thermometers, their properties, why they are good or bad, and the different use cases.  

The standard thermometer, against which all other thermometers are calibrated, is based on the pressure of a gas in a fixed volume. Figure shows such a constant volume gas thermometer; it consists of a gas-filled bulb connected by a tube to a mercury monometer.

$-{39}^{\circ }\mathrm{C}$

${450}^{\circ }\mathrm{C}$  Pressure at the temperature being measured, $-{270}^{\circ }$  pressure when bulb in a triple point cell.