Conversion of temperature scale

The Celsius scale and the Fahrenheit scale are different temperature scales. If the temperature of an object is measured and expressed on a Celsius scale, then we want to state the temperature of the object on the Fahrenheit scale then we change it from the Celsius scale to the Fahrenheit scale. In this section, we learn to change or convert a temperature scale.

At a pressure of 1 atm, the temperature of the freezing point for the Celsius thermometer = 0 oC, while the Fahrenheit thermometer = 32 oF. Conversely, at a pressure of 1 atm, the temperature of the boiling point for a Celsius scale thermometer = 100 oC, while a thermometer with a Fahrenheit scale = 212 oF.

To make it easier for you to change the Celsius scale to the Fahrenheit scale or change the Fahrenheit scale to Celsius scale, remember 0 oC = 32 oF and 100 oC = 212 oF. On the Celsius scale, between 0 oC and 100 oC is 100o. While on the Fahrenheit scale, between 32 oF to 212 oF there are 180o.

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3.1 Convert the Celsius scale to the Fahrenheit scale

Conversion of temperature scale 1

To get the temperature on the Fahrenheit (T) scale, multiply the temperature in the Celsius (TC) scale first by 9/5 then add 32o.

3.2 Convert the Fahrenheit scale to the Celsius scale

Conversion of temperature scale 2

To get the temperature on the Celsius scale (TC), reduce the temperature on the Fahrenheit (TF) scale by 32o, then multiply by 5/9.

4. Constant-volume gas thermometer

Previously we have learned how to calibrate a thermometer. The thermometer we use is a mercury thermometer that has no scale. If we calibrate two thermometers of different types, such as a mercury thermometer and an alcohol thermometer, the two thermometers scale is the same only at 0 oC (or 32 oF) and 100 oC (or 212 oF). If we use the two thermometers to measure air temperature, the numbers shown by each thermometer are not necessarily the same. Mercury thermometers can show 48 oC, while alcohol thermometers show 46 oC. This is because the speed of expansion of mercury and alcohol is different. Likewise with other types of thermometers, such as bimetallic thermometers, etc. The temperature scale determined in this way depends very much on the nature of the material used.

Because the temperature scale specified using a thermometer usually has a deficiency, we need a standard thermometer. The existence of a standard thermometer helps us to set the temperature scale more precisely, without having to depend on the nature of a material.


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An almost perfect thermometer is a constant-volume gas thermometer. The working principle of a constant-volume gas thermometer is, the Gas volume is maintained so that it is always fixed or unchanged. Now, when the temperature increases, the gas pressure also increases. In pipe 1 and pipe two there is mercury. The gas volume is maintained so that it is always fixed, by raising or lowering the pipe two so that the mercury surface in pipe 1 is still at the reference sign. If the temperature increases, the gas pressure in the tube also increases. Therefore pipe two must be lifted higher so that the gas volume is always constant. Gas pressure can be known by reading the height of the mercury column (h) in the pipe 2. If using a manual method, just remember the mercury column is 760 mm = 1 atm (1 atmosphere). In a sophisticated constant-volume gas thermometer there is a pressure counter. Gas-filled containers have also been designed so that the gas is always in a fixed volume. So the measured only changes in pressure.


5. Kelvin Scale

If the slash in the chart above is drawn to cut the T oC axis, then when the gas pressure is 0, the gas temperature = -273.15 oC. Maybe we think that the temperature of a gas at a pressure of 0 varies depending on the type of gas in the constant-volume gas thermometer tube. Based on the experiment, although the gas type is different when the gas pressure becomes zero, the temperature is always valued at -273.15 oC. Thus, we can use this temperature as a standard temperature scale (also known as absolute zero temperature). This absolute zero temperature is known as the absolute scale or Kelvin scale. Kelvin is the name of Lord Kelvin (1824-1907), a former British physicist.

On this scale, the temperature is expressed in Kelvin (K), not the Kelvin (oK) degree. The interval between degrees is the same as on the Celsius scale, but zero is shifted to 0 K. So 0 K = -273.15 oC and 273.15 K = 0 oC. The temperature in the Celsius scale can be changed to a Kelvin scale by adding 273.15. Likewise, the temperature of the Kelvin scale can be changed to Celsius scale by reducing 273.15.

T (K) = T (oC) + 273.15

T (oC) = T (K) – 273.15

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