Temperature
Scales & Fixed Points
The Origins of Fixed Temperature Points
When the thermometer was first invented
there was no clear understanding of fixed temperature points.
To further confuse the situation the skills required to make
thermometers were not available: it was difficult to make
a thermometer with a bore of consistent diameter along its
length, meaning that no two thermometers were alike. It was
only when people travelled with individual thermometers that
they were able to confirm that the fixed points of water boiling
and freezing were the same in different locations and the
effects of altitude and air pressure were recognised. The
boiling and freezing points of water were chosen as fixed
points because they were easily achievable.
Once the fixed points had been recognised and thermometer making
skills had improved, the way was clear for a widely recognised
temperature scale. Many were proposed and the main ones are
outlined below.
The Fahrenheit Scale
This was the first widely
used temperature scale and it is still in use today, though
it has lost popularity as people have moved towards metric
measurements. |
| Fixed
Points: |
Freezing Point
of Water
Boiling Point of Water |
| Number
of Divisions: |
180 degrees |
| Notes: |
32 was chosen
as the the figure for the lower fixed point as this produced
a scale that would not fall below zero even when measuring
the lowest possible temperatures that he could produce
in his laboratory - a mixture of ice, salt and water.
It is sometimes suggested that Fahrenheit divided his
scale into 100 degrees using blood temperature and his
lowest possible temperature as fixed points - this is
not true. |
The Réamur
Scale
This scale is not
in use today, but its development was significant as
it was the first scale to use 0° as the freezing
point of water. |
| Fixed
Points: |
Freezing Point
of Water
Boiling Point of Water |
| Number
of Divisions: |
80 degrees |
| Notes: |
The Réamur
scale is not in use today. |
The Celsius Scale
While Celsius's original metric
scale had to be inverted before it came into common use,
it still bears his name today. The Celsius scale is widely
used today for general, engineering and meteorological
purposes. |
| Fixed
Points: |
Freezing Point
of Water
Boiling Point of Water |
| Number
of Divisions: |
100 degrees |
| Notes: |
In 1742 Swedish
scientist Anders Celsius chose 0 degrees for the boiling
point of water, and 100 degrees for the freezing point.
A year later, the Frenchman Jean Pierre Cristin (1683-1755)
inverted the Celsius scale to produce the Centigrade scale
used today (freezing point 0°, boiling point 100°).
By international agreement in 1948 Cristin's adapted scale
became known as Celsius and is still in use today. |
The Kelvin Scale
The Kelvin is the standard SI
unit of thermodynamic temperature in use today.
It is most commonly used by physicists. |
| Fixed
Points: |
Triple Point
of Water
Boiling Point of Water |
| Number
of Divisions: |
100 degrees |
| Notes: |
In 1848 Sir
William Thomson, Baron Kelvin of Largs, Lord Kelvin of
Scotland (1824 - 1907) proposed the absolute temperature
scale with zero degrees being the theoretical lowest temperature
possible where molecular motion ceases. Kelvin defined
1 Kelvin degree as being equal to one Celsius degree.
|
Fixed points in use today
The practical temperature scale
in use today is maintained by the General Conference on Weights
& Measures (Paris). The most recent temperature scale was
devised in 1990: The International Temperature Scale of 1990
or ITS-90 for short.
ITS-90 covers 16 fixed points, being the melting, freezing or
triple points of various substances: Water, Mercury, Gallium,
Indium, Tin, Zinc, Aluminium, Silver Hydrogen, Neon, Oxygen,
Argon, Copper and Gold. Water apart, all of these are elements.
These fixed points give a range of temperatures at which a thermometer
can be calibrated from, for example, the triple point of Hydrogen
at -259.3467°C (13.8033 K ) to the freezing point of Gold
at 1064.16°C ( 1337.33K ).
Triple Points
As well as the boiling
or melting point of a substance, the triple point of a substance
can be use as a fixed point. This is the temperature at which
that substance exists in its solid, liquid and gaseous state
all at the same time.
Water
Triple point cell in use in the Brannan Calibration
Laboratory
|
The triple point of water
is the most important fixed point on ITS-90 as it is the
sole fixed point which is common to ITS-90 and the Kelvin
Thermodynamic Temperature Scale.
In the Brannan
Calibration Laboratory we regularly use the triple
point of water to recalibrate thermometers for our customers
- its temperature is 0.01°C ( 273.16K ) and it can
be recreated with great accuracy.
To recreate the triple point conditions we use a high
powered refrigeration unit and a triple point cell which
has been made in accordance with the ITS-90 specifications.
Using a triple point cell the triple point of water can
be reproduced with an accuracy of +/- 0.00001°C |
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