Feb 032013
 

How are thermometers invented?


Like most inventions, the thermometer wasn’t invented in a day and by just one person. It is assumed that the first attempts to connect the warming of a body with a fluid, which increases its volume in a cylindrical container, was made by the ancient Greek scientist Heron of Alexandria, who lived in the second-third century AD. Or at least we have this information today. His thermoscope, improved by Galilei in 1597, was not meant to measure heat, but to increase the volume of water during heating. The thermoscope represented a small glass ball with a small glass tube, welded to it. Upon heating of the glass globe and placing the end of the tube in a water container, after some time the air in the hollow glass ball begins to cool and the tube draws in part of the water in which it is immersed and, conversely, if we re-heat the glass ball, the air within will expand and it will push the water in the glass tube out. It is obvious that this thermoscope cannot measure temperature because it does not have a graduated scale. It only gives information about the degree of heating of the body, but even this information is conditional because the water level in the tube will depend not only on the temperature but also on the atmospheric pressure at the time of the experiment. About a century later the device was improved by Florentine scientists who draw the air from the flask and sealed it by a water tank at its lower end. Moreover, they lined a value scale on the tube. As a result, the temperature could be measured not only qualitatively but also quantitatively. Later they substituted the water with a little alcohol and removed the tank. The principle of this device, primitive by today’s perspective, was based on the body’s expansion. The constant values in both ends of the scale represented the temperatures in the hottest summer day and the coldest winter day. Later Lord Bacon, Robert Fludd, Cornelis Drebbel and others continued working on the improvement of this device. Without telling the whole story of the improvements of the thermometer over the years, we will only note that in order to make comparison between different thermometers, two main variables in thermometer scales appeared. One is the absolute zero, and the other – the freezing point of water. The scale of Celsius degrees (Anders Celsius – a Swedish astronomer, geologist and meteorologist 1730-1744) is offset by 273.15 degrees in relation to Kelvin (William Thomson or Lord Kelvin – an English physicist and engineer 1824-1907), where 1°C = 1°K. In fact, there is a deliberate mistake here, as Kelvin degrees are not indicated by a small zero, but simply by „K“ and in the international SI system there are only Kelvin degrees or just kelvins because the kelvin is not even referred to as a degree.

 

The point in the scale corresponding to -273.15°C is zero Kelvin (0 K). This is the absolute zero in physics, in which the movement of molecules reaches its minimum. And 0°C is the temperature at which water freezes under normal conditions. However, if you go to the United States you will encounter a temperature scale, measuring degrees by Fahrenheit, named after the German physicist Daniel Gabriel Fahrenheit, 1686-1736. This scale is full of conditions and is hardly used except in the US and Belize. 1°C = 1 K = 1.8°F.

The zero by Fahrenheit is offset by 32°F up on the Celsius scale, i.e. 0°F = 1.8°C + 32 and the boiling point of water is at 212°F. There is 180°F between the boiling point and the freezing point of water. If you are wondering why, the explanation is very subjective – 0°F corresponds to the lowest temperature measured during the winter of 1708/1709, and 100°F is chosen by Fahrenheit to be the body temperature of a horse (!), which is about 37.8°C.
Now, I guess, it becomes clear why Fahrenheit scale has not achieved much popularity and application.
Types of thermometers

Liquid thermometers
Liquid thermometers are based on the principle of changes in the volume of the liquid, which fills the thermometer. This is usually alcohol or mercury. Mercury is a highly toxic liquid metal, whose vapors cause damage to respiratory airways and cell membranes. The European Commission prohibited the manufacture and sale of mercury thermometers in all EU countries since the beginning of 2009, which started the production of galinstan thermometers. Galinstan is a liquid alloy of gallium, indium and tin.

Visually galinstan thermometers look the same as mercury thermometers, but galinstan is nontoxic. However, the cost of a galinstan thermometer is about 10 times the cost of a mercury thermometer.
termometer

Mechanical thermometers
Mechanical thermometers work with rolled metallic or bimetallic plates.
termometer1

Digital thermometers

The active part of digital thermometers is a conductor, whose resistance changes with the change of the ambient temperature. A large part of these thermometers have thermocouples of two metals with different conductivity, depending on the temperature, but the most accurate and unchanging over time are resistance thermometers, which contain platinum wire or platinum with a ceramic base.

The most commonly used alloy is the so called platinum 100 – PT100, which at 0°C has resistance of 100Ω or platinum 1000, which at 0°C has resistance of 1000Ω. The dependence of the resistance on the temperature is almost linear in the temperature range of -200 to +850°C, and this quality is used for graduating the thermometers.

Optical thermometers
They are based on the change of the properties of optical fibers when the temperature changes. Generally they change their degree of light transmission as well as their range.

Infrared thermometers
In order to measure the temperature, the tactile part of an infrared thermometer doesn’t have to be in contact with the body, whose temperature it measures. These thermometers are more and more used in industry, medicine, and recently in households.

Gas thermometers
They are devices for measuring the temperature, whose operating principle is subject to the Charles’s law. In 1703 Charles found that at uniform heating of any gas there is uniform increase in pressure, if the volume of the gas is maintained constant. Moreover, the dependence of the pressure on the temperature is linear, so it can be assumed that pressure is a quantitative indicator of temperature. In this way, when connecting a gauge to an air-tight container, filled with gas, the gauge can be graduated for measuring temperature, instead of pressure. The gas thermometers containing hydrogen or helium are the most precise ones.
It is also possible to classify thermometers according to their purpose of use but this is not the subject of this article.

 

Home weather-stations

By definition, weather-stations represent a combination of various devices for measuring weather parameters. The main devices of this type are:
thermometer – for measuring the air temperature;
barometer – for measuring the atmospheric pressure;
hygrometer – for measuring humidity;
anemometer – for measuring the speed and direction of wind;
rain gauge – for measuring rainfall, etc.
meteostancia

The building, where meteorological observations are conducted is also called weather-station.
In this article, however, we will have a look at the so called home weather-stations, which are used in households.

Ever since the 19th century, in public buildings and ships’ dining rooms there were not only clocks and thermometers but also massive barometers placed in solid wooden hulls. As electronics was slowly entering households, compact digital weather-stations began to appear on the market. Through one or more sensors they indicate the inside and outside temperature, the humidity and the atmospheric pressure, and analyzing the data through a small processor, they make a weather forecast for one day ahead. These weather-stations operate by replaceable batteries or are plugged into the mains.

meteo
Often this small digital device, except for weather sensors is equipped with a digital alarm clock with a calendar, displays the moon phases, etc. These multifunctional devices provide multiple data on one display, thus facilitating the daily lives of millions of people, and are also quite affordable.

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