Instrumentation Temperature Measurement Thermometers Fahrenheit Celsius Kelvin

OddMix.com - Technology Note - 090730 - Karl Nagy

Fig 1 - Liquid thermometer [6 KB]
Fig. 1 - Liquid thermometer

Heat is the most abundant form of energy. During any energy conversion the unavoidable loss of the process is heat. Heat energy always flows from the direction of hotter to colder place or object. Heat is a very noticeable and subjective sensation. All of us are equipped with temperature sensors and at one time or another felt either "hot" or "cold". The sensation we are experiencing as hotness or coldness is called temperature.

Fig. 2 - Bymetal thermometer [8 KB]
Fig. 2 - Bymetal thermometer

Temperature is an important property of all matters around us in nature. All objects have some temperature. Most living, warm-blooded animals generate heat and have a closely regulated body temperature. In humans this body temperature is around 36 degree Celsius. All of us are familiar that changes in the normal body temperature usually signify the beginning of an illness.

In the last 50 years the wold population has doubled from three to over six thousand million (billion in the US) people. Each human body an average generate and loads the ambient with 100 Watts heat each hour. In addition, all human activity generates large amount of heat. Since the beginning of the Industrial Revolution this heat energy used to be provided by burning of fossil fuels. The remnants of the original coal and oil deposits are being burned faster than ever by inefficient overpriced gas-guzzler vehicles. Most power plants generate electricity by burning fossil fuels. The byproduct of an efficient power plant is usually and typically 60-70% heat. This waste heat is usually twice as much as the desirable output in electricity.

Until recent times, there was no easy way to accurately measure temperatures. With the current state of the art, thermometers are all around us, and taken for granted. They are in many forms, using different methods to measure temperature. The first thermometers were made in the seventeenth century. They were usually used some type of fluid for operation. A German physicist Gabriel Daniel Fahrenheit constructed the first mercury thermometer in 1714. The US is the only industrial country still using this way outdated Fahrenheit scale Fig. 1 and 2.

Fig. 3 - Digital thermometer [6 KB]
Fig 3 - Digital thermometer

Europeans and scientist everywhere (even in the USA) are working with Celsius degrees for over a hundred years. In 1742 the Swedish astronomer Andrew Celsius made his currently used and different temperature scale. In that practical - often referred to as Centigrade - system the freezing water marks the 0-degree point and the boiling point of water is located at 100 degree. The absolute temperature or Kelvin scale (named after William Thompson, later in 1848, Lord Kelvin), used by the scientific community is also based on the Celsius system. The Kelvin temperature is easily obtained by adding 273 to the Celsius temperature. By definition, normal room temperature is at 24 degree Celsius or 24+273=297 degree Kelvin.

The comfortable temperature range for us humans is quite narrow. Most homes, apartments and workplaces are at least heated and some are also cooled and kept within this comfort zone. A preset temperature within this comfort range is usually automatically regulated within a fraction of a degree by a simple temperature-measuring device called a thermostat. Thermostats sense the ambient air temperature within the house and if that falls below the preset value, the device outputs a signal to activate the heater to provide more heat. Modern thermometer with a solid-state sensing device is shown on Fig. 3.

The study of the flow of heat is the subject of the branch of physics called thermodynamics. This word is coined from the Latin words of flow and heat. Thermodynamics is most often used in energy conversion and power related computations. Interestingly, most student dread thermodynamics a lot more than the much harder college course in electromagnetic fields.

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