In 1714, the scientist and inventor Daniel Gabriel Fahrenheit imagined the first reliable thermometer, using mercury instead of a mixture of alcohol and water. For the very first time was created a thermometer using mercury, whose Dilation coefficient is high, the production quality provides a finer scale and the reproducibility is larger. Ten years later, The mercury thermometer is adopted worldwide, and Daniel Gabriel Fahrenheit offers a temperature scale that now (slightly adjusted) bears his name.
Then, in 1742, it was the scientist Anders Celsius which, after years of research, subjects a new scale for the mercury thermometer, of which The boiling point is zero And The water freezing point is 100 degrees. This scale, whose boiling and freezing points have been reversed, you know it because its use is common around the world: the Celsius degree.
Doctor Herman Boerhaave was the first to apply Mercury thermometer measurements in clinical practice; His work has initiated a correlation between the different states of body temperature and the symptoms of a patient.
Today there are many thermometers, ranging from the infrared thermometer, to gallium, passing by High precision thermometers, etc ... used for measure the temperature On different measurement beaches, in different trades.
Characteristics of a thermometer #1 Thermometric materials ⚗️
That you need a thermometer for Measure the room temperature as part of a domestic use or whether you are a chef and you need a Kitchen thermometer As part of your work, you will find a wide variety of empirical thermometers based on the properties of materials.
The latter are based on the constitutive relationship between pressure, volume and temperature their thermometric material; For example, mercury expands when heated. If this pressure/volume/temperature relationship is used, a thermometric material must have three properties:
- Its heating and cooling must be quick : First, when a certain amount of heat enters or leaves the material, the latter must expand or contract until reaching, either its volume or its final pressure. Then it must reach its final temperature practically without delay; Part of the incoming heat is considered to modify the volume of the body to constant temperature, it is called latent heat of expansion at constant temperature ; the rest is considered to be modifying body temperature at constant volume, and is called Specific heat with constant volume. Some materials do not have this property and take some time to distribute heat between the change in temperature and volume.
- Its heating and cooling must be reversible : The material must be able to be heated and cooled indefinitely (often by the same increment and decree of heat) and always return to its pressure, its original volume and temperature.
- Its heating and cooling must be monotonous : throughout the temperature range for which it must operate its pressure or its volume are constant.
Unlike water which does not have these properties and therefore cannot be used as a material for thermometers, Gas have all these properties. Therefore, these are Thermometric materials appropriate. Their role is essential in the development of thermometry.
Characteristics of a thermometer #2 The primary and secondary thermometers 🧪
A thermometer is called primary or secondary depending on how the gross physical quantity it measures corresponds to a temperature.
Primary thermometers: The measured property of the material is so well known that the temperature can be calculated without any unknown quantity. Examples of these are thermometers based on the state equation of a gas or on the speed of sound in a gas.
Secondary thermometers: Knowledge of the measured property is not sufficient to allow a direct calculation of the temperature. They must be calibrated; The thermometers can be calibrated either by comparing them with other calibrated thermometers, or by comparing them to fixed points known on the temperature scale. The best known of these fixed points are the points of fusion and boiling of pure water.
Characteristics of a thermometer #3 Resolution, precision and reproducibility 🔬
Resolution of a thermometer Responds to what degree fraction it is possible to read. For high temperature work, it may be possible to measure only at the near or more 10 ° C. Clinical thermometers and many electronic thermometers (Baby front thermometer, contactless thermometer, Heard, Healing, infrared thermometer, etc…) are generally readable at 0.1 ° C. Special instruments such as probes type tips, can give readings to the thousandth of degrees. However, this temperature display, whether digital thanks to an LCD screen or not, does not mean that reading is true or exact; It only means that very small changes can be observed.
The accuracy of a calibrated thermometer is given to a known and precise fixed point (that is to say that it gives a true reading) to this point. Between the fixed calibration points, the interpolation is generally carried out in a linear manner. This can give significant differences between the different types of thermometers to points distant from the fixed points. For example, the expansion of mercury in a glass thermometer (as found for Axillary or rectal temperature intake) is slightly different from the resistance change of a platinum resistance thermometer, so these two will be slightly disagreement.
Reproducibility of a thermometer Is particularly important: does the same thermometer give the same reading for the same temperature? A reproducible temperature measurement means that comparisons are valid in scientific experiences and that industrial processes are consistent. So if the same type of thermometer is calibrated in the same way, its readings will be valid even if they are slightly imprecise compared to the absolute scale.
An example of Reference thermometer Used to check the others according to industrial standards would be a platinum resistance thermometer with a digital display at 0.1 ° C (its precision) which was calibrated in 5 points (−18, 0, 40, 70, 100 ° C ) and whose accuracy is ± 0.2 ° C.
The correctly calibrated, used and maintained glass thermometers can reach a measurement uncertainty of ± 0.01 ° C in the range from 0 to 100 ° C.
Choose your thermometer
There are a multitude of ways of Choose the right thermometer ; depending on its characteristics Of course (thermometer with or without contact, laser thermometer, etc.), its use (whether you are an individual or a professional) or even its features (Multifunction, recorder, memorization, waterproof, automatic stop, silent mode, etc.). To find out more about the thermometer, do your research directly in our guide or not waste any more time and Call on an expert!
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