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Choisir son thermomètre en fonction des caractéristiques

Choose your thermometer according to the characteristics

In 1714, scientist and inventor Daniel Gabriel Fahrenheit devised the first reliable thermometer, using mercury instead of a mixture of alcohol and water. For the very first time, a thermometer was created using mercury, whose coefficient of expansion is high, the production quality provides finer scale and reproducibility is larger. Ten years later, the mercury thermometer is adopted worldwide, and Daniel Gabriel Fahrenheit proposes a temperature scale that now (slightly adjusted) named after him.

Then, in 1742, it was the scholar Anders Celsius who, after years of research, submits a new scale for the mercury thermometer, whose boiling point is zero and the freezing point of water is 100 degrees. This scale, whose boiling and freezing points have been reversed, you know it because its use is common throughout the world: the degree Celsius.

Physician Herman Boerhaave was the first to apply mercury thermometer measurements in the clinical practice; his work initiated a correlation between different states of body temperature and a patient's symptoms.

Today there are many thermometers, ranging from infrared thermometers, gallium, to high precision thermometers etc. used to measure temperature over measurement ranges different and in different professions.

Characteristics of a thermometer #1 thermometric materials ⚗️

Whether you need a thermometer to measure the ambient temperature for domestic use or you are a chef and need a kitchen thermometer for your job , you will find a wide variety of empirical thermometer types based on material properties.

The latter are based on the constitutive relationship between pressure, volume and temperature of their thermometric material; for example, mercury expands when heated. If this pressure/volume/temperature relationship is used, a thermometric material must have three properties:

  1. Its heating and cooling must be rapid: First, when a certain amount of heat enters or leaves the material, the latter must expand or contract until it reaches either its volume is 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 at constant temperature, it is called latent heat of expansion at constant temperature; the rest is considered to modify the temperature of the body at constant volume, and is called specific heat at constant volume. Some materials do not have this property and take a long time to distribute the heat between the change in temperature and volume.
  2. Its heating and cooling must be reversible: the material must be able to be heated and cooled indefinitely (often by the same increment and decrement of heat) and always return to its pressure, volume and temperature original
  3. Its heating and cooling must be monotonous: over the entire temperature range for which it must operate, its pressure or volume are constant.

Unlike water, which does not have these properties and therefore cannot be used as a material for thermometers, gases have all these properties. Therefore, they are suitable thermometric materials. Their role is essential in the development of thermometry.

Characteristics of a thermometer #2 primary and secondary thermometers 🧪

A thermometer is called primary or secondary based on how the gross physical quantity it measures corresponds to a temperature.

Primary thermometers: the measured property of matter is so well known that temperature can be calculated without any unknown quantity. Examples of these are thermometers based on the equation of state 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; thermometers can be calibrated either by comparing them with other calibrated thermometers or by comparing them to known fixed points on the temperature scale. The best known of these fixed points are the melting and boiling points of pure water.

Characteristics of a thermometer #3 resolution, precision and reproducibility 🔬

The resolution of a thermometer responds to what fraction of a degree it is possible to take a reading. For high temperature work, it may be possible to measure only to within 10°C or more. Clinical thermometers and many electronic thermometers (baby forehead thermometer, non-contact thermometer, ear thermometer, infrared thermometer, etc…) are generally readable at 0, 1°C. Special instruments, such as probe-type tips, can give readings to the thousandth of a degree. However, this temperature display, whether digital via an LCD screen or not, does not mean that the reading is true or accurate; it only means that very small changes can be observed.

The accuracy of a calibrated thermometer is given at a known fixed point and accurate (i.e. it gives a true reading) at that point. Between fixed calibration points, the interpolation is performed generally linearly. This can give significant differences between the different types of thermometers at points far from the fixed points. For example, the expansion of mercury in a glass thermometer (as found for the axillary or rectal temperature measurement) is slightly different from the change of resistance of a platinum resistance thermometer, so these two will slightly disagree.

The reproducibility of a thermometer is particularly important: does the same thermometer give the same reading for the same temperature? Reproducible temperature measurement means comparisons are valid in scientific experiments and 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 inaccurate compared to the 'absolute scale.

An example of a reference thermometer used to verify others to industry standards would be a platinum resistance thermometer with a digital readout at 0.1°C (its accuracy) which has been calibrated at 5 points (−18, 0, 40, 70, 100 ° C) and whose accuracy is ± 0.2 ° C.

Properly calibrated, used and maintained liquid-in-glass thermometers can achieve a measurement uncertainty of ±0.01°C in the range of 0 to 100°C.

Choose your thermometer

There are a multitude of ways to 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 functionalities (multifunction, recorder, storage, waterproof, automatic shutdown, silent mode, etc.). To learn more about the thermometer, do your research directly in our guide or waste no more time and call an expert!

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