Choose your thermometer according to the characteristics

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Choisir son thermomètre en fonction des caractéristiques

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, a thermometer was created using mercury, whose coefficient of expansion is high, the production quality provides a finer scale and the reproducibility is bigger. Ten years later, mercury thermometer is adopted worldwide, and Daniel Gabriel Fahrenheit proposes a temperature scale which now (slightly adjusted) bears his name.

Then, in 1742, it was the scientist Anders Celsius who, after years of research, submits a new scale for the mercury thermometer, of which the 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 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 the infrared thermometer, to the gallium, through the high precision thermometers, etc… used for measure the temperature over different measurement ranges and in different trades.

Characteristics of a thermometer #1 thermometric materials ⚗️

Whether you need a thermometer to measure ambient temperature as part of a home use or whether you are a chef and need a kitchen thermometer as part of your job, you will find a wide variety of types of empirical thermometers based on material properties.

These 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:

  1. Its heating and cooling must be fast : First, when a certain amount of heat enters or leaves the material, the latter must expand or contract until it reaches 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 change 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 still return to its original pressure, volume and temperature.
  3. Its heating and cooling should 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, these are thermometric materials appropriate. 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 depending on how the gross physical quantity it measures corresponds to a temperature.

Primary thermometers: the measured property of matter is so well known that the 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 answers 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 (forehead thermometer for babies, non-contact thermometer, ear, 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 and accurate fixed point (i.e. it gives a true reading) at that point. Between fixed calibration points, 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 axillary or rectal temperature measurement) is slightly different from the resistance change of a platinum resistance thermometer, so these two will be slightly at odds.

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. Thus 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 reference thermometer used to check others to industry standards would be a platinum resistance thermometer with a digital readout to 0.1°C (its accuracy) that 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 ; in terms of his 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 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!