Basics of thermometry

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Base de la thermométrie

Learn more about the basics of thermometry

Thermometers are designed to measure different types of physical characteristics, but the five most common are: bimetallic devices, liquid expansion devices, resistance temperature devices - RTDs and thermistors, thermocouples and infrared radiation devices. 
Experts in measuring give you all the secrets of these little technological gems!

Thermometer Technologies Explained



Have dial displays. The dial is connected to a coil spring in the center of the probe. The spring is made of two different types of metal which, when exposed to heat, expand in different but predictable ways. The heat expands the spring, pushing the needle onto the dial. Bimetallic thermometers are inexpensive and usually take a few minutes to reach temperature. Not to mention that their entire metal coil must be immersed in the material being measured to get an accurate reading.

Liquid thermometers


And bimetallics are mechanical thermometers that require no electricity to operate. Bimetallic thermometers lose calibration very easily and must be recalibrated weekly, or even daily, using a simple screw that rewinds the metal coil.

Electronic thermometers


RTDs, thermistors and thermocouples: measure the effects of heat on electronic current. Resistance devices, RTDs and thermistors, take advantage of the fact that electrical resistance responds to temperature changes in predictable patterns.

The relatively inexpensive thermistor and high precision RTD measures resistance in a resistor attached to an electronic circuit to measure temperature.

Thermistors typically use ceramic beads as resistors, while RTDs often use platinum or metal films.

With thermistors, resistance decreases with temperature and with RTDs, resistance increases.

Thermistors and RTDs can have a higher degree of accuracy than thermocouples, but their range is limited in comparison and they are generally not as fast.

Thermocouples work on the principle that when connected to two different metals over a distance with a temperature difference, an electronic circuit is generated

The generated circuit voltage changes with temperature variations in a predictable manner.

THE thermocouples common solder together nickel and chromium - Type K, copper and constantan - Type T or iron and constantan - Type J and place the solder on the probe tip of the thermometer.

Since thermocouples only generate voltage if there is a temperature difference along the circuit (and the temperature difference must be known to calculate a temperature reading), thermocouples either have a cold junction where part of the circuit is brought to the ice point (0°C/32°F) or electronic cold junction compensation which facilitates calculation. thermocouples can detect temperatures over wide ranges and are generally quite fast.

Infrared thermometers


A type of thermometry that measures the amount of infrared energy emitted by a substance and compares this value to a predictable curve to calculate the temperature.

Thermometry Concepts


Speed, or response time, is another important consideration when choosing a thermometer. Some thermometer technologies are faster than others and, depending on the application, extra seconds or fractions of a second can make all the difference.

Generally, electronic thermometers are faster than mechanical thermometers such as liquid mercury thermometers or dial thermometers. Thermocouple sensors are faster than resistance sensors like thermistor or RTD, and reduced tip probes are faster than standard diameter probes because the sensor is closer to the material being measured and the mass of the sensor is more small and therefore more responsive to temperature changes.
The actual response time of a thermometer varies depending on the particular substance and range of measured temperatures.


The quality of a thermometer depends on the temperatures it takes. The accuracy of the thermometer is therefore of the utmost importance. Slight increases or decreases in temperature can have profound effects on the growth of bacteria, the flexibility of plastics, the interaction of chemicals, a patient's health, and more, and electronic thermometers with digital displays make measuring easy of the temperature to the nearest tenth. degree or less.
Accuracy is generally expressed in ± a certain number of degrees or ± a certain percentage of the full reading.

The United Kingdom Accreditation Service (UKAS) allows calibrated thermometers and their temperatures to be traced against a national standard, giving the user a guarantee of accuracy.


The resolution of the thermometer refers to the smallest readable measuring increment from this one.
A thermometer that displays temperature to hundredths of a degree, for example 30.26°, has a greater resolution than a thermometer that only displays tenths of a degree, for example 30.2°, or whole degrees 100° .

Although resolution differs from precision, the two should be considered as going hand in hand. A thermometer accurate to ±0.05° would not be as useful if its resolution was only in tenths of a degree, for example 0.1°. Likewise, it could be misleading for a thermometer to display hundredths of a degree on its screen, if its traceable accuracy is only ±1°.

Temperature range

The range describes the upper and lower limits of the measuring scale of a thermometer. Different types of thermometers and sensors tend to perform better in different measurement ranges. Some specialize in extremely hot or very, very cold temperatures. Some have a wider range. Often, a thermometer will have different accuracy or resolution specifications in the center of its range and at its outer limits.

Specification tables require careful reading. The better you have an idea of ​​the temperature range you are most likely to measure, for example cooking temperatures between 149 and 204°C, the more easily you can select a technology that works best within that range.

Learn more about thermometer features

Thermometers may have many different features that make monitoring and recording temperatures easy ; Which ones you'll need generally depends on your application. Learn more about each feature to find the ones that work best for you.

Explanation of thermometer features

Maximum / Minimum


Logging maximum and minimum temperatures is a very useful feature, especially when trying to determine whether a target has been maintained within designated temperature limits over an extended period of time - such as logging data.

Thermometers with Max/Min functionality display the highest and lowest temperatures encountered. Some mechanical thermometers do this with physical markers that increase or decrease over time, but Max/Min is more common with electronic instruments. *Note that electronic instruments with Max/Min often do not have an Auto OFF function since powering off an instrument resets its Max/Min records.



Hold is a feature that allows you to freeze a displayed measurement (usually a digital reading) for later reference.



Differential Records - Diff, displays the product of subtracting the minimum temperature encountered from the maximum temperature encountered, showing the range of deviation over a period of time.



Average Temperature Records - Avg, simply averages all measurements encountered over a period of time.



High and Low Alarms – Hi/Lo, alerts you by flashing, beeping or even sending you an email or text message when a reading has gone above or below a certain preset temperature.

Automatic shutdown


Auto-off is a feature that turns off the instrument after a specified amount of time to protect battery life. Some units also offer the ability to disable and change the period of time the thermometer turns off. Use this feature for more extensive measurements.

Learn more about sensors

The sensor is the probe type. It exists three main types, and which one you choose generally depends on the type of accuracy, reliability, and temperature range you need.


RTD / Pt100


The sensor of a thermoelectric thermometer, consisting of electrically conductive circuit elements of two different thermoelectric characteristics connected at a junction.

Type K


A common thermocouple sensor combining two wires composed primarily of nickel and chromium and using voltage variation to calculate temperatures, known for its wide temperature range and affordability typical of industrial applications.

Accuracy Specifications

All probes/sensors thermocouple Type K are manufactured from Class 1 Type K thermocouple wire, as detailed in British Standard BS EN 60584-1:2013, and meet the following accuracy specifications:

±1.5°C between -40 and 375°C
±0.4% between 375 and 1000°C

High precision Type K thermocouple probes/sensors (indicated on relevant product pages with the “high precision” icon)
High precision ETI Type K probes are manufactured from Class 1 Type K thermocouple wire which is chosen for improved accuracy and performance and meets the following accuracy requirements specification:

±0.5°C between 0 and 100°C

Type T


A more specialized thermocouple sensor combining two wires made primarily of copper and constantan and using voltage variation to calculate temperatures known for greater accuracy and durability, typical of medical or pharmaceutical applications.

Accuracy Specifications

All Type T thermocouple probes/sensors are manufactured from Class 1 Type T thermocouple wire, as detailed in British Standard BS EN 60584-1:2013, and meet the following accuracy specifications:

±0.5°C between -40 and 125°C
±0.4% between 125 and 400 °C

Type J


A specialized thermocouple sensor combining two wires composed primarily of iron and constantan and using voltage variation to calculate temperatures – more limited in its range at higher temperatures but known for its sensitivity.

Acronym for Resistance Temperature Detection. RTD/PT100 probes consist of a flat film or wire wound platinum resistance sensor element. The measured value changes depending on the measured temperature.

These probes use the variation of resistance (usually platinum) to calculate temperatures known for their high accuracy over a wide temperature range and low drift, typical of high precision applications such as calibration.

Accuracy Specifications


PT100/RTD probes/sensors are manufactured from Class A 100 Ω (ohms) PT100/RTD detectors, as detailed in IEC 60751 (2008), and meet the following accuracy specifications:

±0.15°C ±0.2% between -200 and 600°C

A common thermal sensor that uses the predictable variation of resistance to an electric current with changes in temperature to calculate temperatures.

Accuracy Specifications


Thermistor probes/sensors NTC for all thermistor probes manufactured are as follows:

±0.4°C between -20 and 100°C
±0.3°C between -10 and 0°C
±0.2°C between 0 and 70°C
±0.4°C between 70 and 100°C

Learn more about Bluetooth features

The secure data transmission Temperature control is vital to the safety of food processing and food service operations.
This is what makes Bluetooth thermometers an ideal choice, we offer many solutions across our Bluetooth range. Our range offers professionals in the food industry speed, accuracy and reliability when it comes to keeping digital records of temperatures – an absolute must for businesses to operate safely and remain compliant.

Infrared basis

THE infrared thermometers are very fast, generally giving a reading in a fraction of a second, the time it takes for the thermometer's processor to complete its calculations. Their speed and relative ease of use have made infrared thermometers safety tools invaluable in the food service industry, manufacturing, HVAC, asphalt and concrete, laboratories and countless other industrial applications.

Infrared thermometers are ideal for taking surface temperature measurements remotely. They provide relatively accurate temperatures without ever having to touch the object you are measuring.

Infrared Technologies Explained

Mica lens


Mica lens thermometers such as the RayTemp 38 are the most commonly used type in industrial settings. They have more rigid mineral-based ground lenses.

This allows them to:

  • Take precise measurements at much higher temperatures, above 1000°C.
  • Be approximately half as sensitive to thermal shock effects caused by sudden changes in ambient temperature as Fresnel lens thermometers.
  • Be more accurate at greater distances – above a 20:1 distance. target ratios

Mica lens thermometers are often equipped with one or two lasers to help guide both the orientation of the thermometer and the estimation of the measured field of view. Mica lens thermometers, however, are the most fragile of infrared technologies. They often come with carrying cases because they are more likely to crack or break if dropped. They are generally the most expensive and still need to acclimate to extreme ambient temperatures for 10 minutes or more before giving accurate readings.

Fresnel lens


Fresnel lens thermometers, such as the RayTemp 8 , are the most commonly used type in the food industry.

Unlike the mica lens, the Fresnel thermometer lens is usually made of plastic, which offers several key advantages:

  • Less expensive than mica lens thermometers
  • More durable and more resistant to drops than mica lens thermometers
  • Can deliver narrow spot diameters at a greater distance than lensless thermometers
  • Generally more accurate at a distance of 6" to 12" than other technologies

Fresnel lens thermometers often come with laser guides to help direct your measurement. However, the plastic Fresnel lens has a narrower temperature range than the more versatile mica lens. It is also more sensitive to inaccuracies due to sudden changes in ambient temperature, called thermal shock, than other types of infrared thermometers.

If, for example, you carry your Fresnel lens thermometer from room temperature into a freezer to take measurements of frozen foods, the sudden drop in temperature can actually change the shape of the lens as the plastic contracts with the cold. Most Fresnel lens thermometers display error alerts when this happens and give erroneous readings until the lens has had a chance to acclimate to the new environment. Similar distortions occur in the upper temperature range within the specifications of a Fresnel lens thermometer.

The good news is that letting your Fresnel lens thermometer sit in the new ambient temperature for 20 minutes or more before taking your measurements can significantly reduce distortions due to thermal shock.

No lens


Lensless thermometers, such as IR pocket infrared thermometer , use a reflective funnel design to focus infrared energy on the thermopile rather than a lens.

Not having a goal at all has distinct advantages:

  • Usually less expensive
  • More sustainable
  • Generally smaller and easier to handle
  • More precise in cold spaces

Since there is no lens between the electromagnetic waves emitted by a surface and the thermopile of the thermometer, there are no significant contraction or expansion effects on lensless thermometers. In most units, an internal sensor compensates for the effect of ambient temperature on the electronic components themselves, so you can literally go from a hot room straight to a sub-zero freezer and start taking measurements without to wait for.

The important caveat about lensless thermometers is that their distance-to-target ratio or DTR is always 1:1 or less. This means that you should hold lensless thermometers as close to the target surface as possible when taking measurements. Thermometers without a lens are not as well suited for taking measurements from a distance.

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