Facts about temperature


Brief temperature facts

Learn about different methods for measuring temperature, criteria that influence the choice of sensor, how to pair the right instrument and sensor and ensure the right sensor design for the measurement task.

Two main methods.

The measurement of temperature in or on different objects can be broadly divided into the following two methods:

  • Air, insertion and dip measurements
  • Surface temperature measurements

Air-entry and dip measurements.

A measurement in air may involve finding out the indoor temperature of a home. To measure the perceived temperature, the operational temperature is measured with a globe thermometer. This thermometer takes into account both the air temperature and the temperature of surrounding surfaces such as windows and walls.

An insertion measurement is a measurement when the object to be measured is penetrated, for example during food inspections. In these cases, an insertion thermometer is used.

Doppler measurements are made in liquids to check, for example, that the temperature is at least 50°C and thus avoid legionella.

Surface temperature measurements.

When it comes to the measurement of surface temperatures, there are many applications for which contact measurement is the best method of measurement. However, there are other areas where non-contact measurement is more appropriate, such as using an IR meter. A combination of both measurements in a single instrument is therefore preferable.

Choosing the right sensor.

The measurement task determines the choice of sensor type. The most appropriate temperature sensor shall be selected according to the following criteria:

  • Measurement area
  • Accuracy
  • Design
  • Response time
  • Resistance

Testo has a wide range of sensor elements and transducers, to deliver the sensors you need in your applications:

  • Thermistors (NTC)
  • Resistance sensors (PT 100)
  • Thermocouple

Thermistors (NTC)

Temperature measurement with thermistors is also based on a temperature-dependent change in the resistance of the sensor element. Unlike resistance thermometers, thermistors have a negative temperature coefficient (resistance decreases with increasing temperature). Properties and tolerances are not standardized.

Resistance sensors (Pt100)

When measuring with resistance sensors, the temperature-dependent resistance change in the “resistance” of platinum is utilized.

A constant current is applied to the measuring resistor. Measurement is then made of the voltage drop, which changes with the resistance value via temperature. The basic values and tolerances for resistance sensors are defined in IEC 751.


Temperature measurement with thermocouples is based on the thermoelectric effect. Thermocouples consist of two wires welded together. The wires are made of different metals or alloys. The basic thermoelectric voltage values and the maximum tolerances for thermocouples are defined in the IEC 584 standard. The most common thermocouple is NiCR-Ni (type – designation K).

rule of thumb

Thermocouple sensors are fast and have a wide measuring range. Resistance and NTC sensors are slower but more accurate. The wider the measurement range, the more general the applications.

Measurement area

First cross out the sensor type that does not match your measurement range. The diagram below shows the measurement ranges for different types of sensors.

Temperature measurement: measuring ranges different sensor types

Temperature measurement diagram different sensor types

Accuracy: sensor type.

Select the sensor type in the diagram or table that has the desired accuracy for the application.

Temperature accuracies table
Data for thermocouples according to EN 60584-1 (formerly IEC 584-1). Two values are displayed. A fixed value in °C and a formula.
The largest value always applies. Data for Pt100 in accordance with EN 60751 (formerly IEC 751). There is no standardization for NTC sensors.

Accuracy: thermocouple.

For thermocouples, accuracy class 1 applies in the measuring range -40 … +1000 °C.
In the range -200 … -40,1 °C, class 3 = ±2,5 °C or 0,015 ltl.

Temperature chart accuracy thermocouple

Choosing the right temperature sensors and instruments.

Highest accuracy

The testo 735-1 and 735-2 have a simple, menu-driven function with the highest accuracy. In addition to the fast and reliable thermocouple sensors, the following sensors can be connected: Pt100 sensors, which comply with EN 60751 (formerly IEC 751) or selected Pt100-based high precision sensors with 1/10 class B accuracy. When compared to standard precision sensors, with their high accuracy Pt100 sensors, these precision sensors are ten times more accurate. With a class B sensor that has an error tolerance of ±0.3 + 0.005 x I temperature I, this results in an error of only ±0.03 + 0.0005 x I temperature I.

Pt100/NTC – accuracy

Temperature accuracy Pt100 NTC

Which sensor type should be chosen for a particular instrument?

Now you can choose which instrument is suitable for your application, by selecting the appropriate sensor type, based on the measurement range and accuracy. Some of Testo’s instruments have other functions, besides displaying measurement values, to help you solve the measurement task. Select the features that are important to you and the corresponding instruments from the product pages.

Which sensor fits which instrument?

The right sensor design for the mission.

Response time: t99 = Time taken for the sensor to display 99% of the temperature change.

Temperature immersion sensorsDoppler donors
(TE type K/J/T, Pt100, NTC) for measurements in liquid, but also for measurements in gas and air.

Temperature insertion sensorInserting sensors
(TE type K/J/T, Pt100, NTC) for measurements in plastic or semi-solid materials. Also works for air, gas and liquid.

More information

  • The indicated response time* t99 is measured in moving liquid (water) at 60 °C.
  • In general, the narrower the sensor, the faster the response time, which means less time in the target.
  • The narrower a sensor is, the more care should be taken with it.
  • The sensor should be inserted into the object of measurement at least 10 x the diameter of the sensor for thermocouple sensors and 15 x the diameter for Pt100 and NTC type sensors for correct measurement results.
  • Thermocouple sensors can be manufactured with very small diameters (down to 0.25mm) and are therefore ideal for fast measurements and for measurements in or on small objects.
  • Only resistance sensors with a diameter >2 mm can be manufactured at low cost. They are usually more accurate than thermocouple sensors.

Sensor material
The probe tube of the thermocouple-type immersion sensors is made of Inconel (2.4816). Stainless steel V4A (1.4571) is used for the sensor tube in all other sensor types. Resistance to corrosive substances is usually sufficient due to the high quality materials used. Testo has sensors with glass protection tubes for use in highly corrosive substances.

Temperature air sensorAir sensors
(TE type K/J/T, Pt100, NTC) To facilitate quick measurements, the sensor is usually in an unprotected location.

  • The indicated response time* t99 is measured in a wind tunnel at 2 m/s and 60 °C.
  • Dip/insert sensors can be used for air measurements. The response time is 40 to 60 times longer than the stated value measured in water.

Temperature Surface temperature Proximity sensorsBerth providers
Sensor designs: TE type K/T/J, PT100, NTC. With flat measuring tip for measurements on smooth, flat surfaces. For optimal performance, we recommend silicone-based heat conduction paste (Tmax 260 °C).

Robust construction.
Higher sensor accuracy (Pt100 and NTC).

Longer response time.
Careful handling is required.

Only suitable for smooth surfaces and objects with high heat capacity, e.g. large metal objects.

Temperature Surface temperature Proximity sensorsProximity sensors – Resilient thermocouple bands

We recommend the patented measuring head with resilient thermocouple bands for quick measurements, even on uneven surfaces. The thermocouple bands measure the actual temperature of the target within a few seconds:

  • Easy to use (without silicone-based heat conduction paste).
  • Fast measurement results.

Further information

  • The indicated response times* t99 are measured on polished steel plates at 60 °C.
  • Specified accuracy is the sensor accuracy.
  • The accuracy of your application depends on the nature of the surface (roughness), the material of the measured object (heat capacity and thermal conductivity), and the accuracy of the sensor. If you want to know the deviations of your measuring system, you can get a calibration certificate issued by Nordtec. These calibrations use specially adapted equipment for accurate and traceable measurements.

Nordtec Instrument AB can issue accredited calibration certificates in the measuring range – 100…+1200 °C in its own calibration lab or at the customer’s site. Also for higher and lower temperatures via our main supplier Testo KG.

*Response times are indicated in the technical data for each product.

Temperature measuring instruments from Nordtec.

Measurement facts testo 925 surface temperature

Manual instrument

Testo has a wide range of portable hand instruments for temperature measurement. Ranging from small pocket instruments to more advanced instruments for highly accurate measurements with various calculation functions.

See the range

Measurement facts testo 174T life

Temperature loggers

Again, there are many different logs to choose from. From small compact ones to larger multi-channel ones with internal or external sensors. Intuitive software for programming and reading.

See the range

testo Saveris software

Remote monitoring

We have 4 monitoring systems in our program – all with their special features that are adapted for different applications in e.g.. indoor climate, construction, food, museums, etc.

See the range