Response time temperature probe

Here you will find all the important information about the response time of temperature probes.

What is the response time of temperature probes?
What factors influence the response time of temperature probes?
How is the response time specified?
What are the different types of temperature measurement?
How do we measure response times?
What does a response time curve look like?

What is the response time of temperature probes?

The response time tx is the time it takes for a temperature probe to react to a change in the external, applied temperature and to reach a certain percentage value x of this temperature. x = 100% would mean that the final temperature, i.e. the external, applied temperature, has been reached exactly.

To determine the exact response time of a temperature probe, the temperature probe is abruptly exposed to the temperature T1 at a temperature T2 and the response time tx is determined, which indicates how long it takes for the sensor inside the temperature probe to "see", i.e. reach, x% of the medium temperature T2.

What factors influence the response time of temperature probes?

The response time of a temperature probe is essentially dependent on the thermal mass (heat capacity) of the temperature probe, its (internal) construction and, crucially, the immersion depth. The medium in which the temperature is to be determined acts as an energy transmitter; the higher the specific heat capacity of the medium, the faster energy is transmitted to the temperature probe in the form of heat.

Brief explanation: In physical terms, thermal mass is the heat capacity of the temperature probe. It indicates the amount of heat that the entire temperature probe immersed in the medium can absorb for a given temperature change. The greater the heat capacity of a component, or in this case of a temperature probe, the more energy is required to heat the component or the probe.

The internal structure also has a decisive influence on the response time: an internal structure with good thermal conductivity ensures rapid temperature equalisation between the external medium temperature and the sensor element inside the temperature probe and thus a good, i.e. short, response time overall. An internal structure with poor thermal conductivity, e.g. due to air pockets inside the probe, increases the response time of the probe considerably.

The immersion depth is the third important parameter that significantly influences the response time. A sufficient immersion depth is necessary to keep the heat dissipation "to the rear", i.e. to the surroundings, negligibly small. Otherwise, the heat dissipation will delay the response time considerably and even prevent the final temperature from being reached - the probe would be wrongly classified as "measuring inaccurately". An insufficient immersion depth is the most frequent cause of excessively long response times; the undesirably long response times are then often spontaneously attributed to the probe itself instead of correctly correcting the immersion depth, e.g. by adjusting the installation of the probe in the medium to be measured.

The applied temperature jump delta T = T2 - T1, i.e. the temperature change from the initial temperature T1 to the final or medium temperature T2, does not influence the response time tx, contrary to frequently held assumptions, as can be proven both theoretically and in (carefully constructed and conducted) experiments. However, secondary effects (unwanted or unrecognised heat dissipation to the outside) ensure that in practice the impression is created that the delta T is an influencing variable.

The probe assembly:

In order to achieve the shortest possible response time, any temperature change must be transferred as energy transfer as quickly as possible, i.e. via materials that conduct heat well, to the sensor element. By sensor element is meant any type of temperature-dependent resistor. The sensor element, or simply sensor for short, is therefore the heart of a temperature probe. The temperature probe "packages" the sensor in a sleeve inside which the sensor is connected to an electrical cable. By means of this electrical cable, the resistance can be read out at any time and thus, conversely, the temperature of the medium can be deduced from the measured resistance value. A temperature probe can only measure its own temperature, or more precisely, its sensor temperature; it must therefore be ensured that the sensor element is thermally coupled as well as possible to the medium to be measured.

The lower the thermal mass of the probe, the faster a temperature change can heat up the temperature probe and penetrate into the sensor element. The thermal mass is determined by the specific heat capacities of the materials used for the probe and their absolute masses. The specific heat capacity is a purely material-dependent measurand; the absolute mass is essentially determined by the diameter of the protection sleeve, its length and wall thickness. This means that comparatively long stainless steel sleeves with a larger diameter lead to significantly longer response times compared to short copper sleeves with a smaller diameter, all other things being equal.

But the internal design of the probe also has a significant influence on the response time. The actual temperature sensor is enclosed in a protective tube (i.e. the sleeve), also to protect this sensitive component. The better the sensor is thermally coupled to the protective tube, the faster a temperature change can be "sensed" by the sensor, i.e. detected.

The short response time that is usually required is counteracted by the fact that heat can never be completely prevented from escaping via process fittings or the cable, for example. Here, too, the greatest possible immersion depth is an effective means of minimising the influence of heat dissipation on the response time.

As a general rule, a temperature probe whose sensor is particularly well thermally coupled to the medium to be measured, but at the same time is very well thermally insulated "to the rear" from the environment, leads to the shortest response times.

The medium to be measured

The response times vary with the medium to be measured, for example, air or water have very different thermal conductivity properties due to their very different densities. Due to the higher thermal conductivity of water, the response times in water are significantly shorter than those in air, with the same probe assembly. In order to compare different types of sensors with regard to their response times, it must be ensured that the measurements are carried out in exactly the same medium.

Installation and immersion depth

The correct installation or insertion of the temperature probe into the medium to be measured plays an essential role in obtaining a fast response time.

It is important to ensure good thermal coupling to the medium to be measured. With temperature probes installed directly, i.e. in contact with the medium, significantly shorter response times can be achieved than with probes installed in an immersion sleeve. With the latter, air gaps, which have a thermally insulating effect, can hardly be prevented. In any case, any interface between materials, for example between the medium and the immersion sleeve and between the immersion sleeve and the temperature probe, acts as a heat transfer brake and thus leads to longer response times.

The immersion depth of the probe generated by the installation is also of great importance for the response time and even for the achievable accuracy of a probe. Ensuring sufficient immersion depth ensures optimum heat transfer from the medium to the sensor element inside the probe and negligible heat dissipation "to the rear" via the sleeve or cable. In addition to the required short response time, the required accuracy in the measurement can also be achieved in this way. In short: the greater the immersion depth of the probe, the smaller the effect of the unavoidable heat dissipation on the response time of the probe.

In general, the deeper the probe can be immersed, the better. A rule of thumb for the minimum immersion depth is that it should be at least 15 times the diameter of the sensor sleeve. However, this rule of thumb is only a rough guide. There are optimised probes that provide excellent response times at much shallower immersion depths.

A more or less direct coupling of the probe to the measured medium also significantly influences the response time: For surface measurements on pipes, where the temperature probe is only on the outside of the pipe, but the water temperature in the pipe is to be determined, the response time is considerably longer than when the temperature probe is installed directly in the pipe and thus has direct contact with the medium (water).

How is the response time specified?

The response time is given as standard in t50, t63, t90 and t99.

The number after the t indicates the percentage of temperature change reached up to that point.

T50 = Time until the probe reaches 50% of the temperature change.

T63 = Time until the probe reaches 63 % of the temperature change.

T90 = Time until the probe reaches 90 % of the temperature change.

T99 = Time until the probe reaches 99 % of the temperature change.

What are the different types of temperature measurement?

Surface temperature measurement

Here the temperature of the outer layer of a solid material is measured.

Suitable types: Contact probe, surface probe, cross band probe.

Fastest type: cross band probe (t63: < 0.8 s t99: < 3 s)

Air temperature measurement

Here the temperature of the air is measured:

Suitable types: Air temperature probe

Liquid temperature measurement

Here the temperature of liquids in basins or pipes is measured.

Suitable types: Cable probe or Screw-in probe

Fastest type: cable probe or cable thermocouple with smallest possible diameter.

How do we measure response times?

We measure the response times in different media in our laboratory under defined conditions. This gives us reproducible results.

Example of a response time measurement for cable and screw-in sensors in water:

From room temperature (approx. 25 °C) to 60 °C at a flow velocity of 0.2 m/s.