Surface temperature measurement

Discover everything you need to know about precise surface temperature measurement in our guide.

We have compiled for you what you need to look out for when selecting, purchasing and operating your surface probe.

• What is a surface temperature measurement?
• How does a surface measurement work and what problems can occur?
• Mass ratio - measured object and probe
• Thermal conductivity
• Surface condition
• What measuring methods are available for surface measurement
• Non-contact measurement (e.g. infrared)ontaktlose Messung (z.B. Infrarot)
• Contact surface measurement
• Optimization of the response time
• What are the different types of surface measurement?
• Difference between PT elements and thermocouples such as the cross band probe

What is a surface temperature measurement?

Surface temperature measurement refers to the measurement of the temperature on the outer surface of an object or material. The measurement can be carried out using different temperature measurement techniques, e.g. contact measurements using PT sensors and thermocouples or non-contact measurements using infrared thermometers. The measured surface temperature provides an indication of the thermal state of an object or medium, which is relevant in numerous applications in industry, construction, medicine and research. Precise surface temperature measurement can be crucial for optimizing processes and ensuring product quality.

How does surface measurement work and what problems can occur?

The characteristics of the object to be measured play a decisive role in the temperature measurement of solid bodies. This section therefore begins by explaining the effects of these characteristics.

Mass ratio - measuring object and probe

To begin with, it is important that the object to be measured has sufficient mass to contain enough energy compared to the specified heat capacity of the thermometer. Otherwise, the thermometer could absorb so much heat from the object that it cools down noticeably, resulting in a temperature measurement that is systematically too low. It is not only the total heat capacity of the object that is important, but in particular the heat capacity of the volume surrounding the measuring point on the surface of the object must be sufficiently large.

In short, the object to be measured should be as large as possible in relation to the temperature probe. Or vice versa, the temperature probe used to measure the surface temperature should have the smallest possible thermal mass.

Thermal conductivity

When measuring the temperature of solid bodies with contact thermometers, a certain minimum thermal conductivity of the object is required. When a contact thermometer is placed on the surface of such a body, heat is extracted from it, which initially leads to cooling of the object at the point of contact. If the object's thermal conductivity is too low, the heat extracted by the thermometer cannot be adequately replenished. As a result, the internal temperature field of the object is strongly disturbed by the thermometer and large temperature gradients occur around the contact point. This systematic error means that the measured result is always too low, especially with poor heat conductors such as glass, porcelain, ceramics, plastics or paper. Particularly when determining the temperature of electronic components in plastic or ceramic housings, significant errors can occur, especially due to the low mass and thermal conductivity. It is therefore particularly important to use very small and low-mass thermometers with low thermal conductivity for such applications.

The contact surface of the temperature probe should be as large as possible.

Surface properties

Determining the temperature on rough surfaces is a challenge. For example, two apparently "flat metal surfaces" (polished) only actually touch each other directly on around 1 % of their total surface area. Most of the heat exchange must therefore take place via these few points of contact, while between them there is only heat exchange by radiation and/or the medium in between. For this reason, the heat transfer resistances between rough surfaces are often several orders of magnitude greater than with polished surfaces. This condition can be improved by the use of thermal conduction pastes.

These 3 factors have an effect on the response time and measurement accuracy.

What measuring methods are available for surface measurement?

Non-contact measurement (e.g. infrared)

In non-contact measurements, the surface of the object is analyzed using sensors such as infrared thermometers or thermographic cameras. These instruments record the infrared radiation emitted by the surface and convert it into temperature data. Based on the intensity of the radiation and other parameters, conclusions can be drawn about the temperature.

Non-contact surface measurement offers some clear advantages but also disadvantages in relation to contact measurement:

The advantages include, the very fast measurement. With non-contact measurement, no additional heat is drawn from the measurement object because no contact probe needs to be heated by the measurement object. Only the heat radiation of a surface is recorded, and it is not necessary to touch the surface.

The biggest disadvantage, however, is that not all surfaces are suitable for infrared measurement. With reflective surfaces, such as metals, the measurement usually works poorly or not at all. Reflective surfaces reflect the infrared radiation and accurate measurement is then usually no longer possible. The influence of sources of interference can also often not be ruled out. The result is measurement value deviations or strong fluctuations in the measurement result; for the end user, these incorrect measurements are often difficult to distinguish from regular measurement results.

Contact surface measurement

With contact-based measurements, a sensor is placed directly on the surface of the object to be measured. The sensor then records the temperature directly by touching the surface to be measured. Different measuring methods can be used for the measurement, such as the resistance of platinum (for PT sensors) or the thermoelectric voltage (for thermocouples).

Contact surface measurement also has advantages and disadvantages.

The most significant disadvantage is that the probe must absorb the thermal energy of the measured object, the measured object must have a certain mass and the heat transfer takes some time.

The advantages are that reflective surfaces can also be measured. Contact surface measurement is not as susceptible to interference as non-contact infrared measurement.

The key to successful measurement with contact temperature probes lies in selecting the right temperature probe. It should be noted that thermocouples are usually significantly faster than PT sensors, for example. PT sensors usually have to be installed in a protective tube, which means that the heat transfer from the measured object must first be transferred to the protective tube and from there to the sensor itself, usually by means of a heat conducting pad. This long chain leads to a loss of response speed and often means that the final temperature is no longer reached. Thermocouples can be constructed in such a way that they rest directly on the surface and do not require a sensor tube. This makes them significantly faster and also significantly better at reaching the final temperature.

In surface measurement, the thermocouple is therefore usually ahead in terms of response time and reaching the final temperature and, taking all influencing factors into account, can still provide the more accurate measurement result despite a higher basic error. In surface measurement, therefore, more factors than the specified basic tolerance of the measuring element must be considered.

In the vast majority of applications, thermocouple-based cross band probes are clearly the best choice for contact surface measurement. You can find our cross band probes here.

Optimizing the response time

It is important to be aware of the above-mentioned influences on surface measurement; these physical conditions cannot be overridden. In most cases, however, the measurement can be optimized with knowledge of the influencing factors and a measurement result can be significantly improved with the right choice of sensor.

The first step is to find the right mass ratio between the probe and the object to be measured. In most cases, the measurement object is already fixed, so the probe must have a low thermal mass ratio.

In the second step, the correct measuring point must be found; the largest possible, flat contact surface is particularly advantageous.

You can find out more about the response time here.

What different types of surface measurement are there?

We offer different temperature sensors and thermocouples for surface measurement:

Cross band probes

• Cross band probe with probe shaft
• Cross band probe with screw-in thread
• Cross band probe with ring magnet
• Cross band probe with counter nut

Contact probes

Our contact probe bar magnets are available with silicone cable, PFA cable and glass fibre cable.

Our contact probes with horseshoe magnet are available with PVC cable, silicone cable, PFA cable and glass fibre cable.

Our contact probes with screw connection are available with PVC cable, silicone cable, PFA cable and glass fibre cable.

Our angle contact probes are available with PVC cable, silicone cable, PFA cable and glass fibre cable.

Our contact probes with block are available with PVC cable, silicone cable, PFA cable and glass fibre cable.

Our contact probes with cable lug are available with PVC cable, silicone cable, PFA cable and glass fibre cable.

Our angle contact probes are available with silicone cable, PFA cable and glass fibre cable.

Our stepped contact probes are available with PVC cable, silicone cable, PFA cable and glass fibre cable.

Our self-adhesive contact probe is available with PVC cable.

We offer our pipe contact probes with housing, PVC cable, silicone cable, PFA cable and glass fibre cable.

What is the difference between PT elements and thermocouples such as the cross band probe?

In fact, PT elements normally offer higher accuracy than thermocouples. However, the mass of the probe plays a significant role in surface measurement. PT elements usually have to be installed in a protective housing and therefore tend to have more mass, which leads to a significantly slower response to temperature changes. Due to the high heat dissipation caused by the higher mass, the final temperature is usually no longer reached. For this reason, thermocouples with their lower mass are usually far superior for surface measurement. cross band probes specially developed for surface measurement provide the best results for contact surface measurement.