Non-contact temperature measurement
Basics of using infrared sensors
A guest contribution by Martinus Menne*
providers on the subject
Temperature is one of the physical variables that is probably measured most frequently in industry, as numerous applications show. The non-contact temperature measurement with infrared technology is of particular importance.
The potential areas of application for non-contact temperature measurements with infrared sensors are extremely diverse. Such scanners have a special position among optical sensors, as they convert the infrared radiation emitted by objects into an electrical signal. This signal is amplified and transformed into a linearized measured value proportional to the object temperature. The measured value can be output as a changing or analog signal.
No problems with high thermal radiation
Infrared sensors are used in areas where conventional sensors cannot be used, for example because their coupling distance is too small, they are disturbed by the thermal radiation of an object, or they are subject to excessive contamination due to environmental conditions. The most common task for non-contact infrared sensors is to determine the surface temperature of objects that are difficult to access or that are moving, which may also have a high level of thermal radiation.
Principle of infrared temperature measurement
Depending on its temperature, each body emits a certain amount of infrared radiation, the intensity of which adapts or changes as the temperature changes. The wavelength range of this thermal radiation used in infrared measurement technology is between 1 µm and 20 µm. The intensity of the thermal radiation emitted by an object or substance depends both on the temperature and on the radiation properties of the material to be examined. The emissivity ε is the corresponding material constant that describes the ability of a body or object to emit infrared energy. This material-dependent constant is known for most substances and can be between 0 and 100 percent. An ideal radiating body therefore has an emissivity of 1.0, while the emissivity of a poorly radiating body, such as a mirror, is 0.1.
Components of an infrared thermometer
As already mentioned, infrared thermometers are optoelectronic sensors that register the infrared radiation emitted by a body without contact and are therefore completely wear-free and calculate its surface temperature on this basis. An IR thermometer essentially has the following components:
- Electronics for amplification, linearization or signal processing
The properties of the optics have a significant influence on the beam path of an IR thermometer, which is characterized by the relationship between the distance to an object and the size of the measuring point. The function of optics as an important factor for non-contact temperature measurement will be discussed in more detail below.
The spectral filter is used to select the wavelength range that is relevant for the temperature measurement. The task of the detector together with the subsequent processing electronics is to convert the intensity of the emitted infrared radiation into electrical signals, for example to set a switching output on a sensor.
Compact devices with measuring ranges up to +1,030 °C
Non-contact temperature measurements, such as those offered by Ipf electronic, are available as compact units or as two-part systems. The former is essentially characterized by the fact that the entire evaluation electronics are located in the sensor. The various device versions differ in particular in the built-in optics and the maximum adjustable switching thresholds or measurement ranges from +300 °C to +750 °C. All units in stainless steel housing (IP67) have a diameter of 60 mm, a switching frequency of 10 Hz and are designed for ambient temperatures from -20 °C to +75 °C. An exception is the OI12C758, the smallest solution among the compact devices. This robust dwarf for the terminal connection has a measuring range from -40 °C to +1,030 °C and is designed for ambient temperatures up to +80 °C.
Higher measurement ranges through separate evaluation
In the two-part systems, the measuring head is separated from the evaluation electronics. In actual use, the electronics can be installed in a thermally less critical area, while the sensor or measuring head itself can withstand much higher ambient temperatures of up to +180 °C or +250 °C (OI98E239) compared to the compact units.
According to the company, the measuring heads in the two-part systems from Ipf electronic are among the smallest in the world with a high optical resolution of 22:1. The measuring ranges of the systems (from -40 °C to +975 °C) are scalable and can be configured using the buttons on the evaluation unit. An exception here is the OI98E240, whose measurement range extends from +385 °C to +1,600 °C.
The choice of optics is crucial
The maximum distance between the measuring head and an object to be measured depends on the size of the object and the optical resolution of the IR thermometer. As already mentioned, it is therefore important to choose the right optics for the measuring head. Because it not only determines the possible setting ranges for the switching threshold, but above all also the size of the measuring point via which the temperature on or at an object is determined.
Avoid measurement errors by using the correct measurement location
The detection area of standard focus optics (short: SF) forms a cone from the leading edge of the measuring head. Exceptions are optionally available, special attachment optics with so-called Close Focus (short: CF) for certain device models. These optics focus the beam path up to a certain distance so that even the smallest objects can be detected.
In general, however, one can say: The further away the SF optics on a measuring head is from the measuring area, the larger the area, i.e. the measuring point for which the sensor assesses the temperature. Thereby, it always determines the average temperature in relation to the area in question. To avoid measurement errors, the object should therefore completely fill the measurement point, which is defined by the focus of the optics and the respective distance to the object. This means that the sensor’s evaluation area must always be the same size as or smaller than the measurement object.
Important measured values for reliable processes
At the beginning of the article, it was also briefly mentioned that the measured value provided by an infrared sensor can be output as a switching or analog signal. Among the compact units, the OI12C758 has both a switching and an analog output (0 V…5 V / 0 V…10 V), while the two-part systems all have an additional analog output (0/4. ..20 mA , 0…5/10 V) integrate.
These outputs provide a temperature-proportional signal that can, for example, be used to monitor certain temperature ranges for specific processes, such as the further processing of forged components. Here, the measurements made by an infrared sensor on the glowing metal components are used, for example, to adjust the temperature in a furnace to reliably form the components in a die. Other potential areas of application for the two-part systems from Ipf electronic can be found, for example, in temperature measurements on smoothing or embossing calenders, on automatic casting machines, in laser welding or laser cutting or on wafers in semiconductor production.