This engineering brief demonstrates the operation of a non-contact industrial thermal sensor. The sensor is used to verify the presence of hot glue with a windowed pass/fail discrete output. A thermal camera is used to visualize the process and better understand the operation of the sensor. This is a unique learning opportunity for students as it teaches that temperature measurement has a time-dependent behavior not commonly found in other industrial processes.
Key Takeaways
- The sensor can detect the dispensed hot glue.
- The sensor must be taught using the traditional teach-in pushbutton or an external remote-teach control line.
- The teach-in process is complicated as the glue quickly cools. The thermal imager provides system visualization to support setup and calibration.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Sensors
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 29 Jun 2026
Non-contact thermal measurement may be used in industrial process control. The technique is demonstrated using a Banner M18TB14 temperature sensor along with a Klein TI 290 Thermal Imager as shown in Figure 1.
The temperature sensor is a natural fit for this application. Consider the specifications:
- The hot glue temperature is approximately in the mid-range of the sensor, which is specified as 0 °C to +300 °C (+32 °F to +572 °F).
- The sensor’s narrow field of view allows temperature measurement to be made at a distance. This nearly eliminates the possibility of hot glue fouling the sensor.
- The sensor has a fast response time allowing individual spots of glue to be sensed. With a 20 Hz switching speed, the paper may be moved thereby simulating the speed of a conveyor.
Figure 1: Image of the Banner thermal sensor and Klein thermal imager.
The temperature sensor is used to provide a go/no-go temperature assessment of the dispensed hot glue. Figure 2 shows the glue on paper along with the temperature sensor. Figure 3 provides a thermal image of the process.
Figure 2: Image of the Banner thermal sensor detecting hot glue dispensed on paper.
Figure 3: Thermal image of the hot glue on paper. Note that the glue has already cooled.
Challenges of Sensor Teaching
The sensor must be taught to identify the desired setpoint. The process is similar to the teach-in functionality used in many industrial sensors such as proximity and ultrasonic sensors. In simplistic terms, the sensor is presented the target condition and told to remember the measured analog condition via a button press. From that point forward, the sensor will continually look for the target condition by comparing the measured temperature to the desired setpoint. It will respond by activating its discrete output which is then monitored by a PLC.
The hot glue is an excellent example as it reveals the sensor teaching challenges. Specifically, it exposes the time dependency of the temperature measurement. From the moment the glue is dispensed, it begins to cool. The sensor must be taught within tens of seconds of the glue being dispensed. This fact is clearly exposed via the Klein thermal imager.
Distance-to-Spot Size Sensor Selection
The featured Banner sensor has a narrow distance-to-spot size (D:S) ratio. The concept is shown graphically in Figure 4. It’s helpful to momentarily view the thermal sensor as a light bulb with a focusing lens. In this application, the light would cast a spot of diameter S at a distance D. The featured sensor has a D:S ratio of 14, which yields a spot size of 7 mm at a distance of 100 mm. Likewise, the field of view increases to 71 mm at a distance of 1000 mm.
This is an important distinction as the field of view determines the directional sensitivity of the sensor. The narrow beam is highly directional, requiring careful alignment to see the desired target. This has several advantages:
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The sensor measures the object of interest as opposed to averaging temperature over a wide area.
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All things being equal, the distance between sensor and object may be increased when compared to a lower D:S ratio sensor. This may be advantageous in the hot glue example as the machine has more open space which generally aids in servicing the machine. Also, with greater distance, there is less risk of glue touching the sensor.
Figure 4: Distance-to-spot size illustration.
Parting Thoughts
This hot-glue detection exercise is a good lab for intermediate industrial control technicians and engineers. It is one of the few lessons that clearly reinforces the time dependence of measurement.
Temperature is much harder to teach than a stable physical object. Inductive proximity sensors don’t need to worry about the metal object gradually disappearing while it is being measured.
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About this Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.