The task
A customer wanted additional measuring points along the cylinder of an extruder in order to assess the actual temperature distribution. On a conventionally heated extruder, this is usually straightforward: drill a hole, install the sensor, done.
That was not possible here. The cylinder had a bimetallic construction and could not be drilled afterwards. The requirement was therefore a sensor that would remain securely attached to the outside of the cylinder and reliably measure its surface temperature.
The first attempt held. It just did not read correctly.
We started with a commercially available surface sensor attached to the cylinder by a strong magnet. Mechanically, the solution was immediately convincing: the sensor sat securely, installation took only a few seconds and the cylinder remained unchanged.
Then came the control measurement. We placed a mobile surface sensor directly beside the permanently mounted magnetic sensor at the same point. The magnetic sensor read approximately 21 °C lower.
A sensor that sits reliably but reliably reports the wrong temperature is of no help when assessing a process.
Where did the deviation come from?
In an induction-heated extruder, heat is not generated by external band heaters. An alternating electromagnetic field produces eddy currents and thus heat directly in the cylinder metal. Additional magnetic and metallic components are not neutral in this environment.
The entire heat path from the cylinder surface to the sensing element is also crucial. The thermocouple alone does not determine the measurement result.
Contact area and pressure
How completely, and with what pressure, does the sensor actually contact the surface?
Position of the sensing element
Does the sensing element measure the surface, or mainly the temperature of the housing?
Thermal mass
Additional mass stores and distributes heat and can influence the measuring point.
Heat dissipation
The housing and connection cable can conduct heat away from the actual measuring point.
Mounting materials
Magnetic, metallic and thermally conductive materials react differently to the field and heat flow.
It would be technically too simplistic to attribute the 21 °C solely to the magnet. The deviation resulted from the interaction of the magnetic mounting, induction field, thermal coupling and sensor design.
Why a stronger magnet would not have helped
The task was not to hold the sensor more firmly. It already held. It had to remain magnetically attached while measuring the actual surface temperature under the conditions of the induction field.
We therefore redesigned the complete measuring point: magnetic mounting, surface contact, heat transfer and sensing-element position. Every parameter was matched to the application.
The result: the 21 °C deviation had disappeared.
What the customer gained
The new sensor design measures the extruder surface temperature without drilling into or otherwise modifying the bimetallic cylinder. It can be fitted at the required positions and remains magnetically fixed in place.
Additional measuring points
Existing systems gain further measuring points without mechanical intervention.
Comparison along the cylinder
Temperature zones can be compared directly and unusual areas identified.
Verify the control system
The existing temperature control can be assessed using independent surface measurements.
Retrofit continuous measurement
The measuring point remains permanently in the desired position without a sensor bore.
What this case demonstrates
A standard sensor is not automatically suitable for every measuring task, even if it is mechanically secure. Sensor type, mounting and installation situation must be considered together. This is especially true with induction heating, restricted installation options and processes where a few degrees are critical.
A magnetic sensor that initially worked only mechanically thus became a measuring solution matched to the application.
Do you need additional measuring points but cannot drill? Send us details of the cylinder and material structure, temperature range, heating method and desired measuring position. We will assess the appropriate sensor design.
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