The temperature in fluid, gas, powder or plastic is mostly measured with a penetration probe.
This is a thermocouple that consists normally of two wires out of different metals.
The temperature difference induces electrical voltage inside the metals (thermoelectric circuit) which can be calculated backwards to temperatures from the connected measuring device.
Dependent on the materials, the penetration probe can be used for different temperature ranges.
Penetration probes from type K (NiCr-Ni) can be used in a temperature range between -65 °C to 400 °C.

In the plastic industry, penetration probes are used to check the temperature of the melt that is injected directly out of the nozzle.

Penetration Probe in Plastic Industry

Measuring Technology: Measurement of the melt temperature using a penetration probe type K.

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.