ENGINEERS | How Temperature Sensors Work Inside Melt Pressure Transducers
- Bob
- Aug 30
- 2 min read
If you measure pressure only now, is there a benefit to ADDING temperature as well? Why do people do it?
How Temperature Sensors Work Inside Melt Pressure Transducers
Most melt pressure transducers measure temperature via an integrated thermocouple or RTD (resistance temperature detector).
Typical setup:
The pressure diaphragm contacts the melt.
Behind the diaphragm, a thermocouple tip or RTD element is embedded.
This measures local melt temperature at the sensor location, providing a simultaneous pressure and temperature reading.
Key characteristics:
Fast response to temperature changes (but slightly slower than a separate melt bolt thermocouple due to the pressure diaphragm’s thermal mass).
Provides direct temperature correlation to the exact pressure reading.
Accuracy typically ±1–2 °C, depending on thermocouple type (K-type is common).
Melt Bolt Thermocouples (Separate Temperature Sensors)
Installed independently of the pressure sensor in the extruder barrel or manifold.
Types: K, J, or N thermocouples, usually inserted into a melt bolt or probe.
Advantages:
Can be placed exactly at a critical melt location (mixing zone, die entrance).
Allows faster thermal response in high-flow melts.
Can be replaced without affecting the pressure sensor.
Disadvantages:
Temperature is not exactly at the pressure measurement point → may introduce minor discrepancy in melt viscosity/pressure calculations.
More ports and installation complexity.
Comparison Chart: Integrated Thermocouple vs Melt Bolt
Feature | Integrated in Pressure Transducer | Separate Melt Bolt Thermocouple |
Location | At diaphragm (pressure point) | Anywhere along barrel/manifold |
Temperature Accuracy | Directly correlated to pressure reading | Slightly offset from pressure measurement |
Response Time | Moderate (thermal mass of diaphragm) | Fast, especially in high flow |
Maintenance / Replacement | Must handle as one unit | Can replace independently |
Installation Complexity | Single sensor for P + T | Requires separate port(s) |
Cost | Moderate-high | Moderate, adds extra parts |
Practical Recommendations
Use integrated temperature sensors if:
You need temperature readings at the exact pressure point.
Space is limited (fewer ports).
You want simpler wiring and installation.
Use separate melt bolt thermocouples if:
You need high response to melt changes or want multiple temperature points.
You anticipate frequent sensor replacement or material changes.
You need to monitor temperature upstream or downstream of the pressure point for process optimization.
Hybrid approach:
Some facilities use both: integrated pressure + T transducer for control and additional melt bolt thermocouples for process profiling or troubleshooting.
Summary:
Integrated thermocouples give direct temperature at the pressure measurement point, simplifying control and calibration.
Melt bolt thermocouples give faster, more flexible readings at multiple points but may not match the exact pressure location.
Choice depends on precision needed, maintenance preference, and process monitoring goals.
If you have questions always feel free to contact NNS. We can help with Melt Bolt sensors (both fixed AND retractable) too.
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