Testing the resistance value of a hot runner temperature sensor requires selecting the appropriate method based on the sensor type (e.g., PT100 or thermocouple). The core method is to use a multimeter to measure its resistance value in a power-off state and compare it with a standard value to determine its functionality.
Common temperature sensors in hot runner systems are platinum resistance thermometers (PT100) or thermocouples. PT100 is widely used due to its high accuracy and stability. Its resistance value changes with temperature; the nominal resistance is 100Ω at 0℃, and the resistance increases by approximately 0.385Ω for every 1℃ increase in temperature. Therefore, measuring its resistance value can provide a preliminary assessment of whether the sensor is functioning correctly.
I. Preparation before testing
Power-off operation: Turn off the power to the injection molding machine or temperature control box and unplug the sensor connector to avoid damaging the instrument or causing inaccurate readings due to live measurement.
Tool preparation: Use a digital multimeter, set to the resistance measurement range (Ω), and ensure the battery is fully charged and the probes have good contact.
Ambient Temperature Record
If conditions permit, record the current ambient temperature for comparison with the theoretical resistance value (refer to the table below).
II. Resistance Test Procedure for PT100 Sensor
Temperature (°C) Theoretical Resistance Value (Ω)
Measuring the Resistance Between Two Lines
Contact the two probes of the multimeter to the two signal lines of the PT100 (usually red and white or red and black) and read the resistance value.
Judgment Criteria
The measured value should be close to the theoretical value at the current temperature, with a deviation generally not exceeding ±1Ω (±0.5Ω for high precision requirements);
If the display shows "OL" or infinity, the sensor is open-circuited;
If the resistance value is close to 0Ω, it indicates an internal short circuit;
If the resistance value is stable but significantly deviates from the standard, it may be due to aging or drift.
Heating Verification (Optional) Gently heat the sensor probe with a hot air gun and observe whether the multimeter reading rises slowly. If the resistance increases with temperature, the sensor's dynamic response is normal.
III. Testing Instructions for Thermocouples
Thermocouples cannot be directly judged by resistance method because their working principle is based on the thermoelectric effect to generate millivolt-level voltage. However, the following methods can be used to assist in troubleshooting:
Measure the continuity of the measurement circuit: A normal thermocouple resistance should be several ohms to tens of ohms. If it is "OL", it is open circuit;
A more accurate method is to use the millivolt range to measure its output voltage and refer to the thermocouple calibration table to determine the temperature correspondence.
IV. Precautions
Avoid parallel interference: The connection between the sensor and the controller must be disconnected; otherwise, parallel circuits will cause measurement distortion.
Check the wiring terminals: Oxidized, loose, or contaminated terminals will affect contact resistance and cause misjudgment.
Distinguish pins for multi-point sensors: For sensors with compensation lines or dual outputs, refer to the wiring diagram to confirm the measurement terminals.
