How to Determine if a Sensor Needs Replacement Rather Than Calibration

I. Criteria for Physical Damage

Irreversible Probe Deformation Thermocouple probes are obviously bent, flattened or broken, and interior armor wires are structurally damaged , and normal contact cannot be restored by reinstallation.

Armor Corrosion/Wear-Through: Localized wear-through, high temperature oxidation and peeling of the sensor's armor metal layer which causes green corrosion and immediately exposes the magnesium oxide insulation layer to the high temperature environment resulting in permanent insulation failure.

Connectors seriously oxidized and abraded: The sensor tail connector pins are oxidized and blackened in wide regions, with arc erosion marks, and the pin contact resistance is more than 100Ω; repeated polishing and cleaning cannot restore stable signal connection.

II. Electrical Performance Requirements

Insulation Resistance Failure: Insulation resistance between sensor core wire and armor layer is <1MΩ at room temperature and <0.1MΩ at 320℃ by 500V digital megohmmeter. The insulation failure can not be repaired by calibration.

Complete interruption of signal: The temperature controller always shows the alarm of open circuit of the sensor. The multimeter, set to continuity, shows the thermocouple circuit is totally open and there is no millivolt level thermoelectric signal produced.

Severe deviation in thermoelectric characteristic: The sensor's temperature measurement deviates from the conventional temperature sensor by more than ±5℃ all the time. Even if the maximum compensation parameter input is reversed, the deviation cannot be restored to the ±1℃ tolerable range, surpassing the calibration compensation limit of the temperature controller.

III. Criteria for the Judgment of Dynamic Performance

The sensor temperature measurement response lagged the change of the heater current by more than 2 min when the hot runner was heated up at 50°C/h. The sensor end fully loses its fast reaction capabilities, and the PID control is in a state of constant overheating and uncontrollability.

Irregular signal fluctuation: After eliminating the possibility of grounding loop interference and loose wiring, the temperature measurement of the sensor fluctuates irregularly by more than ±5℃ within 10 minutes, and it is unable to produce steady temperature data at all. Calibration will not remove the signal irregularities.

Abnormal Drift Rate: When the daily temperature measurement is greater than 2°C for three consecutive days, it shows that the internal thermoelectric material of the sensor has been subjected to irreversible high-temperature aging. Calibration will shortly reintroduce significant discrepancies.

IV. Quick Judgment Methods in the Field

Visual Inspection: By visual inspection, check the physical condition of the sensor probe, armor layer and connectors for fast identification of visible damages such deformation, corrosion and ablation.

Insulation Resistance Testing (Segmented) 2.1.5 Use a 500V megohmmeter to test the insulation resistance between the sensor core wire and the armor layer to reliably pinpoint insulation failure locations.

Millivolt Signal Measurement: Put the sensor into a 200 °C standard constant temperature furnace, then use a high-precision millivoltmeter to measure the output thermoelectric potential. Compare the result with a K-type thermocouple standard calibration chart. Deviation above 1mV indicates failure of sensor.

Replacement Verification Method: Replace the sensor in the corresponding channel with a brand new sensor of the same model temporarily. If the data for the temperature measurement quickly stabilizes and qualifies following replacement, the original sensor must be replaced.