I. Observe the instrument display to initially determine the type of fault.
By monitoring the system or observing abnormal indications on the display instrument, the problem area can be quickly identified.
Displaying "infinity" or "open circuit" alarm:
This may indicate a broken platinum resistance wire, a broken lead wire, or loose wiring.
Key checks: Check if the terminal blocks are loose, if the screws are stripped, and if the internal lead wires are broken.
Displaying "bad point" or a fixed temperature value:
This may indicate component damage or signal interruption.
Key checks: Check if the sensor is responding, and if the reading changes with changes in the medium temperature.
Frequent reading fluctuations and irregular variations:
This may indicate electromagnetic interference or poor contact.
Key checks: Check if the shielding ground is reliable and if the screws are loose.
Continuously low measured value or slow response:
This may indicate poor thermal contact or abnormal lead wire resistance.
Key checks: Check if the mounting surface is flat, whether thermal conductive grease is used, and whether the wiring is oxidized.
II. Measure the resistance value with a multimeter to verify the component status.
This is the most direct and effective method and can be completed quickly on-site.
Disconnect the sensor from the instrument to avoid circuit interference.
Use a multimeter in resistance mode (200Ω range) to measure the resistance between the two lead wires.
Judgment criteria:
In a 0℃ environment, the Pt100 resistance should be approximately 100Ω.
For every 1℃ increase in temperature, the resistance increases by approximately 0.385Ω (i.e., every 1Ω increase corresponds to approximately 2.6℃ temperature increase).
If the measured resistance is 0Ω, it may be a short circuit; if it is infinite (OL), it is an open circuit, which basically indicates damage.
Example: At room temperature of 25℃, the normal Pt100 resistance is approximately 100 + 25×0.385 ≈ 109.6Ω. If the actual measurement is far below or above this value, further investigation is needed.
III. Check the wiring and contact status to rule out external factors.
Many "faults" are actually caused by improper installation or maintenance, which should be checked first. I. Check for loose screws:
Use a torque screwdriver to check the tightness of the screws (e.g., 1.2~1.5 N·m for M4 screws) to prevent poor contact due to vibration.
II. Check for oxidation of the terminals:
Remove the terminal cover and observe whether the copper wires and crimping surfaces have green rust or black oxide layers. If so, polish them with fine sandpaper until they have a metallic luster and apply conductive paste to prevent oxidation.
III. Check for intact sealing:
Check if the O-ring is aged or deformed, and if the protective cover is tightened to prevent moisture intrusion leading to reduced insulation.
IV. Test insulation performance to rule out moisture or short-circuit risks
Moisture is a significant cause of platinum resistance failure.
Use a megohmmeter to test the insulation resistance:
Measure the insulation resistance between the lead wires and the metal casing.
The normal value should be greater than 20 MΩ. If it is lower than this value, it indicates a risk of moisture or internal contamination.
Recommended treatment: After disassembly, dry with dry air; fire drying is strictly prohibited; if repeatedly exposed to moisture, replace with a fully sealed product.
V. Verify the shielding and grounding system to eliminate interference factors
Screw-type platinum resistance outputs a weak electrical signal and is highly susceptible to electromagnetic interference.
Check if the shielding wire is connected throughout:
From the sensor end to the data acquisition end, the shielding layer should be continuous without breaks.
Check if the grounding is reliable:
The grounding resistance should be less than 4 Ω.
The grounding post needs to be scraped to remove paint to ensure direct metal conduction.
It is recommended to use single-point grounding at the acquisition end to avoid forming a ground loop.
VI. Use the replacement method to quickly identify the fault source
If the above checks still cannot determine the problem, replacement verification can be used.
Replace the sensor:
If the display returns to normal after replacement, the original sensor is damaged.
Replace the display instrument:
If the instrument is set to PT100 input specifications, and the temperature display is normal after replacement, it indicates that the original instrument is faulty.
