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Complete process for determining the quality of the Bently Nevada 3500 eddy current probe and preamplifier.

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Complete process for determining the quality of the Bently Nevada 3500 eddy current probe and preamplifier.

2026-06-11

Applicable to: 3300XL series probes (8/11/14mm) + 330180 series preamplifiers, with matching 3500 vibration/displacement monitoring cards. The procedure involves five steps: initial visual inspection → power-off electrical testing → power-on voltage verification → TK-3E professional calibration → 3500 system alarm verification, providing a quick and precise fault location process.

I. Visual Physical Inspection (Step 1, Power-off Operation)

1. Probe Inspection:

End face: No bumps, scratches, corrosion, or oil buildup; ceramic sensing surface intact and without cracks. If the end face is damaged, the coil is likely damaged, and it is directly considered faulty.
Cable/Connector: Tail wire without insulation damage, bending, or aging; BNC coaxial connector without oxidation, deformation, or water ingress; threads without stripping.

2. Preamplifier Inspection:

Housing without deformation, water ingress, or oil corrosion; terminals without burning or blackening.
Complete Marking: Confirm the total cable length (5m/9m/14m) marked on the preamplifier. The total length of the probe tail wire + extension cable must match; mismatched lengths will cause sensitivity failure.

3. The coaxial sheath of the extension cable is undamaged, and there is no water ingress or bent needle core at the BNC connectors at both ends; the middle connector is well sealed and there is no oil leakage.

II. Electrical measurement after power failure (multimeter + megohmmeter to distinguish probe/cable faults)

(1) Probe coil conduction resistance (multimeter resistance range)

Disconnect the probe from the extension cable and measure the resistance between the probe BNC inner core and the shield shell:

Qualified standard: 8mm probe 5～15Ω; 11/14mm probe range is close, deviation ≤5% of the original factory value
Fault judgment: Infinite resistance: internal coil open circuit, probe scrapped; resistance ≈0Ω: coil short circuit, probe scrapped; resistance far exceeding 15Ω: lead wire broken, poor contact.

(2) Probe insulation resistance (500V megohmmeter)

Measure the inner core of the probe and the metal shell/armor shielding layer:

Qualified: ≥100MΩ
Fault: insulation <5MΩ → probe is damp, internal insulation breaks down, signal drift, jump.

(3) Extension cable test

Continuity: The inner cores at both ends are interconnected (2~5Ω), and the outer shields at both ends are interconnected (0~1Ω); infinity indicates a broken wire.
Insulation: The insulation between the inner core and the shield layer is ≥100MΩ. If it is lower than the standard, the cable is short-circuited.

(4) Preamplifier insulation rough test

The insulation between the power supply end and the output end of the preamplifier and the shell is ≥100MΩ. If the insulation is too low, it means that the internal circuit is damp and broken down.

III. Power-on static voltage test (distinguishing between good and bad preamplifiers, core on-site method)

Preamplifier wiring definition (three-wire system)

VT: -24V power supply negative (power supply range -17.5~-26VDC, reverse connection is strictly prohibited)
COM: common reference ground
OUT: gap voltage signal output (multimeter DC range to measure OUT and COM)

Step 1:

First confirm that the power supply is normal. Disconnect the probe circuit and only power on the preamplifier. Measure the voltage of VT and COM to be stable at -18~-24VDC; if there is no voltage/the voltage is too low/the polarity is reversed, deal with the power supply first and do not judge that the sensor is damaged.

Step 2: No-load short-circuit test (to determine the condition of the preamplifier separately)

Disconnect the probe/extension cable and short-circuit the BNC inner core and shielding shell of the preamplifier with a metal wire:

Qualified output voltage: -0.6～-0.8VDC
Fault judgment: Voltage outside the range, no voltage, voltage following the supply voltage → Damaged internal oscillation/demodulation circuit of the preamplifier, replace directly.

Step 3: Connect the probe to measure the gap voltage (linear zero-point verification)

Align the probe with a clean carbon steel target surface and slowly advance it to the linear midpoint (standard zero-point gap is about 1.27mm/50mil):

Normal 8mm probe zero-point voltage: -9.0～-10.0VDC
Slowly move the probe away from the target surface: The output voltage should smoothly rise to -2V; when approaching the target surface, it should smoothly drop to -18V, without any jumps or steps throughout the entire process.

Voltage Abnormality Faults:

Constant Output ≈ -24V: Open circuit in probe circuit (broken wire/loose connector/gap exceeding the maximum linear range);
Constant Output ≈ 0V: Short circuit between probe/cable core and shield;
Significant Voltage Drift and Frequent Jumps: Damaged probe insulation, damaged cable shield, aging preamplifier;
Irregular Voltage Changes and Step-like Jumps: Oxidation and poor contact of BNC connector.

IV. Professional Quantitative Judgment of TK-3E Calibrator (Accurate Verification of Sensitivity/Linearity, Mandatory for Unit Annual Inspection)

Match the bracket according to specifications, fix the probe on the micrometer displacement stage, fully connect the probe + matching length extension cable + preamplifier, and connect to the standard -24V power supply.
Zero Point Calibration: Adjust the micrometer to 50mil (1.27mm), the output voltage must fall at the standard zero point (-9.0V±0.5V).
Multi-point linearity test (0～80mil full range divided into 4 points): 8mm probe standard sensitivity 7.87V/mm (200mV/mil), voltage error at each point ≤±0.5% of full range is acceptable.
Fault diagnosis: Linearity deviation exceeding standard, sensitivity drift >10%: probe coil aging or preamplifier circuit drift; non-linear curve, inflection point jump: probe damage or preamplifier damage.

V. 3500 system card status alarm auxiliary judgment

Channel red light constantly on (hard fault Probe Fault): 3500 card detects open/short circuit in sensor circuit, most likely probe disconnection, cable short circuit, or no output from preamplifier.
OK green light flashing/off: preamplifier power supply abnormality or internal damage, circuit self-test failure.
Monitoring screen signal significant drift, fluctuation, or exceeding range: probe insulation failure, preamplifier temperature drift fault, shielding grounding interference.
Comparison and Replacement Method (Rapid On-Site Troubleshooting): Interchange the test channels with a known working probe and cable. If the fault moves with the probe → probe damage; if the fault remains in the original channel → preamplifier or card failure.

VI. Quick Fault Summary and Comparison Table

Infinite coil resistance/0Ω; Probe internal open circuit/short circuit; Extremely low insulation resistance; Probe/cable damp and damaged insulation; Output ≠ -0.6~-0.8V after short circuit BNC; Preamplifier failure; Gap voltage has no smooth change or constant value; Cable open circuit/short circuit; TK-3E linearity/sensitivity severely out of tolerance; Probe aging or preamplifier drift; 3500 channels continuously displaying Probe Fault red light; Loop open circuit/short circuit, segmented resistance measurement for positioning.

⚠️Key Precautions:

The total length of the probe tail wire + extension cable must be consistent with the length marked on the preamplifier. Length mismatch will directly lead to measurement failure.
The shielding layer is only grounded at one end of the preamplifier, and the shielding on the probe side is suspended to avoid ground loop interference causing signal jumps.
When the unit has interlocks, be sure to disconnect the vibration/displacement interlocks before testing to prevent accidental tripping.
Distinguish between "inappropriate installation gap" and "hardware damage": first adjust the gap and clean the joints, then determine if the component is scrapped.

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Complete process for determining the quality of the Bently Nevada 3500 eddy current probe and preamplifier.

2026-06-11

I. Visual Physical Inspection (Step 1, Power-off Operation)

1. Probe Inspection:

End face: No bumps, scratches, corrosion, or oil buildup; ceramic sensing surface intact and without cracks. If the end face is damaged, the coil is likely damaged, and it is directly considered faulty.
Cable/Connector: Tail wire without insulation damage, bending, or aging; BNC coaxial connector without oxidation, deformation, or water ingress; threads without stripping.

2. Preamplifier Inspection:

Housing without deformation, water ingress, or oil corrosion; terminals without burning or blackening.
Complete Marking: Confirm the total cable length (5m/9m/14m) marked on the preamplifier. The total length of the probe tail wire + extension cable must match; mismatched lengths will cause sensitivity failure.

II. Electrical measurement after power failure (multimeter + megohmmeter to distinguish probe/cable faults)

(1) Probe coil conduction resistance (multimeter resistance range)

Disconnect the probe from the extension cable and measure the resistance between the probe BNC inner core and the shield shell:

Qualified standard: 8mm probe 5～15Ω; 11/14mm probe range is close, deviation ≤5% of the original factory value
Fault judgment: Infinite resistance: internal coil open circuit, probe scrapped; resistance ≈0Ω: coil short circuit, probe scrapped; resistance far exceeding 15Ω: lead wire broken, poor contact.

(2) Probe insulation resistance (500V megohmmeter)

Measure the inner core of the probe and the metal shell/armor shielding layer:

Qualified: ≥100MΩ
Fault: insulation <5MΩ → probe is damp, internal insulation breaks down, signal drift, jump.

(3) Extension cable test

Continuity: The inner cores at both ends are interconnected (2~5Ω), and the outer shields at both ends are interconnected (0~1Ω); infinity indicates a broken wire.
Insulation: The insulation between the inner core and the shield layer is ≥100MΩ. If it is lower than the standard, the cable is short-circuited.

(4) Preamplifier insulation rough test

III. Power-on static voltage test (distinguishing between good and bad preamplifiers, core on-site method)

Preamplifier wiring definition (three-wire system)

VT: -24V power supply negative (power supply range -17.5~-26VDC, reverse connection is strictly prohibited)
COM: common reference ground
OUT: gap voltage signal output (multimeter DC range to measure OUT and COM)

Step 1:

Step 2: No-load short-circuit test (to determine the condition of the preamplifier separately)

Disconnect the probe/extension cable and short-circuit the BNC inner core and shielding shell of the preamplifier with a metal wire:

Qualified output voltage: -0.6～-0.8VDC
Fault judgment: Voltage outside the range, no voltage, voltage following the supply voltage → Damaged internal oscillation/demodulation circuit of the preamplifier, replace directly.

Step 3: Connect the probe to measure the gap voltage (linear zero-point verification)

Align the probe with a clean carbon steel target surface and slowly advance it to the linear midpoint (standard zero-point gap is about 1.27mm/50mil):

Normal 8mm probe zero-point voltage: -9.0～-10.0VDC
Slowly move the probe away from the target surface: The output voltage should smoothly rise to -2V; when approaching the target surface, it should smoothly drop to -18V, without any jumps or steps throughout the entire process.

Voltage Abnormality Faults:

Constant Output ≈ -24V: Open circuit in probe circuit (broken wire/loose connector/gap exceeding the maximum linear range);
Constant Output ≈ 0V: Short circuit between probe/cable core and shield;
Significant Voltage Drift and Frequent Jumps: Damaged probe insulation, damaged cable shield, aging preamplifier;
Irregular Voltage Changes and Step-like Jumps: Oxidation and poor contact of BNC connector.

IV. Professional Quantitative Judgment of TK-3E Calibrator (Accurate Verification of Sensitivity/Linearity, Mandatory for Unit Annual Inspection)

Match the bracket according to specifications, fix the probe on the micrometer displacement stage, fully connect the probe + matching length extension cable + preamplifier, and connect to the standard -24V power supply.
Zero Point Calibration: Adjust the micrometer to 50mil (1.27mm), the output voltage must fall at the standard zero point (-9.0V±0.5V).
Multi-point linearity test (0～80mil full range divided into 4 points): 8mm probe standard sensitivity 7.87V/mm (200mV/mil), voltage error at each point ≤±0.5% of full range is acceptable.
Fault diagnosis: Linearity deviation exceeding standard, sensitivity drift >10%: probe coil aging or preamplifier circuit drift; non-linear curve, inflection point jump: probe damage or preamplifier damage.

V. 3500 system card status alarm auxiliary judgment

Channel red light constantly on (hard fault Probe Fault): 3500 card detects open/short circuit in sensor circuit, most likely probe disconnection, cable short circuit, or no output from preamplifier.
OK green light flashing/off: preamplifier power supply abnormality or internal damage, circuit self-test failure.
Monitoring screen signal significant drift, fluctuation, or exceeding range: probe insulation failure, preamplifier temperature drift fault, shielding grounding interference.
Comparison and Replacement Method (Rapid On-Site Troubleshooting): Interchange the test channels with a known working probe and cable. If the fault moves with the probe → probe damage; if the fault remains in the original channel → preamplifier or card failure.

VI. Quick Fault Summary and Comparison Table

⚠️Key Precautions:

The total length of the probe tail wire + extension cable must be consistent with the length marked on the preamplifier. Length mismatch will directly lead to measurement failure.
The shielding layer is only grounded at one end of the preamplifier, and the shielding on the probe side is suspended to avoid ground loop interference causing signal jumps.
When the unit has interlocks, be sure to disconnect the vibration/displacement interlocks before testing to prevent accidental tripping.
Distinguish between "inappropriate installation gap" and "hardware damage": first adjust the gap and clean the joints, then determine if the component is scrapped.