Field Notes: Intermittent Signal Jumps on a 3300 Ceramic Tip Probe - Excellent PLC

Written by Jason Moore – Field Service Engineer, Rotating Machinery

This one took longer than it should have, mostly because nothing failed in a clean, obvious way.

The machine was fine. The rack was fine. The channel never went bad. Yet the vibration plot coming from a Bently Nevada 164517-025-10-02-00 kept showing random spikes that made no mechanical sense.

What the Data Looked Like

Normal vibration levels most of the time
Sudden upward spikes lasting seconds
No repeatable pattern
No alarms, no channel fault indicators

If you only glanced at the trend once a day, you’d miss it entirely.

What We Checked First (And Why It Didn’t Help)

Naturally, we went down the usual checklist:

Rechecked probe gap
Verified target material and surface condition
Swapped monitoring channels
Reviewed historical vibration data

Nothing changed. The spikes followed the probe, not the channel.

The Breakthrough Moment

The clue came during a physical inspection while the machine was still warm.

When the probe cable was gently moved near the conduit entry, the signal flickered. Not enough to trip a fault—just enough to distort the measurement.

That’s when we suspected internal conductor damage.

Root Cause Identified

After removing the probe assembly, we found:

No visible damage on the probe body
Ceramic tip in good condition
Micro-cracks inside the probe cable insulation, likely from long-term vibration and tight bending radius

As temperature changed and the cable shifted slightly, resistance fluctuated just enough to cause signal jumps.

Why This Is Easy to Miss

The 164517-025-10-02-00 probe itself is robust. The weak point isn’t the ceramic tip—it’s the cable section near mechanical stress points.

Because the signal never fully dropped out:

The system never flagged a fault
The probe appeared “mostly healthy”
The problem masqueraded as process instability

How We Fixed It

Replaced the entire probe and cable assembly
Improved cable routing to reduce bending stress
Added strain relief near the probe mounting point
Verified signal stability during heat-up and cool-down

Once replaced, the spikes disappeared completely.

Lessons Worth Remembering

Intermittent faults are usually mechanical, not electronic
Cable health matters as much as probe health
Vibration can damage wiring long before it looks damaged
“Almost good” signals are the hardest to diagnose

Final Note

If a 3300 series ceramic tip probe starts producing random, short-lived spikes, don’t overthink the machine behavior. Wiggle the cable—carefully—and watch the trend.

Sometimes the problem isn’t inside the machine. It’s inside the insulation.

— Jason