Reliability Matters 1.1 – Proximity Probes

When should Proximity Probes be tested for continued effectiveness and accuracy of readings?

In a recent Bently Nevada webinar, it was recommended proximity probes should be changed out once each five (5) years of operation. (Bently Nevada webinar Aug 11, 2020.) It is our observation that exercising a time-based change out is counter productive to the development of a CBM program. Instead of changing out the proximity probes, it would be far more effective to perform an annual visual inspection looking for prox probe damage and to confirm the set up continues to meet specification. A further test of the electrical output will confirm the readings continue to be providing an appropriate output signal and that electrical pulse is again within specification.

When a reading is out of specification, what are the contributing factors?

There are number of contributing factors that will affect the output electrical signature

  1. The gap (space between the face of the probe and the contact point being measured)
    may have changed. The change in the gap will affect the output signature … it will
    either improve or degrade. The change in the output signal will affect the value, may
    affect the alarm ceiling and may also affect any production shutdown system that are
    tied to the probes.
  2. A change in the shape of the measurement point such as a bent or marked contact
    point, or a change in the contact material (such as going from 4140 steel to stainless
    steel), will affect the signature values. In any case, the readings need to be measured
    and the probe recalibrated to identify a consistent output signature.
  3. Moisture – any conductive material that gets into the communication system will affect
    the reliability of the signal that is being monitored.
    From time to time oil can be found leaking from the cable or the cable gland. Oil in
    itself is not an issue with transmission of the electrical pulse. A fluid lock cable gland is
    effective in keeping out oil and moisture from the interconnection points …. moisture
    being more of an enemy to the signature transmission than oil.
  4. One of the cables has been changed out and it is no longer a balanced system.
  5. Every proximity probe is comprised of three components.
    • The proximity probe head or transducer.
    • An interconnecting cable that connects between the prox probe and the driver.
    • A driver that is tuned to the system.

Every proximity probe “system” requires a balanced system that is a measured length. What this means is the entire proximity probe from proximity head to driver must be measured and the driver must be calibrated to both the system length and the type of material that is being monitored. A 5 meter system will not work when the driver is calibrated to 9 meter. Likewise, should you need a 9 meter system and the length happens to measure out at 8.5 feet, the output signals from the proximity probe will be inaccurate as the system capacitance is outside the system requirements.

Always refer to the proximity probe data sheet for accurate component and system specifications when ordering.

Do different metals provide different readings?

The short answer is YES. 4140 steel will yield a different electrical output signature than 316 stainless steel. The driver must be calibrated to the system length as well as the contact material in order to capture an accurate movement signature.

Can we help you with your system?

DTM Consulting Services is a company of equipment reliability technical specialists. We are
available to answer your questions. Feel free to contact us to know how we can help you today!

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