Bently Nevada 3500 vs 3300 XL: How the Monitor Rack and the Proximitor Transducer System Work Together

Spares requests often treat “3500” and “3300 XL” as competing products. They are not. The 3300 XL is the transducer system that sits on the machine and converts shaft motion into a voltage; the 3500 is the monitoring rack that receives that voltage, compares it with alarm and danger setpoints and trips the machine. A 3300 XL proximitor is the sensor/driver in the middle of that chain, and ordering the wrong one, or the wrong length, will give you a channel that reads plausible but wrong numbers.

The Measurement Chain: From Shaft to Trip Relay

An eddy-current proximity measurement has four physical parts and one monitoring layer:

1. The probe, an 8 mm coil tip in a threaded case, mounted through the bearing housing and gapped to the shaft.
2. The extension cable, a coaxial cable with tuned electrical characteristics connecting the probe to the driver.
3. The Proximitor sensor, which generates the RF field, demodulates the eddy-current losses and outputs a negative DC voltage proportional to gap.
4. Field wiring from the Proximitor (power, common, signal) to the monitor.
5. The monitor, for example a Bently Nevada 3500 system rack, which supplies -24 Vdc to the Proximitor, processes the signal into vibration and position values and drives relays and 4–20 mA outputs.

The first three items form the 3300 XL 8 mm transducer system. The probe, extension cable and Proximitor are designed as one calibrated electrical circuit, which is why the total system length matters as much as the part itself.

Key electrical parameters of the 3300 XL 8 mm system

The standard 3300 XL 8 mm system has an average scale factor of 200 mV/mil (7.87 V/mm) when calibrated against an AISI 4140 steel target. The linear range is typically 2 mm (80 mils), starting a short distance from the target face. Because the output is negative and becomes more negative as the gap increases, the probe is normally gapped so the DC gap voltage sits near the middle of the linear range, commonly around -9 to -10 Vdc. Verify the exact linear range and recommended gap on the current datasheet for the system you are installing, because target material, temperature range options and probe type all shift these values.

A shaft of different steel, or with plating or a sleeve, gives a different scale factor. API 670 expects the scale factor to be verified on the actual shaft material or a representative sample, typically with a micrometer-type static calibrator such as a TK-3 or equivalent.

Layer by Layer: What Each Component Does

Probes

Probe family numbers distinguish thread form and mounting style. For example, the 330101 3300 XL probe is the common imperial-thread (3/8-24 UNF) standard-mount probe, the 330103 probe is the metric (M10 x 1) version, and the 330105 reverse-mount probe is installed from inside the machine with the cable exiting outward. The option digits that follow encode unthreaded length, overall case length, total probe cable length, connector/cable option and agency approvals. Read them against the datasheet ordering table.

Where an 8 mm tip cannot be accommodated, the 3300 5 mm probe (330170 family) provides a smaller case. Confirm on the datasheet which Proximitor and extension cable it is specified with before ordering a mixed set.

Proximitor sensors

The 330180-50-05 Proximitor is a frequently requested example. In the 330180-x0-xx structure, the option digits encode the total system length the Proximitor is calibrated for, the mounting option and the agency approvals. A Proximitor calibrated for a 5 m system will not read correctly on a 9 m system and vice versa.

For thrust position on large machines, ramp targets and differential expansion, the 3300 XL 25 mm system uses the 330850 Proximitor. Its scale factor and linear range are very different from the 8 mm system (a much longer range at a lower sensitivity), so a 3500/45 channel configured for one cannot simply be wired to the other.

Total system length

The common 3300 XL configurations are 5 m and 9 m total length. A typical 5 m system is a 1 m probe plus a 4 m extension cable; a 9 m system is a 1 m probe plus an 8 m extension cable, though 0.5 m and longer integral probe cables also exist. The 5 m and 9 m lengths are the ones normally specified on API 670 machinery. The rule is simple: probe cable length plus extension cable length must equal the length the Proximitor is calibrated for.

The 3500 Side: Monitors That Consume the Signal

The 3500 rack accepts the Proximitor output on monitor modules that are configured in software (3500 Rack Configuration Software) for transducer type, scale factor, full scale, alarm setpoints and delays. The modules most often paired with 3300 XL systems are:

• 3500/40M Proximitor Monitor: four channels for radial vibration, thrust position, differential expansion and similar proximity measurements.
• 3500/42M Proximitor/Seismic Monitor: four channels that accept proximity, velocity or acceleration inputs, useful where a train mixes shaft and casing measurements.
• 3500/45 Position Monitor: axial thrust, differential expansion, case expansion and valve position, including the long-range 25 mm systems.
• 3500/25 Keyphasor Module: conditions the once-per-turn reference probe, which is itself usually a 3300 XL 8 mm system looking at a notch or projection.

Buffered outputs on each module let portable analyzers connect without disturbing the protection path. Power supplies (3500/15), the rack interface module (3500/22M) and relay modules complete the rack. For the wider requirements on channel redundancy, voting and testing, see the API 670 machinery protection system guide.

Alternatives for Casing and Balance-of-Plant Measurements

On rolling-element-bearing pumps, fans and motors, shaft relative motion is small and casing velocity is the more useful measurement. The Velomitor 330500 piezoelectric velocity sensor is the usual choice there, wired to a 3500/42M channel or to a stand-alone monitor.

For balance-of-plant machines where a plant DCS or PLC handles alarming, the 990 vibration transmitter and 991 thrust transmitter provide a loop-powered 4–20 mA signal from a proximity probe. They are a sensible option for monitoring, but a single transmitter feeding a DCS analog input does not provide the dedicated protection, voting and first-out functions that API 670 expects for critical machinery. Check on the datasheet which probe and extension cable families each transmitter supports. The 1900/65A general purpose equipment monitor sits between the two approaches: a compact monitor with its own relays for smaller machines that do not justify a full rack.

Replacement Rules: What Must Match

Most field problems after a spare is fitted come from a mismatch somewhere in the chain. Use this table when you raise a requisition or receive a part.

Oman Site Considerations

Compressor and pump trains in Oman often sit in exposed skids where the junction box housing the Proximitors receives direct sun. Summer air temperatures of 45–50 °C plus solar gain can push the inside of a sealed box well above ambient. Check the Proximitor operating temperature range on the datasheet, shade the box where possible and avoid mounting it on hot casings. Extended temperature range probe and cable options exist for hot bearing housings; confirm availability and limits for the specific part number.

Dust and coastal humidity cause intermittent faults at connectors. Use connector protectors or insulating sleeves, keep connectors out of oil and water, and use a probe housing where the probe must be adjustable or replaceable from outside the machine. For hazardous areas, confirm that the probe, cable and Proximitor approvals and the barrier or galvanic isolator arrangement match the area classification. Lead times for specific option codes can be long, so hold a matched probe/cable/Proximitor set for each length in use.

Common Mistakes

• Mixing 3300 and 3300 XL components, or components from different XL lengths, without checking the datasheet compatibility notes.
• Cutting or splicing an extension cable to tidy a route. The cable length is part of the calibrated circuit; coil spare length neatly instead.
• Setting the gap mechanically with a feeler gauge only and not checking gap voltage. Gap electrically with the system powered and record the reading.
• Ignoring runout. Mechanical and electrical runout on the probe track shows up as vibration; API 670 sets limits on combined runout for this reason.
• Forgetting to reload or verify the 3500 configuration after a module swap, which leaves the channel with the wrong scale factor or setpoints.

Replacement and Procurement Checklist

• Record full part numbers from the probe, extension cable and Proximitor, not just the family number.
• Confirm total system length (probe cable plus extension) and that it matches the Proximitor.
• Confirm thread form, case length and mounting style of the probe.
• Confirm agency approvals required for the area classification.
• Confirm target material and whether a field calibration on the shaft material is needed.
• Check the monitor module type, channel configuration and alarm setpoints in the 3500 configuration file.
• After installation, record gap voltage, verify scale factor where practical and check buffered output with a portable analyzer.
• Hold critical spares as complete matched sets for each system length on site.

Sourcing 3300 XL and 3500 Components Through Seven Star LLC

Seven Star LLC supplies Bently Nevada transducers and monitoring hardware for plants across Oman and the GCC, including 3300 XL probes and Proximitors, 3500 rack modules, Velomitor sensors and balance-of-plant transmitters. Browse the Bently Nevada brand page or the vibration monitoring category, and send the full part numbers from the nameplates so the request can be matched option code for option code.

Send your full 3300 XL and 3500 part numbers to Seven Star LLC and we will source the matched components for your machine train.