At a Glance. A Bently Nevada 3300 XL measurement chain is three parts, not one: a proximity probe that faces the shaft, a Proximitor oscillator-demodulator that conditions the signal, and an extension cable that joins them at a fixed system length. Ordering any one of the three without the other two is the most common mistake on this family. The two probes specified most often are the 3300 XL 8mm probe and the 3300 XL 5mm miniature probe, and both feed the 3500/42M monitor in an API 670 protection rack. Everything described here is held in the Bently Nevada range within mechanical automation components.
1. What the 3300 XL chain actually measures
An eddy-current probe does not measure vibration. It measures the gap between its tip and the conductive surface of the shaft. An alternating field from the coil induces eddy currents in that surface, and the loading those currents produce changes the impedance of the coil. The Proximitor turns that impedance change into a voltage proportional to the gap, and the monitor converts the voltage into displacement, then into vibration and position readings.
The practical consequence is that the probe, the cable and the Proximitor form a single calibrated circuit. The parts are sold separately, but they are not interchangeable individually.
2. The three parts of one channel
- Proximity probe. The sensing element that mounts through the bearing housing or a bracket. A typical radial probe is 330106-05-30-05-02-00; where space is very tight, a compact TK-3E with a 3 mm tip is used instead.
- Extension cable. A matched coaxial cable between probe and Proximitor, for example 330130-040-12-05. Its length is part of the electrical specification, not a convenience.
- Proximitor. The oscillator-demodulator that powers the probe and outputs the conditioned signal — for example 330780-50-05 or 330180-51-CN.
Mixing a probe from one system length with a cable and Proximitor from another shifts the scale factor and the null voltage. The machine may still read something; it will simply read the wrong number in the wrong units, which is worse than reading nothing.
3. Tip diameter: 8 mm against 5 mm
Tip diameter is the first decision once the mechanical mounting is fixed, because it sets the linear range.
| Property | 3300 XL 8 mm | 3300 XL 5 mm |
|---|---|---|
| Linear range | 0.5 to 4.0 mm | 0.25 to 1.5 mm |
| Typical cable lengths | 1 m to 9 m | 1 m to 9 m |
| Electrical interface | Standard 3300 XL | Same as the 8 mm probe |
| Usual application | Radial vibration and axial position on large turbomachinery | Tight radial envelope, small shafts, short measuring range |
The 8 mm probe is the default for steam and gas turbines, compressors and large motors, because the wider tip tolerates more shaft runout and thermal growth before the target leaves the linear band. The 5 mm miniature probe exists for the case where the mounting boss is small or the available radial depth is limited; its shorter range means the gap must be set with less margin for error.
Substituting one for the other
If the housing was designed for an 8 mm probe, do not fit a 5 mm probe to save space: the reduced range and the different target area both change the measurement. The reverse substitution is equally unsafe when the boss depth cannot accommodate the 8 mm body. Tip diameter follows the machine drawing, not the stock list.
4. Connecting the chain to a protection system
Where the conditioned signal goes depends on whether the machine is protected, monitored, or simply watched.
| Requirement | Typical module | Notes |
|---|---|---|
| API 670 protection on critical turbomachinery | 3500/42M | Four-channel vibration, axial position and differential expansion |
| Rack power | 3500/15 | AC and DC variants, half-height module |
| General-purpose monitoring of non-critical machines | 1900/65A | Six channels, optional Modbus or Ethernet |
| Two-wire loop into an existing DCS or PLC | 990-04-70-01-01 | 4-20 mA vibration transmitter |
On the 3500 route, the four-channel monitor is the interface between the probes and the logic that trips the machine, and it accepts both 8 mm and 5 mm 3300 XL probes. On the general-purpose route the 1900/65A and the transmitter trade channel density and protection functions for a much smaller installed cost. Both are legitimate answers; choosing between them is a protection philosophy decision rather than a technical one.
5. Reading the part number before ordering
Bently Nevada part numbers encode the mechanical and electrical options as dash-separated fields. A number such as 330106-05-30-05-02-00 is not a single article description; it is a configuration. Two lines that look similar in a quotation can differ in cable length, connector series or agency approval, and those are exactly the differences that stop a chain from calibrating.
Older part numbers such as 3300/10 and 3300/48 follow the same logic for the earlier 3300 platform. A handful of articles in the same catalogues are not probes at all: a two-wire transmitter such as 990-04-70-01 and a monitoring instrument such as 177230 sit alongside the probes and cables but play different roles in the loop.
6. Ordering checklist
- Mounting and tip diameter. Fixed by the machine drawing: 8 mm for the standard boss, 5 mm only where the design calls for it.
- Linear range against expected movement. Check that shaft excursion plus thermal growth stays inside the probe's linear band.
- Cable length. Match probe, extension cable and Proximitor to one system length. Do not improvise with a cable that happens to be on the shelf.
- Connector type and environment. Confirm the connector series and the temperature rating at the probe location.
- Downstream module. Decide between a protected rack channel, a general-purpose monitor and a loop-powered transmitter before ordering the probes.
- Traceability. Critical machines normally require documentation with the parts; state that requirement on the enquiry.
Send the machine tag number, the existing probe part numbers and the rack position, and we will confirm the complete chain — probe, cable and Proximitor — against the Bently Nevada range as one matching set with a single quotation.