TL;DR: The Bently Nevada 3500/25 is a half-height, two-channel Keyphasor module. It powers and terminates one Keyphasor transducer per channel, converts that analogue signal into a clean digital pulse, and passes it around the rack on the backplane so monitors can derive rotative speed, 1X amplitude, phase and tracking-filter values. A rack can hold two /25 main modules stacked in one slot – four channels normally, eight in a paired TMR arrangement. The current part is the Enhanced module 149369-01, a direct plug-in replacement for the legacy 125792-01. TenBN stocks modules and I/O variants; the availability table is below.
What is a Keyphasor, and what does the 3500/25 do with it?
Keyphasor is the name Bently Nevada gives to “a once-per-turn or multiple-event-per-turn pulse from a rotating shaft or gear used to provide a precise timing measurement”. Without it you know how much a shaft is moving; with it you know where on the shaft that movement is, which is what turns a vibration reading into a phase angle, an orbit or a Bode plot.
The 3500/25 creates that pulse. It accepts an analogue signal from a Keyphasor transducer – a proximity probe or a magnetic pickup – detects the moment a designed notch or projection on the shaft passes the tip, and conditions it into a digital indicating output. Those outputs are routed to the monitor modules through the rack backplane “for use in speed, phase, tracking filter, and other measurements”. The module also “provides power and termination for up to two Keyphasor transducers”, so the transducer does not need its own supply.
3500/25 specifications
From Baker Hughes datasheet 141532 (Enhanced Keyphasor Module).
| Channels per card | 2 |
|---|---|
| Accepted transducers | Proximity probes (Proximitor output) or magnetic pickups – passive or powered |
| Input range, non-isolated | +0.8 V to -21.0 V |
| Input range, isolated | +5 V to -11 V |
| Input impedance | 21.8 kΩ minimum |
| Transducer supply | -24 Vdc, 40 mA maximum per channel |
| Speed range, input | 1 to 1,200,000 cpm (0.017 Hz to 20 kHz) |
| Speed range, output | 1 to 99,999 cpm |
| Auto threshold | Minimum 2 V peak-to-peak; minimum 120 rpm (2 Hz) |
| Manual threshold | Selectable 0 to -20 Vdc, minimum 500 mV peak-to-peak, above 0.017 Hz (1 rpm, one event per revolution) |
| Hysteresis | 0.2 to 2.5 V |
| Indicating outputs | 2 buffered front (coaxial) + 2 rear (Euro Style), load 504 Ω maximum |
| Rack total | Up to 4 Keyphasor signals; 8 when two modules are paired for TMR |
| Power draw | 3.2 W typical |
| Main module size / weight | 119.9 x 24.4 x 256.5 mm / 0.34 kg |
| I/O module size / weight | 241.3 x 24.4 x 103.1 mm / 0.46 kg |
| Operating temperature | -30 to +65 °C (0 to +65 °C with Internal Barrier I/O) |
| Storage / humidity | -40 to +85 °C; 95% non-condensing |
| Approvals | cNRTLus Class I Div 2; ATEX / IECEx / CSA Class I Zone 2; T4 at Ta = -20 to +65 °C; EN 61000-6-2, EN 61000-6-4, EN 61010-1, RoHS |
Accuracy figures in the datasheet are stated at +25 °C, so treat a threshold set on a cold machine and a threshold read back in a warm room as two different things.
Compatible racks and slots
The 3500/25 main module takes one half-height front slot, which means it can share a full-height position with a second /25 stacked above it – and it can go in any of the 14 general-purpose slots of a standard 3500/05 rack, not just the reserved left-hand end. A special mounting adapter is used when you fit it into a full-height slot. Two main modules maximum per rack; the rear I/O module takes one full-height rear slot.
The channel budget follows from that geometry: two cards x two channels = four Keyphasor signals per rack, or eight where the two cards are paired for redundant (TMR) measurement, giving primary and secondary outputs for the same shaft mark.
Installation, target and wiring
Most field problems with Keyphasor start at the target, not the module. Bently Nevada’s application guidance is that the notch or projection should be deliberately designed in, positioned away from high-torque areas, radiused at the edges, and mounted radially rather than axially, so that thermal growth and rotor float do not change the gap. Minimum width and length are 1.5 times the probe tip diameter, minimum depth is 0.06 in (1.5 mm), and the step must move the Proximitor output by at least 7 V. Record the target’s axial location at build, and keep a spare Keyphasor transducer on site.
Wiring follows the I/O variant you ordered. Internal Termination I/O takes field cable direct; External Termination I/O works with ET terminal blocks (128718-01 / 128726-01) and pre-terminated multi-core cable (129530-AAAA-BB in 5, 7, 10, 25, 50 and 100 ft lengths) – and ET blocks must not be used with Internal Termination I/O. Barrier-equipped I/O is installed per drawing 138547, non-barrier per 149243 or 149244, and hazardous-area designs should be read against the 3500/04 Internal Barriers datasheet (141495) before ordering. Proximity-system cable is a 3-conductor shielded triad in 0.5, 1, 2, 3, 5 and 9 m standard lengths; a 5 m system is the one that meets API 670 without special treatment.
Feed the Keyphasor output into a monitor channel dedicated to dynamic capture – a 3500/42M channel is the usual choice – so that the same pulse that drives speed logic also time-stamps your waveform records.
Indicators and diagnosis
The module carries two LEDs, and one of them reads the opposite way to what people expect: the datasheet defines OK as indicating “a fault has been detected in the Keyphasor Module”, so read it as a fault annunciator, and TX/RX as evidence the card is talking to the rack interface module.
No failure-code table is published for the 3500/25, so diagnosis is done by reasoning about the signal path. A missing pulse with a healthy probe usually means the threshold no longer matches what the target delivers – thermal growth, a changed gap, or a target that is too shallow to move the Proximitor output by 7 V. A PM-based measurement that only appears above a certain speed is normal behaviour, not a fault: a passive magnetic pickup needs more than 200 rpm (3.3 Hz) before it generates usable output, so low-speed turning-gear measurements need a proximity probe instead. And where speed is being read but phase looks wrong, check whether the rack is fitted with Isolated I/O – see the FAQ below.
Ordering codes
Order as 3500/25-AA-BB-CC.
| A = 01 | One half-height two-channel card (order two of these for four channels) |
|---|---|
| A = 02 | Two half-height cards, ordered as one line item |
| B = 01 / 02 | Internal termination I/O / external termination I/O |
| B = 03 | Internal barrier with internal termination I/O |
| B = 04 / 05 | Isolated internal termination (PM) / isolated external termination (PM) |
| C = 00 / 01 / 02 | No approval / cNRTLus Class I Div 2 / ATEX-IECEx-CSA Class I Zone 2 |
The module and I/O part numbers you will see on racks: Enhanced Keyphasor Module 149369-01 (direct plug-in replacement for legacy 125792-01); Keyphasor I/O 125800-01 (internal) and 126648-01 (external); isolated I/O 125800-02 and 126648-02; barrier I/O 135473-01.
3500/25 parts we can supply
Part number
| 3500/25 149369-01 | 3500/25 149369-01 Enhanced Keyphasor Module · In stock · View |
|---|---|
| 3500/25-01-01-02 | 3500/25-01-01-02 Enhanced Keyphasor Module · In stock · View |
| 125760-01 | 3500/25 Keyphasor Module · In stock · View |
Frequently asked questions
What does the 3500/25 Keyphasor module do?
It creates the once-per-turn timing pulse the rest of the 3500 rack depends on. It supplies power and termination to a Keyphasor transducer, watches for the designed notch or projection on the shaft, and converts that event into a digital pulse that the rack backplane distributes to monitor modules for speed, phase and tracking-filter measurements.
Can the 3500/25 use both probes and magnetic pickups?
Yes – proximity probes and magnetic pickups are the two accepted input types, with no other sensor type listed. Take care with the isolated I/O variant though: it was created specifically for magnetic pickup use, and with a Proximitor it needs external power. Passive magnetic pickups also need more than 200 rpm before they produce a usable signal.
How many Keyphasor channels can one 3500 rack have?
Four normally and eight in a redundant arrangement. Each 3500/25 is a two-channel half-height card, and a rack accepts a maximum of two of them stacked in the same slot position. Pairing the two cards gives TMR operation with primary and secondary outputs.
Is 149369-01 a drop-in replacement for 125792-01?
Yes. Baker Hughes states that the legacy Keyphasor module PWA 125792-01 “is completely replaced by the updated 149369-01 module” and describes 149369-01 as a direct plug-in replacement, downward form-fit-and-function compatible with the older card.
What threshold should I set on a Keyphasor input?
Automatic thresholding needs at least 2 V peak-to-peak and will not track below 120 rpm (2 Hz). Manual threshold is selectable from 0 to -20 Vdc with a minimum signal of 500 mV peak-to-peak and works down to one event per revolution at 0.017 Hz. Hysteresis is adjustable from 0.2 to 2.5 V. As a design rule, size the target so the Proximitor output swings by at least 7 V across the step – that gives you threshold headroom for gap drift and contamination.
Why does my Keyphasor phase reading look wrong with isolated I/O?
Because isolated I/O is not intended for phase work. The datasheet says it was created specifically for magnetic pickup applications, that with a Proximitor it requires external power, and that its intent is primarily to measure shaft speed rather than phase – it introduces a speed-dependent phase shift. If you need phase, use non-isolated internal or external termination I/O.
Which slot does the 3500/25 go into?
Any of the rack’s general-purpose slots. It is half height, so one card occupies half a full-height position and two can be stacked, with a special mounting adapter used in a full-height slot. Unlike the power supplies, it does not need the reserved left-hand positions. The rear I/O module takes one full-height rear slot.
Can I use external termination blocks with an internal termination I/O module?
No. The datasheet states that ET terminal blocks cannot be used with Internal Termination I/O, and that ET blocks and cable are ordered separately from the module. Choose the B position – 01 internal or 02 external – when you order, based on whether field cable lands directly on the rack or on a remote termination panel.
Order a 3500/25 or its I/O module
Send the code from the front of the module, including the I/O variant, and we will confirm what is on the shelf, the equivalent Enhanced replacement if you are still running 125792-01, and whether your barrier or isolated requirements change the part number. Prices are quoted per enquiry and are not published here.
Related modules in the same rack: 3500/42M Proximitor/Seismic Monitor, 3500/22M TDI and the 3500 system complete guide.