Understanding Peak Hold
Peak hold is a measurement and display mode in vibration analysers in which the instrument continuously watches the incoming signal and retains the maximum value — or, for a spectrum, the maximum amplitude in every frequency bin — encountered during the measurement period. As fresh readings arrive, the display updates only when a new maximum appears; otherwise the previous peak is “held” and kept on screen. The result is a cumulative maximum envelope that captures the highest vibration levels a machine experienced, even when those levels lasted only a fraction of a second. That makes peak hold the natural tool for catching intermittent faults, transient events during start-up and shutdown, and the worst-case levels reached under variable operating conditions — events a steady RMS reading would quietly average away.
1. Definition: What is Peak Hold?
Most routine vibration measurements report a value that represents typical behaviour over the averaging window — useful for trending, but blind to brief excursions. Peak hold takes the opposite stance: it remembers the single highest level seen since the measurement began and refuses to forget it until reset. Because a damaging event can be over in milliseconds, this “remember the worst” philosophy is what guarantees a transient is not missed during periodic monitoring or a one-off acceptance run.
The mode applies equally to a single overall reading and to a full frequency spectrum. In the latter case each spectral line keeps its own independent maximum, so the displayed trace is an envelope of the highest amplitude ever reached at each frequency — not a snapshot of any one instant. This is invaluable for capturing transient frequency components that flash up only at a particular speed or load.
2. How Peak Hold Works
Basic operation
The underlying algorithm is deliberately simple — a running comparison:
- Initial measurement: the first vibration value is recorded and displayed.
- Continuous comparison: every subsequent reading is compared against the stored peak.
- Update if higher: if the new value exceeds the stored peak, it replaces it.
- Hold if lower: if the new value is equal to or below the stored peak, the stored peak stays on display.
- Cumulative maximum: the end result is the highest value seen since measurement began.
Application to spectra
- Peak hold can be applied across the entire frequency spectrum, not just to a single overall level.
- Each frequency bin holds its own maximum independently of the others.
- The display becomes an envelope of maximum amplitudes across all frequencies.
- This is especially useful for capturing transient frequency components that appear only briefly — for example a sideband that flares as a machine sweeps through a resonance.
3. Where Peak Hold Earns Its Keep
3.1 Transient event capture
Brief, high-amplitude events are exactly what peak hold is built to record:
- Start-up / shutdown: vibration peaks as the rotor passes through its critical speeds, where amplitude can momentarily spike during run-up and coast-down.
- Load changes: transient vibration when load is suddenly applied or shed.
- Process upsets: vibration spikes triggered by process disturbances.
- Intermittent problems: faults that come and go, such as loose parts or an intermittent rotor rub.
3.2 Variable-condition monitoring
- Variable-speed equipment: captures the maximum reached anywhere across the speed range.
- Cyclic loading: records the worst vibration during a load cycle.
- Varying process conditions: retains peak levels across operational swings.
- Long-term maximum: the single highest level over weeks or months of running.
3.3 Machinery acceptance testing
- Run the machine through its full operating range, then read the held maximum.
- Peak hold captures the highest vibration reached at any condition during the test.
- Confirm the maximum never breached the contractual specification.
- The held value documents the worst case for the acceptance record — a useful entry in any diagnostic report.
3.4 Intermittent fault detection
- Faults that surface only occasionally and would be missed by spot readings.
- Loose components that rattle intermittently.
- Temperature-dependent problems that appear only once the machine is hot.
- Load-dependent issues that show up only under specific duty.
4. Peak Hold Compared with Other Measurement Modes
Peak hold is one option among several, and choosing the right one matters. The distinctions below are worth keeping straight — and it helps to be clear on the difference between peak amplitude, peak-to-peak and RMS before reading peak-hold data.
Peak hold vs. RMS
- Peak hold: the maximum value; catches transients and can sit far above the average.
- RMS: the average energy content; ignores brief peaks and represents typical levels.
- Relationship: peak hold is always greater than or equal to RMS; the ratio between them is a clue to how much impacting or transient activity is present (closely related to the crest factor).
Peak hold vs. true peak
- Peak hold: holds the maximum over an extended period — minutes, hours or days.
- True peak: the instantaneous maximum within a single waveform capture, lasting only seconds.
- Consequence: peak hold can be considerably higher, because it sweeps up every transient over the full period rather than one short record.
Peak hold vs. average spectrum
- Peak-hold spectrum: the maximum amplitude at each frequency across many successive spectra.
- Average spectrum: the mean amplitude at each frequency, computed by averaging successive blocks.
- Use case: peak hold for chasing transients; averaging for pulling repeatable peaks out of the noise floor.
5. Advantages and Limitations
Advantages
- Captures transients: it never misses a brief event and records the maximum even if it occurred hours earlier — essential for intermittent problems.
- Worst-case documentation: it shows the highest vibration experienced, a conservative figure for safety and design purposes that proves the machine stayed within limits.
- Simple implementation: the algorithm is trivial, needs minimal processing, and is available in virtually every modern instrument and data collector.
Limitations and considerations
- No time information: peak hold does not record when the maximum occurred, so you cannot correlate the peak to a specific operating condition or know whether it was recent or weeks old. Logging a timestamp alongside it helps.
- Outlier sensitivity: a single anomalous spike — a measurement error or an external knock on the sensor — permanently corrupts the held value until reset, and may not represent real machine behaviour.
- Hides average behaviour: a machine might run quietly 99% of the time yet show an alarming held peak from one rare event; peak hold alone gives no sense of typical operation, so it should accompany, not replace, average readings.
6. Best Practices
When to use peak hold
- Monitoring equipment through start-up and shutdown cycles.
- Variable-speed or variable-load machines.
- Known or suspected intermittent problems.
- Acceptance testing across the full operating range.
- Long-term watch (weeks or months) to catch the occasional peak.
When to use other modes
- RMS / average: routine monitoring and trending of typical levels.
- Instantaneous: real-time observation of current vibration.
- Min / max: when both extremes over a period are of interest.
A combined approach
The most informative practice is to record both peak hold and RMS together: peak hold reveals the worst case, RMS reveals normal operation, and the ratio between them quantifies the degree of transient activity. This pairing is the backbone of a thorough condition-monitoring programme.
Reset strategy
- Reset peak hold at the start of each measurement session for a clean run-specific maximum.
- Or maintain a long-term peak hold with a deliberate monthly or quarterly reset.
- Always document when the value was last reset, otherwise the held number cannot be interpreted correctly.
7. Peak Hold in Practical Field Work
In the field, peak hold turns a quick walk-round into a transient trap. A portable two-channel analyser such as the Balancet-1A can hold the maximum 1× amplitude while a fan or pump is run up to speed, so an intermittent peak as the rotor crosses a resonance is recorded rather than lost between samples. Once the dominant fault is identified — most often unbalance — the same instrument is used to field-balance the rotor in its own bearings and verify the result, while peak hold confirms no hidden transient remains during the post-balance run-up. Used this way, peak hold and steady-state measurement complement each other: one guards against the worst moment, the other describes the everyday condition.
