Vibration readings from a Vibrex™, EV2K+, or 8500C Plus rarely show up in the unit you actually think in. A maintenance manual sensitivity is written in ips, the analyzer displays mils, and a spec sheet quotes g. They all describe the same physical event — the amount a point on the airframe or rotor is moving — but which one you're looking at, and whether it's peak or RMS, changes the number significantly without the actual vibration having changed at all.
Three Different Things Are Being Measured
Displacement, velocity, and acceleration aren't three units for the same quantity — they're three different physical quantities, related by how fast the vibration is oscillating:
| Quantity | Common units | What it emphasizes |
|---|---|---|
| Displacement | mils, microns | How far the point physically moves — dominant at low frequency |
| Velocity | ips, mm/s | Energy of the vibration — the standard for rotor unbalance |
| Acceleration | g, m/s² | Force driving the vibration — dominant at high frequency |
That's why unbalance-driven rotor and propeller vibration is conventionally read and trended in velocity (ips or mm/s) rather than displacement or acceleration — it's the quantity that stays roughly constant in magnitude across the RPM range typical rotors and propellers operate in, which makes trending and threshold comparisons meaningful from one flight to the next.
Why RPM Has to Come Into the Conversion
Converting between displacement, velocity, and acceleration isn't a fixed multiplier — it depends on the vibration's frequency, because displacement, velocity, and acceleration are successive derivatives of the same sinusoidal motion with respect to time. For a rotor turning at a known RPM, the dominant vibration frequency is the 1/rev rate (the RPM itself), so converting a 1/rev velocity reading into an equivalent displacement or acceleration reading requires that RPM as an input — leave it out, or use the wrong RPM, and the converted figure is wrong even though the arithmetic is technically correct.
Peak vs RMS: Same Signal, Different Number
A sensor reading of a sinusoidal vibration can be expressed as its peak value (the maximum excursion) or its RMS value (the root-mean-square, a kind of time-averaged magnitude). For a pure sine wave, peak is about 1.41× the RMS value — so quoting the same vibration as "0.30 ips RMS" or "0.42 ips peak" both describe the identical physical motion. Mixing them up when comparing a reading against a manufacturer's threshold is one of the most common ways a perfectly good balancing job gets mis-read as out of limits, or vice versa.
Convert Once, Read the Whole Picture
BalancePro's Vibration Unit Converter takes one reading — in whichever unit you have it — plus the rotor's RPM, and returns the equivalent value across every displacement, velocity, and acceleration unit at once, peak and RMS. It's built for exactly this: translating between what your analyzer displays, what the maintenance manual specifies, and what a spec sheet quotes, without doing the derivative math by hand every time.
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Open Vibration Unit ConverterBalancePro is an experimental, unofficial calculation aid provided for reference purposes only. It has not been certified, approved, or endorsed by any aircraft, engine, or component manufacturer, nor by any civil aviation authority, and is not a substitute for approved maintenance data or manufacturer-published sensitivities. Independently verify all results before use.