Short answer: a healthy IEPE sensor sits at the bias voltage on its datasheet, often around 10–12 V. Near 0 V usually means a short; near the supply voltage means an open circuit; in range but wandering points to moisture, a loose connector or a damaged amplifier.
What bias voltage is
An IEPE (ICP®-type) accelerometer has a small amplifier inside. The data collector or monitoring system powers it with a constant current, typically 2–10 mA, through the same coax or twisted pair that carries the signal. The amplifier settles at a DC level, the bias output voltage (BOV), and the vibration signal rides on top of it as an AC voltage.
Because the bias depends on the sensor, the cable and the supply all working, it is a quick go/no-go test of the whole path, with no shaker needed.
How to measure it
- On the monitoring system. Most online systems report bias (or “gap”) per channel and alarm when it leaves a window. Trend it: a slow change is an early warning.
- With a data collector. Many portable analyzers show bias when IEPE power is on.
- With a meter. A DC voltmeter across the signal and common at the sensor connector, while the sensor is powered by its constant-current source. A meter alone, without IEPE power, reads nothing useful.
Let the sensor settle for a few seconds after power-up. Bias rises from 0 V as the coupling capacitors charge.
What the reading means
| Bias reading | Likely cause | What to check |
|---|---|---|
| About 0 V | Short circuit, or no power | Crushed or wet cable, shorted connector, sensor amplifier failed short, IEPE power switched off |
| At the supply (compliance) voltage, often 18–24 V | Open circuit | Disconnected or broken cable, failed termination, sensor amplifier failed open |
| In range, steady | Healthy | Nothing, but bias doesn’t prove sensitivity: verify on a shaker periodically |
| In range but wandering | Moisture, intermittent connection, thermal transients | Connector seals, cable flexing points, ground loops, nearby heat sources |
| Steady but outside the datasheet window | Damaged amplifier (shock, ESD, over-temperature), or wrong supply | Compare with a known-good sensor on the same channel; check current and compliance settings |
A “ski-slope” spectrum, with energy rising toward 0 Hz, often appears when the bias is jumping, for example after a heavy shock saturates the amplifier. Treat it as a sensor or cable fault, not machine vibration.
Bias and signal headroom
The vibration signal swings above and below the bias. It can’t go below roughly 1 V (the amplifier’s floor) or above the supply’s compliance voltage minus about 1 V, so the smaller of those two gaps limits the largest signal before clipping. A sensor biased at 12 V on a 24 V supply has roughly 10–11 V of swing either way; at 100 mV/g that is around 100 g peak.
Clipping creates false harmonics and broadband noise. If you measure high levels, such as impacts or high-frequency gear mesh, check the headroom with the tool below.
Bias checker
Check a bias reading
Enter what you measured and the sensor’s datasheet values.
The AT2040 powers IEPE sensors and shows bias and live sensitivity on the same screen while it vibrates the sensor, so one test confirms the cable, the amplifier and the sensitivity. See also how to calibrate an IEPE accelerometer.
FAQ
What is a normal bias voltage for an accelerometer?
It depends on the model. Many industrial IEPE accelerometers specify a bias around 10–12 V, within a window such as 8–14 V. Always check the datasheet for your sensor.
What does 0 V bias mean?
Usually a short circuit in the cable, connector or sensor, or the IEPE power supply is off.
Why does my bias read the supply voltage?
That indicates an open circuit: a disconnected or broken cable, a failed connector, or a sensor amplifier that has failed open.
If the bias is good, is the sensor calibrated?
No. Bias shows the amplifier and cable are working, not that the sensitivity is correct. Verify sensitivity on a calibrated shaker.
What causes a ski-slope spectrum?
Low-frequency energy that rises toward 0 Hz usually comes from a bias that is shifting, often after amplifier saturation from shock, or from thermal transients and poor connections.
Related: How often should accelerometers be calibrated? · How to choose a portable vibration calibrator