Ioniser Verification: Decay Time and Offset Voltage
An ioniser is the only static control device that puts charge into the air, which is why it is also the one that is most often installed and then forgotten. Two numbers describe whether it works: how quickly it neutralises a charge, and how much residual voltage it leaves behind. Both change as the emitters wear and the airflow drifts, and both are measured with the same instrument. This article covers the measurement methods and the maintenance decisions that follow from them.
What an Ioniser Is For
An ioniser produces positive and negative ions that are carried by an air stream to a charged surface, where they neutralise the static charge that has accumulated. It is used where a part cannot be grounded: moving webs, plastic housings, and boards that are handled on insulating surfaces.
It is not a substitute for grounding. A wrist strap or a conductive work surface drains charge through a resistance, while an ioniser works only within the air stream and only for as long as it is running, so the two controls address different situations. The rest of the plan is described in our notes on <a href="https://www.gopcba.com/esd-floor-wrist-strap-testing/” title=”floor and wrist strap testing”>floor and wrist strap testing.
Decay Time and How It Is Measured
Decay time is the interval in which a charged plate monitored at a standard distance falls from one defined voltage to another, most commonly from 1000 volts to 100 volts. The measurement is made with a charged plate monitor placed at the working distance, usually 300 millimetres, in the centre of the ion stream.
The figure is compared with whatever the process requires rather than with a single universal number, and a common target for assembly benches is under 20 seconds and a tight target is under 10. Measuring at a different distance changes the answer, so the distance belongs in the record.
The plate, not the sensor, defines the measurement. A monitor used with the plate facing away from the ion stream reports a decay time that belongs to the surrounding air rather than to the ioniser.
Offset Voltage and Balance
Offset voltage is the residual voltage left on the plate after the ion stream has had time to act, and it measures the imbalance between the positive and negative ion production. A balanced ioniser leaves the plate close to zero, while an unbalanced one charges a floating part to a steady voltage that can exceed the damage threshold of a sensitive device.
Typical acceptance is plus or minus 35 volts, with tighter limits of plus or minus 20 volts for very sensitive work. Offset is affected by emitter condition, by airflow and by the proximity of grounded metal, so it is measured at the same distance and the same orientation as the decay time.
Emitter Types and Their Failure Modes
Corona emitters use sharp points that produce ions from a high voltage, and the points accumulate a deposit of oxides and airborne contamination that reduces output and unbalances the field. Cleaning restores the output up to a point, and beyond that the point is worn and the bar has to be replaced.
Pulsed direct current systems switch polarity rapidly, which keeps the emitters cleaner and produces a lower offset, while alternating current systems are simpler and cheaper to maintain. Whichever type is installed, the emitter condition is the variable that changes the measurement between one verification and the next.
A visual check of the emitters before measurement explains most unexpected results, because a single contaminated point changes the balance of an entire bar.
Airflow, Distance and Coverage
Decay time falls as airflow rises, so a bar that is starved of air by a blocked filter or a failing fan will look weak even with clean emitters. Airflow is verified separately, and the filter is part of the maintenance schedule rather than an accessory.
Coverage falls with distance and with the size of the area to be protected. A bar sized for a 300 millimetre bench will not protect a 1 metre conveyor, and the measurement in the middle of a wide line can be acceptable while the edges are outside the requirement.
Verification Frequency and Records
Verification frequency is set by the control plan, with quarterly measurement typical and monthly measurement where the process is sensitive or the environment is dirty. An additional check follows any cleaning, repair or relocation of the equipment.
The record should carry the location, the distance, the airflow setting, the decay time, the offset voltage and the date of the last emitter service. Without the distance and the airflow, two measurements of the same ioniser cannot be compared, and the trend is the part that predicts failure.
Effect on Sensitive Devices
The device classifications that matter here are the human body model and the charged device model, because they describe how a part is actually damaged. A charged device model event occurs when a part itself is charged and then contacts a grounded surface, and an ioniser that leaves a high offset voltage can create exactly that situation.
The consequence is that an unbalanced ioniser can do harm rather than good, which is why the offset voltage is measured at every verification and not only when the decay time looks poor. The two figures together describe the device, not the bar.
Common Misapplications
The most common error is using an ioniser to compensate for an insulating material that should be replaced with a dissipative one, which leaves the process dependent on a single piece of equipment. The second is measuring once at the centre of the bar and assuming the whole area is covered.
A third is treating ionisation as a substitute for grounding during service or rework, where a wrist strap is not worn because the bar is running nearby. The ioniser reduces the charge on a floating part; it does not protect a person or a tool that is already at a different potential.
Integration with the Control Plan
An ioniser belongs to the same electrostatic protected area as the benches it serves, so its boundaries, its measurement points and its frequency are documented with the rest of the plan. Where it covers a conveyor, the measurement points follow the product path rather than the bar.
The plan should also state who acts on a failed measurement and what the interim control is, because an ioniser that is out of specification is often left running while a repair is arranged. A spare bar and a defined replacement interval keep that decision from becoming a delay. How the surrounding area is arranged is discussed in our floor zoning notes.
FAQ
What decay time should an ioniser achieve? Under 20 seconds from 1000 to 100 volts at the working distance is a common requirement, and under 10 seconds is typical for sensitive assembly, measured at the same distance every time.
Why does offset voltage matter if the decay time passes? A fast but unbalanced ioniser charges a floating part to a steady voltage, and that voltage is what damages a charged device model sensitive component.
How often should the emitters be cleaned? Clean them at every verification and inspect them in between, because a single contaminated point changes the balance and the output of the whole bar.
