Vibration Amplification in ATEX Zones:
Safe-Distance Measurement Without Zone Entry

ATEX zone classification exists to manage the risk of ignition in environments where flammable gases, vapours, mists or combustible dusts may be present. The zones — 0, 1 and 2 for gas and vapour, 20, 21 and 22 for dust — define the probability of an explosive atmosphere being present, and consequently the requirements for equipment and personnel operating within them.

The vibration health of equipment in these zones matters as much as anywhere else in the plant. A pump with a developing imbalance fault or a loosening support structure in a Zone 1 area will deteriorate on the same timeline as an identical installation in a non-classified area. The challenge is not the measurement itself but the logistics of making it safely within the permit-to-work framework.

The Overhead of Zone Entry for Vibration Measurement

A route-based vibration measurement programme in a classified area involves steps beyond the measurement itself. Entry to a Zone 1 or Zone 2 area typically requires a permit-to-work, a gas test or continuous personal gas monitor, appropriate PPE, sometimes a second person as observer, and coordination with the control room to confirm operating conditions. On a straightforward measurement point, the permit and entry overhead can take longer than the measurement.

For portable accelerometer measurements, the technician must also physically attach the sensor — magnetic mount, threaded stud or direct contact — which requires close approach to running equipment in an environment already presenting explosive atmosphere risk. Facilities with large numbers of measurement points in classified areas frequently find that the condition monitoring programme is limited in practice by the available time and capacity for zone entry, not by the measurement technology itself.

Permanently installed ATEX-rated sensors eliminate the need for route entry, but introduce their own considerations. The sensors must be certified for the zone, cabling must be intrinsically safe or otherwise suitably protected, and routine inspection or replacement of sensors still requires zone access. For facilities with a large number of relevant assets spread across classified areas, fully instrumenting everything is often not practical within the maintenance budget.

How Vibration Amplification Works at a Zone Boundary

Vibration amplification measures the surface motion of a structure by analysing video of the target. The camera is positioned at a distance from the equipment — typically between 2 and 15 metres — and the processing amplifies sub-pixel motion produced by the machine during normal operation. The camera does not need to be in contact with the equipment, and in most plant layouts it does not need to be inside the classified zone.

Provided there is a clear line of sight from a location outside the zone boundary to the equipment under assessment, the measurement can be made without any personnel entry into the classified area. No sensors are attached, no cabling is introduced into the zone, and no equipment that could act as an ignition source is placed in the classified environment. The machine remains at full operating load throughout.

Where a fence, barrier or open mesh separates the measurement position from the zone, feasibility depends on the specific installation. A coarse mesh with apertures significantly larger than the spatial resolution needed is generally workable; solid glass, dense mesh or close lattice will attenuate the signal. In practice, most process plant layouts include positions from which a compressor, pump or fan installation is visible from outside the zone boundary without obstruction.

What Vibration Amplification Can and Cannot Detect

From a position outside the zone, vibration amplification can capture the operational deflection shape of any equipment surface that is visible to the camera. This includes: overall vibration amplitude at the housing and support structure; spatial distribution of vibration energy across the installation; structural resonance conditions of the housing, support frame or connected pipework; relative motion between components such as shaft, housing and baseplate; and mounting looseness visible as non-linear relative motion between the machine base and the structure beneath it.

It cannot provide the high-frequency signal content needed for rolling-element bearing envelope analysis or gear mesh diagnostics. The camera frame rate limits the measurable frequency range to low-frequency structural motion — bearing defect frequencies, which typically sit between 1 kHz and 30 kHz, are not accessible by this method. For bearing condition monitoring in ATEX zones, permanently installed intrinsically safe accelerometers with appropriate cabling remain the correct solution. Vibration amplification is a complement to that approach, not a substitute.

Practical Requirements

The measurement requires adequate illumination of the target surface. Natural lighting or existing process lighting is sufficient in most installations. Supplementary lighting may be needed in enclosed areas, which introduces considerations around the suitability of lighting equipment for the zone. Reflective or highly polished surfaces can create artefacts if specular reflections enter the camera field of view; painted or matt surfaces perform better.

Camera stability is important: the measurement computes motion relative to the camera position, so any camera movement during the recording period introduces error. A stable tripod on a firm surface outside the zone is standard practice. Recording duration is typically 30 to 120 seconds at the operating condition of interest — well within the duration of a normal zone observation or external inspection.

What It Does Not Replace

Vibration amplification from outside a zone boundary does not replace continuous monitoring. If the requirement is to detect a developing bearing fault over weeks, with automatic alarming when a threshold is exceeded, that requires a permanently installed sensor with intrinsically safe certification. Vibration amplification is a periodic measurement tool — it characterises the machine's vibration behaviour at the time of measurement, not how that behaviour changes between sessions.

It also does not eliminate zone entry entirely. Equipment in classified areas still requires periodic physical inspection, lubrication, and maintenance that cannot be performed from outside the boundary. What changes is the frequency and purpose of entries specifically for vibration assessment — reducing the number of routine measurement entries and, where a vibration complaint has been raised, allowing an initial investigation to be conducted without entry before deciding whether closer inspection is warranted.