Some powders in Additive Manufacturing, especially metal powders, can react explosively with oxygen and can present a potentially serious risk to life. For such a risk to exist, the powders must be present in an explosive atmosphere with an ignition source, such as the laser which is at the heart of the Laser Beam Powder Bed Fusion (PBF-LB) Additive Manufacturing process, explains Process Sensing Technologies (PST), headquartered in Ely, Cambridgeshire, UK.
An ‘explosive atmosphere’ is a mixture of air, under atmospheric conditions, and flammable substances in the form of gases, vapours, mists, or dusts, where combustion spreads to the entire unburned combination after ignition has occurred. In addition, a ‘potentially explosive atmosphere’ is one which could become explosive due to local and operational conditions, which could be part of the process or due to a machine fault.
In the European Union, the Machinery Directive (2006/42/EC)  requires machinery to be designed and constructed in a way that avoids any risk of explosion posed by gases, liquids, dust, vapours, and other substances produced or used, or by the machinery itself. Practically, this can be achieved by either making the machinery sufficiently robust so that any explosion is contained within it, avoiding external hazards (in the same way as an internal combustion engine), or by eliminating ignition sources in accordance with the ATEX directive (2014/34/EU) .
Additive Manufacturing machines employing lasers or other high energy systems, have an ignition source intrinsic to the design, as well as the presence of a fuel in the form of the powder. Explosion prevention relies on the principle of an inert atmosphere, ensuring the concentration of oxygen in the atmosphere is reduced below the limiting oxygen concentration (LOC) of the powder used when the ignition source is present.
Inert atmospheres to prevent explosions is also covered by the ATEX directive (1999/92/EC) , where atmospheres are classified into zones depending on when an atmosphere capable of sustaining an explosion is expected to occur:
- Zone 20: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, for long periods, or frequently (> 1000 hrs/yr)
- Zone 21: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur occasionally during normal operation (10 to < 1000 hrs/yr)
- Zone 22: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is unlikely to occur during normal operation but, if it does, will persist for a short period (< 10 hrs/yr)
- Safe Area: A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is unlikely to occur under any condition for a period that would pose an intolerable risk to the employer/operator
Responsibility for deciding whether an area is a particular zone or safe area, and for maintaining the area at that zone or safe area, belongs to the employer/operator of the equipment.
Oxygen analysers, such as the Ntron SIL02 available from PST, are critical in aiding the employer/operator to ensure their zones or safe areas are maintained reliably to achieve at least tolerably safe working conditions. The oxygen analysers may be used as part of the basic process control system – for control and operation of the inerting system – or separately, as an independent safety system to monitor the correct functioning of the basic process control system and initiate a suitable response in the event that a fault is detected.
The Ntron SIL02 oxygen analyser has been designed to specifically address Additive Manufacturing requirements for a low cost SIL2 rated oxygen analyser for inertisation applications. The complete system (analyser and sensor) meets the requirements of IEC 61508 SIL2. A fast response, high repeatability and no sensor drift make this analyser a low maintenance solution that delivers reliable performance for safety critical, process control applications.
 Directive 2006/42/EC of the European Parliament and of the Council on machinery, and amending Directive 95/16/EC
 Directive 2014/34/EU of the European Parliament and of the Council on the harmonisation of the laws of the Member States relating to equipment and protective systems intended for use in potentially explosive atmospheres
 Directive 1999/92/EC of the European Parliament and of the Council on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres (15th individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC)