In confined space entry, tank cleaning, and sandblasting, workers rely on Supplied-Air Respirators (SAR), also known as Type C respirators. These systems deliver Grade D breathing air through a long, heavy-duty hose from a remote compressor. The most common, lethal field error workers make is allowing the hose to bend sharply around structural steel or doorframes, causing Flow-Restriction Asphyxiation.
SAR systems operate on strict volumetric flow requirements-usually a minimum of 4 to 6 cubic feet per minute (CFM) at the facepiece, regulated by a valve at the waist or the mask. The physics of fluid dynamics dictate that airflow through a tube is inversely proportional to the resistance. When a worker drags a 100-foot hose around a sharp 90-degree beam corner, the hose bends beyond its Minimum Bend Radius. The internal lumen collapses, creating a severe restriction.
This restriction causes a massive pressure drop downstream. The compressor is still pumping air, but it cannot force the required volume through the pinched hose. The regulator at the waist tries to compensate, but the flow starves. When the worker exerts themselves and demands more air, the SAR cannot deliver.
The terrifying result is that the mask goes into negative pressure. The worker inhales against a vacuum, and the exhalation valves cannot clear the CO2-rich air fast enough. The worker experiences air starvation, panic, and rapid hypercapnia (carbon dioxide poisoning). In a panic, they may rip the mask off inside the confined space, inhaling the lethal atmosphere instantly.
The Maintenance Protocol: Never route an SAR hose around sharp corners or allow heavy equipment to rest on it. Use wide-radius hose bridges or suspension hooks to ensure the hose maintains its circular cross-section at all times. Before entry, perform a flow-check by breathing deeply through the mask while a partner watches the air-supply pressure gauge; if the gauge needle fluctuates wildly or drops during inhalation, the hose is restricted. If you feel any resistance or "pulling" on your lungs, do not walk further-trace the hose immediately, as you are slowly being choked by the geometry of the bend.
