Gas monitors for welding and hot work: what to measure

Which gases does welding produce?

OSHA’s welding fact sheet lists the process gases: nitric oxide, nitrogen dioxide, carbon monoxide, ozone, phosgene, hydrogen fluoride and carbon dioxide. Ozone and nitrogen oxides come from the arc acting on the air around it. Carbon monoxide can form during the work, and OSHA calls it a serious asphyxiation hazard. Phosgene and hydrogen fluoride come from chlorinated degreasers and fluoride fluxes in the path of the arc; the standard requires degreasing to be kept out of the way of welding for that reason. The shielding gases themselves, argon, helium and carbon dioxide, displace oxygen in the air and can suffocate a worker in a confined or enclosed space.

The limits those gases are held to are the 1910.1000 limits: carbon monoxide 50 ppm over 8 hours; nitrogen dioxide a ceiling of 5 ppm; ozone 0.1 ppm over 8 hours. NIOSH recommends lower: 35 ppm CO with a 200 ppm ceiling, 1 ppm nitrogen dioxide for 15 minutes, and a 0.1 ppm ceiling for ozone. The fume metals, iron oxide, manganese, zinc oxide and hexavalent chromium, have their own limits and are measured by sampling, not by a gas monitor.

What should the monitor read, and when?

1. Before the arc: LEL and oxygen, from outside the space. Test the tank, pit or room for flammables and oxygen before anyone goes in and before the torch is lit. For a container, test after cleaning and venting, at the bottom and in any hollow. A reading above 10 percent LEL stops the job; purge and retest.
2. During the work: oxygen and CO on the welder. In a confined or enclosed space the shielding gas and the CO build up around the work. A four-gas monitor clipped to the welder’s collar reads oxygen falling and CO rising and alarms before either reaches the limit. Continuous monitoring of a permit space is required; the monitor is how it is done.
3. Arc or torch work in an enclosed space: add NO2. Nitrogen dioxide has a 5 ppm ceiling and is not on the standard four-gas set. A single-gas NO2 monitor on the welder, or a multi-gas unit with an NO2 sensor, covers it. Ozone is rarer to monitor and has its own sensor where it is a known problem.
4. Pumped unit with a probe for the re-check. When the job stops for a break or a shift change, a sample-draw monitor with tubing reads the bottom of the vessel before re-entry without anyone leaning in.
5. Fire watch with the LEL reading. The half-hour watch after the work is also the time pooled vapor can drift back toward hot metal. The LEL reading on the watch’s monitor stays in view.

What about construction sites?

The construction welding standard carries the same rules in its own numbers: welding, cutting or heating in a confined space requires general mechanical or local exhaust ventilation, airline respirators where ventilation cannot be provided without blocking the way out, and an outside attendant in communication with the welder; oxygen shall not be used for ventilation. Confined spaces on construction sites fall under Subpart AA, which has its own atmospheric testing and monitoring duties.