Group B : Atmospheres containing Butadiene, ethylene oxide, propylene oxide, acrolein, or hydrogen (or gases or vapors of equivalent hazard to hydrogen, such as manufactured gas).
EESiFlo North America, Terminology and Glossary: Explosion Proof & Hazardous Areas, Pennsylvania, USA 2002, flowmeterdirectory.com/flowmeter_terminology, (Mrtm/ Explo Proof and Haz Areas - Amer) {561}