Because the resulting O2 concentration of 7 Vol% is to close to the required concentration of O2 we have to do this twice times to get an Oxygen level in the range of 2 to 3 %.

• 因为氧气的体积浓度为7%25，接近燃烧所需要的氧气浓度值，所以必须进行两次这样的操作，使用氧气浓度范围为2-3%25。