Minnesota Boiler License Practice Exam 2026 – All-in-One Guide to Passing with Confidence

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What is the approximate percentage of oxygen by volume in air?

21%.

The approximate percentage of oxygen by volume in air is around 21%. This value is based on the composition of the Earth's atmosphere, which is primarily made up of nitrogen (about 78%) and oxygen. The remaining percentage consists of other gases, such as argon and carbon dioxide. The 21% figure for oxygen is well-established and is crucial for various applications, including combustion processes and supporting life, as it provides the necessary oxygen for respiration in humans and animals. Understanding the composition of air is essential for anyone involved in boiler operations, where the combustion efficiency and emissions are influenced directly by the available oxygen.

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14%.

79%.

18%.

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