A vapor sample is drawn into the combustible...

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Question 1 of 205eccef9ebacbc34ba4d465a9

A vapor sample is drawn into the combustible gas indicator shown in the illustration. The needle deflects to the explosive range of the scale and then returns to rest at 20%. What does this indicate? See illustration SF-0002.

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🔍 Key Concepts

• Combustible gas indicator (CGI) scales and what they are actually calibrated to measure • Difference between percent gas by volume and percent of the lower explosive limit (LEL) • What it means when the needle briefly goes into the explosive range and then settles back to a lower value

💭 Think About

• Ask yourself: Is this type of meter normally marked in actual percent gas, or in percent of some reference value such as LEL? • What would it mean physically if the atmosphere were actually above the upper explosive limit—how would the meter behave? • Why might the needle first swing high (into the explosive range) and then fall back and stabilize at a particular percentage?

✅ Before You Answer

• Check how most portable CGIs are calibrated (what the scale numbers represent). • Verify whether a reading that settles at 20% would normally be interpreted as 20% gas by volume or 20% of LEL. • Consider whether a condition above the upper explosive limit would give a stable reading at 20% or cause some other indication.

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