The flammable limits of benzene are 1.4% to 8% by volume in air. The instrument shown in the illustration is used to sample a tank that had contained benzene and a 90% reading is obtained. What percentage of benzene vapor would be indicated at the sampling point? See illustration SF-0002.
• Combustible gas indicators (CGI) like the one shown typically read in percent of the lower flammable limit (LFL), not direct percent by volume in air. • Benzene’s lower flammable limit (LFL) is given as 1.4% by volume in air; the upper limit is 8%. • To convert a CGI reading (as % of LFL) to actual volume percent of vapor in air, you multiply the LFL by the instrument reading as a decimal.
• What does a 90% reading on this type of meter usually represent: 90% of what? Is it 90% of the full flammable range (1.4%–8%) or 90% of the lower limit only? • If the LFL is 1.4% by volume and the instrument shows 90% of LFL, how do you express that concentration as a decimal fraction (like 0.00xx)? • After you find the percentage by volume, compare it to the answer choices: which one matches your calculated value when written as a decimal fraction instead of a percent?
• Confirm that the scale on the illustration is marked in % of LFL (common for CGIs) and not in straight percent by volume. • Double‑check your percent-to-decimal conversion: 1% by volume equals 0.01 as a decimal fraction. • Verify you multiplied the LFL (1.4%) by 0.90 (for 90%) and not by 90.
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