Question 7

The Correct Answer is B **Explanation for Option B (Correct Answer):** Span gas is a precisely mixed gas standard, containing known concentrations of one or more specific gases (e.g., carbon monoxide, methane, or oxygen) diluted in a background gas (like nitrogen or air). It is essential for the **span calibration** of gas analyzers or detectors. Span calibration involves introducing this known concentration of gas to the sensor to confirm (or adjust) that the instrument reads the correct value for a specific target concentration, thereby ensuring accurate measurements across the instrument's operational range. **Explanation for Incorrect Options:** **A) To ensure an inert atmosphere during the span of time when a system is being brought to a dry air condition from a flammable atmosphere:** This process describes gas replacement or inerting (often using nitrogen or CO2) to manage explosive risks. Span gas is a calibration standard, not a bulk inerting agent. **C) To find the zero point on a gas analyzer:** The zero point (or baseline) is established using **zero gas**, which is typically ultra-pure air or nitrogen containing none of the target gas being measured. While zeroing is part of the overall calibration process, span gas is specifically used to verify the upper measurement range (the "span"), not the zero point. **D) To purge tanks in a gas-freeing process:** Purging tanks requires large volumes of inert gas (like nitrogen) or ambient air to displace hazardous vapors. Span gas is too costly and supplied in volumes far too small to be used for industrial purging operations.