# Step-by-Step Solution ## Problem Restatement You need to determine the correct method to check for a leak in a vacuum stack. The options are: A) Apply foam and watch for decay B) Apply soap solution and watch for decay of bubbles C) Apply soap solution and watch for increase in bubbles D) Add a chemical smoke and watch for smoke to be drawn into leak ## Step 1: Understanding the Context - **Vacuum stack:** A pipe or chamber under negative pressure (lower than atmospheric pressure). - **Leak detection:** The goal is to find where air is entering the vacuum system. ## Step 2: Analyzing Each Option ### Option A: Apply foam and watch for decay - **Foam decay** is typically used in systems under positive pressure, not vacuum. - Not a common vacuum leak test. ### Option B: Apply soap solution and watch for decay of bubbles - In a vacuum, leaks **pull air in**. Applying soap may cause bubbles to be drawn in, but "decay of bubbles" is not a standard indication. ### Option C: Apply soap solution and watch for increase in bubbles - For **positive pressure**, leaks **push** air out, creating bubbles. - In a **vacuum**, air is drawn **in**, so bubbles forming is unlikely. ### Option D: Add chemical smoke and watch for smoke to be drawn into leak - If you produce smoke near the suspected leak, in a vacuum, the leak **draws smoke in**. - You can visually see smoke pulled into the leak — a standard method for vacuum systems. ## Step 3: Selecting the Best Answer - **Option D** matches standard vacuum leak detection techniques. ## Final Answer **D) Add a chemical and watch for smoke to be drawn into leak.** This is the correct and commonly used method for checking leaks in vacuum systems. The smoke is visibly drawn into the point of leakage, making it easy to locate the leak.