Oilers/Plant Tenders (HHC) Civil Service Practice Exam

What is the equivalent absolute pressure when a compound gage reads 14 inches of vacuum and the barometer reads 29.76 inches of mercury?

1. 15.76 inches of mercury

2. 13.76 inches of mercury

3. 29.76 inches of mercury

4. 14 inches of mercury

The correct answer is: 15.76 inches of mercury

To determine the equivalent absolute pressure from a vacuum gauge reading, you need to understand the relationship between gauge pressure and absolute pressure. Absolute pressure is the total pressure measured relative to a perfect vacuum, while gauge pressure is measured relative to atmospheric pressure. In this scenario, the compound gauge reads 14 inches of vacuum, which means it indicates a pressure that is 14 inches below atmospheric pressure. The barometric reading indicates the current atmospheric pressure, which is given as 29.76 inches of mercury. To find the absolute pressure, you would add the barometric pressure (29.76 inches of mercury) to the vacuum reading converted to a positive pressure. Since the gauge reads 14 inches of vacuum, it indicates that the absolute pressure is lower than the atmospheric pressure by that amount. Thus, you can express this mathematically as: Absolute Pressure = Barometric Pressure - Vacuum Pressure So, it can be calculated as follows: Absolute Pressure = 29.76 inches of mercury + 14 inches of vacuum Absolute Pressure = 29.76 inches of mercury + 14 inches Absolute Pressure = 15.76 inches of mercury This calculation confirms that the equivalent absolute pressure is indeed 15.76 inches of mercury, making it the accurate