Final answer:
The normal reading of the black hand on the duplex air gauge should fall between 14 and 98 psi, representing gauge pressure. To get the absolute pressure, one must add the atmospheric pressure, usually about 14.7 psi, to the gauge reading.
Step-by-step explanation:
The normal reading of the black hand on the duplex air gauge should be approximately in the range specified, anywhere from 14 to 98 pounds per square inch (psi). When considering pressure measurements, it's important to distinguish between gauge pressure and absolute pressure. For example, if a tire gauge reads 34 psi, that is the gauge pressure. The absolute pressure is what you actually have in the tire, which includes the atmospheric pressure, typically about 14.7 psi at sea level. Therefore, the absolute pressure in this case would be 34 psi + 14.7 psi = 48.7 psi. This concept is critical to understanding how pressure devices, including gauges, function, as the pressure results in a force that is converted into a readable output.