Final answer:
The reading of the black hand on a duplex air gauge indicates the actual air pressure. While the pressure reading range can start at 28 lbs and go up to 100 lbs, the normal operating pressure will be specific to the system's specifications.
Step-by-step explanation:
The question pertains to the reading of the black hand on a duplex air gauge when operating normally. Duplex air gauges are instruments used to measure pressure in various industrial and engineering applications. The gauge typically has two pointers, a black and a red hand. The black hand indicates the actual air pressure within the system, while the red hand can be set as a marker or warning indicator.
It is common for these gauges to measure pressures in a range that can start as low as 28 lbs and go up to 100 lbs or beyond, depending on the specifications of the gauge and the requirements of the system it is being used in. In practice, the normal operating range for such a gauge will be determined by the specifications for the equipment it is monitoring.
The black hand of the duplex air gauge is used as an ammeter. The reading on this gauge depends on the full-scale reading. If the full-scale reading is 20.0-A, the reading should be approximately 28-100 lbs. If the full-scale reading is 100-mA, the reading should be the same as well.