Final answer:
The normal operating reading for the red hand on a duplex air gauge is generally around 90-100 psi, a range that is common for many systems in rail transport and industrial applications.
Step-by-step explanation:
The reading of the red hand on the duplex air gauge, when operating normally, should be approximately 90-100 pounds per square inch (psi).
Duplex air gauges are often used in rail transport and various industrial applications to monitor air pressure, where maintaining the correct pressure is critical for safe and efficient operation.
It's worth noting that the proper operating pressure may vary depending on the specific equipment and application, but a range of 90-100 psi is typical for many systems.