The normal reading of the black hand on a duplex air gauge should be within the range of 28-100 lbs, but without further context or system specifications, an exact value cannot be provided from the options given.
The question pertains to the reading of the black hand on a duplex air gauge, which is a device used to measure pressure. While the question specifies a range from 28 to 100 lbs for the normal operating pressure, it does not provide enough context to determine the exact value within this range. Consequently, without further context about what normal operating conditions entail for the specific gauge in question or the system it is used in, it is not possible to provide a definitive answer from the options given (28, 50, 75, 100 lbs).
Gauges, in general, are calibrated to show zero at atmospheric pressure and display positive values when the pressure is greater than atmospheric. This could be a contributing factor in determining the normal operating reading for a pressure gauge. However, due to insufficient information provided, we cannot confidently choose among the multiple-choice options.
So, the normal operating reading on a duplex air gauge pressure would typically be within the specified range, but the exact pressure depends on the specific application or system requirements, which are not detailed in the question.