In flow measurement, which statement correctly describes the function of a weir?

In flow measurement, which statement correctly describes the function of a weir?

• A. It measures flow by head over a crest
• B. It measures flow by detecting magnetic field changes
• C. It measures flow by thermal conductivity
• D. It measures flow by visual observation of water level only

Answer: (A)

A weir works by creating a known water surface head as flow passes over its crest. The discharge is determined by how high the water is above the crest—the greater the head, the more water flows over. In formulas for a sharp-crested weir, the flow rate is related to the head raised to a 3/2 power and the weir’s length, with a coefficient that depends on the weir shape and conditions. This direct link between head over the crest and flow makes this method specifically about measuring head to determine flow. The other methods described aren’t how a weir functions: detecting magnetic field changes is how magnetic flow meters measure flow in pipes; measuring by thermal conductivity is a different technique used for some gases; simply observing water level without relating it to a calibrated head-discharge relationship wouldn’t yield an accurate flow rate.

A weir works by creating a known water surface head as flow passes over its crest. The discharge is determined by how high the water is above the crest—the greater the head, the more water flows over. In formulas for a sharp-crested weir, the flow rate is related to the head raised to a 3/2 power and the weir’s length, with a coefficient that depends on the weir shape and conditions. This direct link between head over the crest and flow makes this method specifically about measuring head to determine flow.

The other methods described aren’t how a weir functions: detecting magnetic field changes is how magnetic flow meters measure flow in pipes; measuring by thermal conductivity is a different technique used for some gases; simply observing water level without relating it to a calibrated head-discharge relationship wouldn’t yield an accurate flow rate.