Why does a v-notch weir will give a more accurate indication of low flow rates than a rectangular weir?

Question

This is an engineering question I must solve. I know that a v-notch weir will provide a more accurate determination of flow rate than a rectangular weir but I don't know why.

Answer 1

because it's more accurate to work out the cross-section area of a shallow triangle than of an even shallower rectangle !

Answer 2

Consider this:
A rectangular weir with a width of 2 ft trying to measure a flow rate of less than 1 gallon per minute vs a rectangular weir with a width of 2 inches and the same flow rate. The height of the liquid on the two inch weir plate would be much higher therefore a higher resolution at low flow rates.

A V-notch weir plate works similarly low flows require a much higher head (level) to produce the same flow as rectangular wier plate.

Answer 3

Draw a graph of the two type of notches for the same given flows. You will see a much larger slope on the v-notch, thus it is more accurate to measure the larger differences.