Referring to Figure 9.4 (a), determine the maximum measurable velocity if the maximum Pitot tube scale length

Question:

Referring to Figure 9.4 (a), determine the maximum measurable velocity if the maximum Pitot tube scale length is 25 cm. The pipe fluid is water, which extends all the way to the manometry fluid, which is mercury with a specific gravity of 13.6. Estimate the maximum discharge in the 10-cm-diameter pipe. Why is it just an estimate?