From Figure 24.2,
V = D tan q1
and V-s = D tan q2
Thus, s = D ( tan q1 - tan q2 )
-------------------- Equation (24.4)
-------------------- Equation (24.5)
Therefore R.L. of Q = (R.L. of P + h) - V – r -------------------- Equation (24.6)
where, h is the height of the instrument, r is the staff reading corresponding to lower vane.