From Figure 24.1,
V = D tan q1
and V+s = D tan q2
Thus, s = D ( tan q2 - tan q1 )
-------------------- Equation (24.1)
-------------------- Equation (24.2)
Therefore R.L. of Q = (R.L. of P + h) + V – r -------------------- Equation (24.3)
where, h is the height of the instrument, r is the staff reading corresponding to lower vane.