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\large
$$
\left( {\Delta h_{FCM} A_{float} \rho g} \right)L_{float\;lever\;arm}  = \left( {\Delta h_{stock} \rho gA_{orifice} } \right)L_{valve\;lever\;arm} 
where \Delta h_{FCM} is the change in water level in the constant head tank, 
$$
{latex}