They not only meter the right amount of liquid, but they are also instrumental in controlling the uniformity of the spray over the entire width of the sprayer boom. Nozzle flow rate depends on the size of the nozzle orifice and the pressure. Usually, flow rate increases as pressure increases. In other words, doubling the pressure does not double the flow rate. The pressure must be increased by a factor of 4 to double the flow rate.
Manufacturers supply tables of nozzle flow rates at various pressures.
Most of these tables, however, are developed by measuring the flow rate of water through the nozzle. Flow rates of other liquids and chemical solutions differ from the flow rate of water. Most farm chemicals are more dense, or viscous, than water and have lower flow rates. This is one reason why sprayers must be calibrated for each type of liquid applied. ISU extension publication PM-817 outlines a simple calibration procedure.