There are three common modes of control: manual, digital pulse and analog input. Manual is simple control based on manually adjusting stroke length and frequency to obtain the desired flow rate. Digital pulse is a discrete contact or optical signal that can be read by the control module and converted to strokes per minute. Many chemical injection pumps have multipliers and dividers for the discrete contacts, allowing greater flexibility in the output range proportional to the pulse signal. Analog control is typically a 4-20 milliamp signal, which varies the stroke frequency in a linear fashion proportional to the output of the device, such as a flowmeter or controller. For example, a pH controller may be set to pH 7 which is equal to 4 mA and pH 9 is equal to 20 mA. The pump has 0-360 strokes per minute. A pH of 8 would be the middle of the scale equal to 12 mA and the pump would be running at 180 strokes per minute.