Changes in flow and pressure can cause fluctuations in the level of chlorine passing through the membrane. These changes, in turn, will affect the reading.  All existing amperometric sensors need constant flow, usually by using a waste stream, to maintain their accuracy and calibration. Water consumption by water treatment sensors is often overlooked. Unless a sensor is certified to NSF61, the water that touches the sensor cannot be put back into a potable water tank. One drinking water utility estimated water usage of 14.7 million L/yr (3.8 million gal/yr) for eight of its facilities when using conventional amperometric sensors — just to measure chlorine.