Flow sensors can be used to monitor the flow from a valve each time it is actuated. They work by watching for flow every time a valve is actuated and then transmitting a signal to the system . Once the system has confirmed flow, it doesn’t monitor the flow sensor again until the next valve cycle. As a result, if the duration is quite long, and flow is confirmed within the first .5 second, the system would not detect it if the flow stopped after that time until the next cycle. Another limitation is that the sensors can not differentiate between the flow of fluid or air. As a result, if the fluid reservoir runs dry so air is flowing through the valve instead of fluid, it would not create an alarm. In this case, a low level switch in the fluid reservoir would detect the problem. One other limitation is that since a flow sensor requires some time to sense flow, it is usually not possible to detect flow for a duration of less than about 250 milliseconds.