Apparatus and method for semiconductor substrate processing are presented. For devices such as valves used for semiconductor substrate processing especially a process like ALD, there is a need to monitor and control the exact time taken from the signal to open and close the valves so that delay times may be controlled. In an embodiment, an apparatus comprising a reactor, a valve, a process controller and a valve monitor system is presented. The process controller may be operationally connected to the valve and may be provided with a memory. The sensors may be either electrical or optical sensors.