'Start value monitor.'

def start(self):

    if self.started:
        self.stop()
    self.old_value = [0 for i in range(self.num_of_params)]
    self.num_of_get_value = 0
    self.log = []
    self.end_event.clear()
    self.start_time = time.time()
    self._get_value()
    self.started = True
    if self.continuously:
        self.monitor = FileFieldMonitor.Monitor(self)
        self.monitor.start()
