I am trying to monitor and trend a signal that has a scale of 1 to 1000. Typically, the signal will trend from about 40 to 280 over a 10 minute period. Periodically, spikes 2 to 4 times the normal signal occur. I need to filter out this signal spikes from the input signal so that the trend is more uniform. I know there has to be a mathematical formula that will allow a range of values to pass and filter those out of the range. But, I also need the range to shift so that the trend is accurate. Any mathematical ideas?