Journal
Why a fifteen-minute interval will not show a three-minute spike
A vibration pen that stores a point every fifteen minutes will not confess a knock that lasted three. We can draw a handsome figure from that file and still miss the event the electrician felt with a hand on the casing.
When a log arrives, the first line we look for is not the highest value. It is the interval. If the question is “did the motor spike when the mill started,” and the interval is a quarter of an hour, the honest reading is that we cannot see the start. We can see whether the quarter-hour that contains the start sat higher than its neighbours. That is a different sentence.
People sometimes ask us to “fill in” a finer line. We will not. Inventing points between stored values is a second instrument, and it is not the one that sat on the machine. If you need the three-minute story, the logger has to keep it. Until then the figure should look as coarse as the file.
If you are preparing a commission, write the interval on the brief in the same words as the logger menu: one minute, five, fifteen, hourly. If the interval changed mid-series, say where. A figure that mixes two intervals without a mark will argue with anyone who was in the room that week.