User’s Guide

Alert Types

delta <=

A delta <= alert is used to detect a rapid change in a sensor reading over a period of time. For example, it may be used to detect a sudden drop in a tank level reading. The key parameters are Check Window, which specifies the time or number of samples over which to check for a sudden drop, and Limit, which is the largest tolerable drop. For example, suppose a delta <= alert is set with Check Window = 0:25 and Limit = -10. Assuming the unit is set to sample every 5 minutes, the check window will include 6 samples taken at 0,-5,-10,-15,-20 and -25 ago minutes. Suppose the readings are as seen below.

0 -5 -10 -15 -20 -25
298 301 302 350 351 353

A drop greater than 10 occurred between 10 and 15 minutes ago. The delta <= alert computes the drop between two samples as shown below.

0 to -20 -5 to -25
-53 -52

Since both drops are <= limit of -10, the alert will be triggered.

The delta <= alert uses two deltas (difference between first and second to last, difference between second and last) from the check window rather than one to filter out spurious readings. Consider the samples shown below.

0 -5 -10 -15 -20 -25
345 301 347 350 351 353

The sample from 5 minutes ago is spurious. The delta <= alert computes the drop between two samples as shown below.

0 to -20 -5 to -25
-6 -52

Not all deltas are <= -10 so the alert will not be triggered. The penalty for filtering out such spurious readings is that the alert will be delayed by one sample period, typically 5 minutes.

For the delta <= alert to function correctly, a check window that includes at leat 4 samples must be specified. If not, the alert will never check or trigger.