ABSTRACT
The cause of error, like beauty, is in the eye of the beholder. An example can illustrate this.
CASE HISTORY 1.1 A Tank Overflow
On a fine early spring morning, operators in a refinery control room received an alarm from a storage-tank level instrument. It indicated a high level in the tank and a potential for overflow. There were two level sensors on the tank, one using the radar principle and the other using a float, a wire (Figure 1.1) and a counterweight. The radar sensor gave the high-level alarm; the float chamber sensor indicated that everything was OK. The radar sensor had failed several times earlier over the previous year; the float chamber sensor had never been known to fail. The operators made no immediate response; the problem was noted to be checked on the next inspection round, and a work-order request was made for maintenance on the radar sensor.
