
65Potline Process Control Failure
of the measurement. Voltage or resistance ranges over, say, one
minute are averaged over ve minutes and then over a day for
every pot. So, any instantaneous or even prolonged noise events
for 10 or 30 minutes are averaged out of the daily results reported
in the smelter database. In fact the noise limits for “noise control”
are usually in the range 80 to 150 ηΩ, or 0.08 to 0.15 μΩ, much
higher when compared to the above scale.
• Comparing the pot-to-pot variation in the box plot, it is evident
that there are some pots that have been extremely unstable over
this two-month period, as well as a geographical group ...