Criterion for cathodic protection

It can now be demonstrated that the copper/copper-sulphate electrode cannot be regarded as a fixed potential against which a voltage can be measured for cathodic protection survey work.  This raises the question of interpretation and analysis of the millions of voltages that have been recorded over the years and are still being recorded world wide.
It is clear that there can be no criterion for the achievement of protection using a voltage that is measured between to floating potentials.  The established way of presenting this is a straight line at -0.850 volts on a graph showing distance X and voltage Y but if both potentials are floating then the -0.850 volts is also floating.
What is needed is a corrosion cell that can be monitored at each location as close to the pipeline or structure as possible.  The arrangement of coupons that is called the Alexander Cell is just this.
This will show the actual electrical equilibrium at which corrosion stops and can be related to the pipe-to-soil voltage taken at the same spot.