I know that in MMA manufacture, 100% H2SO4 was one reagent which was manufactured by mixing commercially available 98% H2SO4 with approx 13% oleum.The mixing is controlled by conductivity. Unfortunately I no longer have the data, but I remember that conductivity depends upon ions in solution,and H2SO4 ionises in aqueous solution to H+ and SO4-- ions.

As sulfuric acid becomes progressively more concentrated,there is less water around and  the concentration of H+ and SO4-- ions becomes less, leading to a fall in conductivity.

Beyond 100% H2SO4, the solution of SO3 exhibits conductivity which increases as the concentration of SO3 increases.

This situation is not good for control, and one has to decide whether to maintain the control regime above or below the minimum point so that the change in conductivity with concentration monotonically increases or decreases within the control regime chosen.  

Tim,

conductivity works up to 99.99% point just short of oleum. It also works in oleum: the problem is that the conductivity meter does not distinguish between the two cases.

Going to oleum is the big worry, because the conductivity meter will produce a false display (in this case weak acid).

Suspect that you are operating in the oleum regime-it gives the impression of a non-linearity on your panel/screen display.

Causes: stratifcation in your pump tank do to poor dilution and mixing.

Cut you dilution water, if you are producing oleum (if you can't see it in the stack already), your indicated acid strength will begin to increase. If you are in the acid regime, the concentration will begin to drop.

If the acid strength rises when you cut dilution, just stay the course until it bottoms out and starts back up. Then you can go back to normal dilution control.

Take a look at your diluter. There are special designs that provide good mixing. If you are just hosing water to the pump tank, then you've got a potential problem.

I can send you some conductivity data. Good data is hard to come by.

I need some information regarding "Contro Tracer" on piping.
Where I can found information?