We use ORP to control the bleach feed in the cooling towers and we use Free Chlorine Sensors in the retorts to feed bleach.
Why the difference?
ORP is less accurate. Reading ORP is like telling time by where the sun is in the sky. Not very precise, but very hard to miss. Free Chlorine Sensors like we have in the retorts are more precise but the sensors are prone to fouling, especially from the kind of dirt that a tower drags in.
ORP sensors are less prone to foul and also more rugged. They can be put in pressured sample lines. Chlorine sensors need to test water that is flowing past it to a drain or open tank. This water loss is not a big deal for retorts that run all time and have massive amounts of water, but for a tower which turns on and off and may be off for a long time, this constant drain can be troublesome.
Also, at the higher pHs, like we run in the cooling towers, the Free Chlorine Sensors are less accurate than they are where the pH is stable and low in the retorts.
In sum, we use Free Chlorine sensors in the retorts because it is generally cleaner water, we need more precision, we don’t mind the water wasted in sampling, and they have their best accuracy where the pH is stable and on the low side. We use ORP in the cooling towers because the foul less often, can take pressure, precision is not as important, and they are not effected by high pHs or pH fluctuations.