Others have commented that ORP sensors are inaccurate. On my IntelliChem, I shift the ORP setpoint based on Free Chlorine measurement. Refilled the pool a week ago (glass tile removal -- empty to avoid shards left behind) and the ORP matching my desired FC decreased to 600mV, when before it had to be 720. Refill water measured 180ppm calcium, standard Vegas water reports about 550ppm TDS. Pool had been about 1100ppm calcium, and planned one more season before refill, but repair changed that.