I just installed a Aqua trol RJ on my 18' above ground. I get a very low salt reading, even after trying to calibrate. The water tests at 3100ppm with two electronic testers at the local pool store. My aqua trol unit shows 2600 as the highest number during calibration. Obviously this is too low for it to operate, so I get the check cell(it's clear and clean) and check salt LEDs on. Any ideas?