Aqua Rite needs constant recalibrating

I have to keep recalibrating the Aqua rite. The instant salinity only displays a dash. The recalibration seems to work for a day and then goes to null again. The problem seems to have appeared when I installed a Hayward variable speed pump. The pump runs at low speed for 12 hours and a high speed for another 12 hours. No gaps in between.