A 10" diameter tank will most commonly contain 1.5 cubic feet (ft3) of media.
Although Salt settings are most often stated as lbs per ft3, many softeners require the total salt amount to be programmed for the specific amount of capacity to be regenerated. For for a softener containing 1.5 of resin, a 6 lb/ft3 setting will require 9 lbs salt total, which will typically regenerate 31,500 grains of useable capacity per cycle.
Lower, more efficient salt settings involve compromise as hardness leakage through the resin will increase in proportion to the salt and capacity settings. Since less salt will regenerate less capacity, the regeneration frequency will be increased, so a greater amount of water will be consumed for regeneration (=lower water efficiency) when considered on a monthly or annual basis.
For the best balance of soft water quality, useable capacity and salt efficiency, 8 lbs/ft3 is usually recommended to regenerate 36,000 grains of useable capacity in 1.5 ft3 resin, which will provide a hardness reduction efficiency of 3,000 grains per lb of salt.
The amount of hardness leakage (= soft water quality) stated in the chart below, is based on the resin initially containing the maximum hardness reduction capacity. Since 1 ft3 resin will typically have a maximum total hardness reduction capacity of 32,000 grains, then 1.5 ft3 will total 48,000 grains capacity.
Because a lower capacity setting will cause regeneration to occur sooner, the resin's total capacity will not be depleted so less salt will be needed to 'top up' the resin's capacity.
As service water flow is downward through the resin, the resin's capacity near the top of the tank will become most depleted, with less depletion occuring progressively downward towards the bottom of the resin bed. Because the backwash cycle will reclassify the resin granules within the tank, many of the most depleted resin granules from the top, will be moved throughout the tank including to the bottom.
Since the brine will become progressively weaker as it flows down through the resin, a low salt dose will more progressively have less regenerative strength as it flows to the bottom of the tank, so the depleted resin closer to the bottom, will not be fully regenerated, and so that resin will not be as effective in reducing hardness, which will result in higher hardness leakage.
Although the resin granules will be reclassified during each backwash cycle, the amount of not fully regenerated granules throughout the tank, will eventually remain consistant for each salt amount, which is how the hardness leakage amount was determined for each salt setting.
