A water softening system, said system comprising: a water treatment tank containing a resin bed for a water softening ion exchange process, the water treatment tank being configured such that water flows into said water treatment tank via an inlet at a bottom of the water treatment tank and such that said water flows in an upflow direction through said resin bed for the purpose of the water softening ion exchange process in the resin bed; and a sensing device arranged in the water treatment tank, said sensing device comprising: a body having a sensing material disposed therein, the sensing material being subject to a change in size in response to an ionic composition of water flowing upwardly therethrough, said water comprising a portion of the water upflowing through the resin bed; an actuating means disposed within the body and associated with the sensing material; wherein the actuating means is movable between a first position and a second position in response to the change in size of the sensing material to thereby mechanically actuate a switch member, such that when the actuating means is in the first position, the switch member allows a portion of the water flowing upwardly through the sensing device to flow through an outlet of the sensing device to thereby hydraulically actuate an ion regeneration process of the resin bed; and when the actuating means is in the second position, the switch member prevents any water flowing through said outlet; wherein the sensing device is arranged in the water treatment tank such that a liquid inlet of the sensing device is positioned below an upper surface of the resin bed such that a portion of the water upflowing through the resin bed will enter the sensing device and thereby be detected by the sensing material 30 of the sensing device prior to ion depletion of the resin bed. a water treatment tank containing a resin bed for a water softening ion exchange process, the water treatment tank being configured such that water flows into said water treatment tank via an inlet at a bottom of the water treatment tank and such that said water flows in an upflow direction through said resin bed for the purpose of the water softening ion exchange process in the resin bed; and a sensing device arranged in the water treatment tank, said sensing device comprising: a body having a sensing material disposed therein, the sensing material being subject to a change in size in response to an ionic composition of water flowing upwardly therethrough, said water comprising a portion of the water upflowing through the resin bed; an actuating means disposed within the body and associated with the sensing material; wherein the actuating means is movable between a first position and a second position in response to the change in size of the sensing material to thereby mechanically actuate a switch member, such that when the actuating means is in the first position, the switch member allows a portion of the water flowing upwardly through the sensing device to flow through an outlet of the sensing device to thereby hydraulically actuate an ion regeneration process of the resin bed; and when the actuating means is in the second position, the switch member prevents any water flowing through said outlet; wherein the sensing device is arranged in the water treatment tank such that a liquid inlet of the sensing device is positioned below an upper surface of the resin bed such that a portion of the water upflowing through the resin bed will enter the sensing device and thereby be detected by the sensing material 30 of the sensing device prior to ion depletion of the resin bed. a body having a sensing material disposed therein, the sensing material being subject to a change in size in response to an ionic composition of water flowing upwardly therethrough, said water comprising a portion of the water upflowing through the resin bed; an actuating means disposed within the body and associated with the sensing material; wherein the actuating means is movable between a first position and a second position in response to the change in size of the sensing material to thereby mechanically actuate a switch member, such that when the actuating means is in the first position, the switch member allows a portion of the water flowing upwardly through the sensing device to flow through an outlet of the sensing device to thereby hydraulically actuate an ion regeneration process of the resin bed; and when the actuating means is in the second position, the switch member prevents any water flowing through said outlet; wherein the sensing device is arranged in the water treatment tank such that a liquid inlet of the sensing device is positioned below an upper surface of the resin bed such that a portion of the water upflowing through the resin bed will enter the sensing device and thereby be detected by the sensing material 30 of the sensing device prior to ion depletion of the resin bed.