What is a SALT WATER HOT TUB and how does it work?

Salt used in spas (and pools), is actually a chemical compound- NaCl Sodium Chloride.  Sodium Chloride is the base element of all chlorine spas and pools.  When you go to your local pool/spa supply store and purchase your chlorine, you are purchasing ACTIVATED CHLORINE.  Active chlorine is one that the chlorine molecule has been freed from the sodium bond and has now become chlorine.  When the chlorine comes in contact with things like bacteria and and other organics, it breaks it down and then reverts back to NaCl. 

MYTH* Salt requires topping up in a salt water hot tub.  No.  Salt is a mineral and does not evaporate with the water nor does it leave on its own.  The only way salt leaves the spa, it is splashed out or there is a leak.

Chlorine (and bromine) are effective sanitizers for pools and spas because of the staying power of these types of chemicals.    Meaning, chlorine and bromine do not just gas off as soon as they leave their perspective vessels ie, the container you purchased, or when they are added to water.  These are very stable elements and are actually quite durable compared to other sanitizers.  They can survive in water as an active chemical.  When you measure your chlorine in your pool or spa, you are measuring FREE CHLORINE, or active chlorine.  According to health Canada, chlorine and bromine are the only suitable and allowable sanitizers for use in a pool and spa. VIEW HEALTH CANADA LAW HERE

A salt system in a spa (or pool) is designed to work by reactivating the NaCl molecules and generate new active chlorine (or bromine).  Typically using graphite electrodes that are either in-line inside the plumbing or a secondary device in the body of water creating an electrolysis action that frees the chlorine molecule, making free and active chlorine.  And since the NaCl is always there, the system simply recycles the action ongoing.  There is very little additional active chlorine needed to enter the spa or pool. 

There is a controller too... The controllers that you see for the salt system, simply control either the strength or the run time of the electrode.  The longer and/or stronger it runs, the more chlorine it will generate.  See Cal Salt Option