Note:  In a salt water chlorinated spas you do not want to worry about the pH. The sodium chloride (salt) will always make it rise, because of the sodium.  You will never get it down and keep it there.  On a salt pool just worry about the TA(total alkalinty) and you will save yourself much grief and aggravation.  

On a commerical spas you will still have to balance the pH because of the health departments being years behind on proper water chemistry in a salt pool.  The goverenment is very slow to react to new technology in water treatment.  If they would see the advantage of a saltwater chlorinator they would change the laws quickly and require them, so bathers do not get any water disease's