The can be digital or analog. They can be low or high amperage. They can be no ripple to high ripple. (I will not sell high ripple models anymore by the way. They can have have thick plates that give off more negative ions or they can have the same thickness of metal plates that is not certified steel (non 316L medical grade steel) that leaches chromium in the water that fools poeple to think they are detoxing when its really the metals leaching that are reacting. The higher the amperage going to the metal plates, the more leaching of the metal into the water. The way to see if you need a higher amperage foot bath is to look at the water at the end of teh session when the rusts sink to the bottom and the bodily wastes float on top. It takes about 10 minutes to really find out after the session to find out if anything is remaining on the top of the water. If nothing came out of the body through "detox" as they claim, the water on top will be nothing floating on top and the particulate will be on the bottom.
The ionic foot spas can also be set to negative mode. This moves all the electricity to the negative pole of the array and it produces positive ions. There are only a few companies that make these type. Many people need to lower their pH of their blood and also bring their body up to a more acidic state when its too alkaline.
NOTE: Some brands are dual polarity and some single polarity. Some only produce negative ions, some produce both at the same time, and some can produce positive ions only.
Does the footbath have a digital display? While this may seem convenient and modern, the manufacturer will have to convert some of the DC current into AC to run the display. This reduces the efficiency of the footbath which requires DC to run the array and will also cause a trickle of that converted AC to flow back or “ripple”.