The total alkalinity is the buffer of pH. If the TA is not correctly balanced then the pH will give you an incorrect reading. pH is controlled by the Total Alkalinity.
Here is a simple way to understand the difference between pH and total alkalinity.
It is often helpful to think of your pH as the thermometer on your heater's thermostat. The thermometer (pH), accurately measures the temperature of your room.
When it is cooler than you want you to adjust the thermostat up. This is the ability to control the temperature. Just like the thermostat, total alkalinity gives you the ability to control pH.
Which is why you always want to test and adjust the total alkalinity before testing any other water balance including the pH level.
You want to keep the total alkalinity readings for your spa water between 80 to 120 PPM.
However, if you believe your pH is a bit high, adjust your TA up slightly above 120 PPM because of chemicals that lower pH, lower alkalinity at the same time.
If the total alkalinity in your spa is not at the correct level it can result in these issues:
Total Alkalinity is too high:
The pH is difficult to regulate.
Scale forms on spa surfaces.
The water becomes cloudy.
Skin and eye irritation including rash.
Total Alkalinity is too low:
The pH is difficult to balance and change quickly. Corrosion on metal equipment. Skin and eye irritation including rash.
So start with the total Alkalinity then balance your pH.
Remember that using a chlorine based sanitiser will change the pH in your spa.