GH is a measure of the divalent atoms in the water. Calcium is the most common divalent atom, and when water authorities quote hardness, they are measuring the amount of calcium in the water. TDS measures all the ions in the water not just calcium. So there is a sort of relationship - if there is a lot of calcium in the water, TDS will be high. But because TDS includes other things as well, you can't convert one to the other.
At one time, I assumed that GH measured everything. My son worked for a water testing company so I asked him. Although he didn't work in that section, he checked for me and said that they just measure calcium as calcium carbonate for hardness and he thought our GH test kits worked the same way.
I don't know if there could be a meter for measuring just calcium in the water. TDS meters work by measuring the electrical conductivity of the water. Water itself has a conductivity of zero and it's the ions in the water that allow electricity to pass through. The amount of current that passes is directly related to the amount of ions. Can you make a machine that measures just one type of ion cheaply enough to use in the home?
