We have a pool with 18% salt (a mixture of NaCl, CaCl2 and MgCl2). Is there a digital device [refractometer] that can measure the % of salt in the water?
I have some concerns about your ability to measure percent salt in a multi-component mixture with a refractometer. You see, refractometers are very good at measuring binary (two-part) mixtures such as sodium chloride and water, where the refractive index value of water is a constant (static or known) and sodium chloride is the variable. However, when there are many different components in a solution, such as sodium chloride (NaCl), calcium chloride (CaCl2), and magnesium chloride (MgCl2), it is difficult to tell what the contribution is that each component makes to the total refractive index.
So, if the refractive index changes, it is impossible to say, with any certainty, which of the component parts has changed. Since a refractometer only measures the total refractive index of a solution and cannot selectively read the refractive index of one particular component. All water soluble fluids look the same to the refractometer. Therefore it will be impossible to get a reading that is meaningful.
The Palm Abbe refractometer does have scales for binary solutions of sodium chloride (NaCl), calcium chloride (CaCl2), and magnesium chloride (MgCl2), but cannot measure a mixture of all three.