I believe these meters measure the conductivity of water. Distilled water is a pretty good insulator, but add a little salt and it becomes a very good conductor of electricity. Any salt, (not just sodium chloride) due to the ionic nature, will enhance the conductivity of the water and as such be shown on the meter. To get a true reading of the salinity change that you are striving for, you really need to get a base line reading and then measure the change. The TDS meters are marketed with one of the features to show how contaminated the water is and then with RO or distilled water how much cleaner the water is. Note I said cleaner, not healthier. Due to most water supplies having some salinity, you can get some reading on tap water, but generally it is well below the readability of the salt meters. TDS measures generally in ppm, where the salt meters measure at 0.1ppt, or 100 times greater.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard