ppm stands for parts per million
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PPM Part per million. But measurements are made in the conductivity of the solution. Conductivity = The ability to pass electricity between electrodes a set distance apart. So basically pure water barely passes electricity and as you up the solution the conductivity goes up and therefore passes more electricity.
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Exactly! It scales with the temperature of the solution too. Cheap TDS meters typically do not include any logic to automatically account for the temperature of the solution. The common ~$15 USD TDS (PPM 500) meter sold on amazon/ebay is only accurate at 77 degrees F as I recall it. If your meter measures your hot and cold taps as 100’s of PPM’s different, that meter isn’t back-calculating TDS by removing heat from the equation.