What's an expensive one cost? As far as aquarium grade, you know what they say, as soon as a manufacturer puts the 'aquarium' label on it, the price doubles. TDS meters are used extensively outside the aquarium trade and I'm sure the basic design has been around for some time. I've never seen one in person, but from the photos etc. on various vendors' web pages they don't look that complex. Like most probe type instruments that's basically it, a metal probe(s) and the associated circuitry inside. Once you've paid the engineer to design the circuitry, the rest is gravy..... and marketing.
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