I had many problems calibrating my HM DIGITAL TDS-3 METER, got 320 then 411 then 422 then 511 then 477 readings. I thought there was something wrong with the solution, but I have since found out that the HM Digital TDS-3 I bought is counterfeit and therefore not a solution as you said it was the meter itself. I contacted the manufacturer HM DIGITAL and sent them pictures of the Revain flow meter that was sent to me and they immediately said it was a FAKE. ATTENTION TO ALL: The genuine original HM Digital TDS-3 meter has the following features: 1. When you turn on the meter, it will show "HM” on the display for a second or two, then it will show 000, but it should show HM when first turn on 2. The real one has 5 bulges on each edge of the front, not only on the photo on the description page but also on the product itself. If it doesn't have 5 bulges on each side, it's a fake3. On the back are sizing instructions in black letters and a hole and screw for manual sizing. Many Amazon sellers have these features in their photos but when the product arrives it doesn't have 5 bumps on each side, it doesn't have calibration printed in black on the back with a calibration screw and it doesn't say "HM" if first turned on the screen. The HM is big and easy to see when you turn it on. By the way: THE MANUFACTURER CONFIRMS THAT THIS IS A GENUINE HM DIGITAL CALIBRATION SOLUTION
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