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Thread: TDS meters... do they drift?

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    Rocketcaver's Avatar
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    TDS meters... do they drift?

    JimScott's question about RO units got me thinking about my water.
    Our tap water comes in at around 200ppm, so I use either collected rain water, or buy RO water at the local Kroger store. That usually measures around 4ppm if I catch the RO unit after a recent servicing.
    I use a cheapo TDS meter bought from Amazon several years ago. Should I worry about it drifting out of calibration?
    My current readings agree pretty well with past readings from various sources, so I'm not worried about the current calibration, I was just curious about how well these things generally hold calibration over several years.
    Last edited by Rocketcaver; 08-04-2014 at 11:49 AM.

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    They can drift, but this is usually the result of calcium on the gauge. Cleaning it from time to time with distilled water usually sorts things out. Since +/- a few ppm is negligible for this hobby, perfect readings aren't really necessary.

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    BigBella's Avatar
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    They do drift; and that's why they are provided with calibration solutions, in a similar vein to pH meters. I calibrate mine every few weeks, especially if a reading seems totally out of whack . . .
    Last edited by BigBella; 08-04-2014 at 04:31 PM.
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    Now that I think about it I do remember the cal solution pack that came with it. I'm sure that was lost long ago. I suppose I could get another one if I start getting odd readings. So far so good though.
    Thanks for the replies.

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    The meter should be recalibrated periodically. They lose calibration with usage. They lose calibration as the batteries wear down. They lose calibration as the circuitry ages. They use calibration when you drop them. They lose calibration if you leave it in the sun. Without a standard how are you going to know if the meter is reading "true"?

    The instruction manual should say which calibration solution to use (NaCl, KCl, 442 etc.). They are readily available online and at hydroponics shops. Just make sure it hasn't expired.
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