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TDS meter.


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The best for our purposes is the handheld HM Digital COM-100 which is the only meter to read down to tenths of a ppm TDS and can also do conductivity too. It is also waterproof , auto temperature compensated and gives you a digital temperature readout..


Next would be the HM Digital TDS-3 which again is handheld so has more functionality and is +/-2% accuracy, reads to 1 ppm and gives you a digital temperature readout too.


The TDS-4 is also a good meter but you lose the temperature function unless you get the TDS-4TM version. I use my temperature function a lot.


Next would be the dual inlines like the DM-1. They are OK, I own two of them, but have some serious drawbacks. For one they are dedicated to two places and cannot be used portable like a handheld so you are stuck with 2 readings, usually RO only and final RO/DI. They are also not as accurate since they are not temperature compensated, they actually sense air temperature and not wate rtemperature so can be significantly off if the air and wate rare not exactly the same temp which is usually never.


Lastly would be the TDS EZ and meters like it. Its usually only 3% accurate, some meters of its type only read by twos and they are not auto temperature compensated so again lack accuracy. They are also not waterproof.


I like the handhelds as I can test my tap water TDS, my ATO storage container, the LFS, my buddies water, and much more. The inlines don't have this functionality.

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I have a Hanna TDS-1 that I bought on ebay for $9 shipped (used) Seems to read good. Its amaziing to see my tap water read 144 and my ro read 0.


Hey AZ, Is there anything i can pick up locally to calibrate the meter?


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Might be tough to find locally but you need a 342 calibration solution. Maybe you have a scientific of school lab supply place around? If not most any of the RO/DI vendors have it for less than $6-8. Some places sell a higher range solution but we are using it for low range like RO and RO/DI so you want to calibrate it closer to that level not 800 or whatever.

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Thanks for the excellent reply,


I'm eyeballing both the HM Digital COM-100 and TDS-3 on Amazon. The COM-100 is a little over twice the price of the TDS-3. Is the COM-100 worth the extra for our purposes? So since it's a conductivity meter, will it replace my hydrometer? If so and if it's very accurate for both, this sounds like the one to get.


Thanks again.

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No it will not replace a hydrometer or refractometer.

Saltwater is over 32,000 PPM TDS while the meter measures up to 10,000 TDS in high range.


To me, yes its worth the difference. I sold my TDS-3 with my old RO/DI and was very happy with it too though. I find the COM-100 to be extremely accurate and us it a lot with my DI water. I have dual DI so I monitor for the least little bit out of either cartridge, I'm a little anal about water quality though.


The COM-100 will be between $50 and $60 and the TDS-3 is $20 to $25 most places.

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Walgreens sells a Cheap hanna that is accurate.

Do you have a link or product code, I couldnt find it on the website

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Define "accurate"?

If its not 2% accuracy and auto temperature compensated its not worth it at any price. You get what you pay for usually and $20-$25 is about the bare minimum for a decent reliable meter.

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Those are HM Digitals not Hanna.

The one on the left is the TDS-3, the one on the right is hard to tell but could be a TDS-4 or EZ? The TDS-4 is OK the EZ is not.

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  • 1 year later...

You have to be very careful. If you look on ebay you will see many TDS-3 meters, many are Chinese knock offs as low as $7 or $8. The real genuine HM Digital TDS-3 is going to cost you $20-$25 at places like Buckeye Field Supply or Spectrapure and will say HM Digital. The clones usually don't list the brand in the title but I did find a few that did mention HM in the text but were not true HM brand.

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