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tds for local RODI water


astroboy

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I'm getting a reading of 9 ppm TDS for my RODI water. Just wondering, is this reasonable for Fairfax county water or is it time to switch out the media?

 

Thanks!

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I'm getting a reading of 9 ppm TDS for my RODI water. Just wondering, is this reasonable for Fairfax county water or is it time to switch out the media?

 

Thanks!

Hi Mark. Are you taking the RO/DI water sample directly from the drip tube after it's been on for 5 minutes or more? Or are you taking the sample from your RO/DI reservoir. If it's coming from your RO/DI unit's output (and not from the reservoir) and you're getting that kind of reading it's probably time to at least change out your DI resin. It may also be time to start looking at replacing your RO membrane. Replace your front end filters and your DI resin first, and then take samples of your source water and at the RO output (pre-DI) after it's been running for at least 5 minutes. You should see at least a 95% drop in TDS. If not, your membrane may be going, too. 

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HI Tom,

 

From the drip tube I'm getting 3ppm; I seem to dimly recall that when I first got the RODI system it was 1... Anyway, I'll monitor it and if it starts to increase I'll know it's time to replace the filters and media,

 

Thanks,

 

Mark

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You should see 0 ppm out of the drip tube. Unless the tube is really long and is contaminated (e.g. has something growing in it), the DI resin should remove any remaining charged ions. That is, unless the resin is exhausted. Have you verified that your TDS meter reads zero when dipped into a clean glass of distilled water? It might be a good check.

 

If you're reading 9 ppm from your storage container and 3 ppm out of the drip tube, I'd trust the 3 ppm number for an assessment of the state of your filters. It could be that, if you're producing RO/DI water in repeated small volumes that you're getting a high TDS burst during each production that's raising the TDS in your reservoir. However, 3 ppm at the DI output is still high. Try taking those other measurements and report back:

 

1) Check the TDS meter in distilled. Verify that it's reading zero.

 

2) Check TDS of your faucet water. (Let the faucet run for a couple of minutes before taking the sample.) We'll assume that this is the same TDS as what's going into your RO/DI unit.

 

3) Check the TDS of the water coming right out of your RO membrane (the one going to the DI stage, not the waste line). This will give you an idea of the %rejection of your membrane. Take this sample after the unit has been running for at least 5 minutes. This waiting allows any TDS spike from the membrane to pass.

 

When was the last time your pre-filters were changed?

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