TDS sensor and RO/DI water

[jonb]

I just bought a "Smart" TDS sensor. My tap water reads 107, and my RO/DI reads 000. The sensor was calibrated at the factory (they even included a hand written card with actual test results), so I haven't bothered calibrating it myself.

Should I believe this? Is this a reasonable reading for the RO/DI water? It seems a little too good to be true.

 

[jonb]

Did I say "Smart"? I meant "Sharp".

 

[tazdevil]

A properly working R/O, DI filter can give you a TDS of "0". I'd recalibrate if you have any doubts, however, if your inline pressure is optimum, the TDS could be 0.

 

[vair]

If my RO/DI does not read zero then I know it is time to change filters. That is what it should read, if you have doubts on the unit borrow someones unit and compare the values.

Dave

 

[tangir1]

The TDS meter read in ppm (parts per million), it sounds like. My water is about 400ppm, while my DI is "0" ppm.

Unlike pH meter and other instrumentation, conductivity-based meter (TDS, etc.) usually does not need to be calibrated as often, but you should calibrate it maybe every few months.

If you are a hard-core water geek, you should only trust a NIST-certified meter and calibration fluids. But for hobby purpose, what you have is more than what most of us expected.

However, most hobby meter give you a "0" when the water conductivity is under-ranged (RO/DI water will do this to all hobby meter that I see), and I am pretty sure it is also true in your case.

You should read Randy's recent article on TDS meter on one of the online magazine. The caveat is that TDS meter does not measure all the impurity in water, and "0" is indication of underranging. But before you freak out by the caveat, be rest assure that your RO/DI probably is working very well. I would check the RO water (before being DI'ed) just to make sure the RO membrane is working properly. In fact, TDS meter is intended for monitoring RO water, and not RO/DI.

Oh, another thing is that the TDS reading can be +/- 10 ppm, so even if the DI output is 5ppm, it maybe just a meter that needs to be recal'ed.

 

[ufans]

How important is it to test your RO/DI water with a TDS meter?

I have a new Maximum HiRes 60gpd system that I pretty much exclusively use for water changes and top-off of my 75gal so I'm really not using it a lot. I figure that the meter could help me determine how often I am replacing the filters but is there another benefit?

BTW> I don't usually drink the RO/DI water but it works great in the coffee maker !

 

[tangir1]

TDS meter is just another gadget. I only use it occusionally, when there is nothing better to do. It is helpful, for example, when you need to trouble shoot your filter.

As mentioned above, TDS meter is not very helpful for RO/DI monitoring. All you see is a big fat "0" which tells you not much info unless your DI is way out of wreck.

For coffee and tea, use RO. It is a waste of money to use RO/DI.

 

[PRC]

I find my TDS meter to be very helpful in determining when to change my DI resin. IME depleted DI resin will actually raise the TDS of the input water, and it will do this before all of the resin in your cartridge color changes. Without a meter you're kind of shooting in the dark.
And, don't bother with DI for drinking water, it is a waste.

 

[tangir1]

>... don't bother with DI for drinking water, it is a waste. ....

LOL. I like your pun on the word "waste."

 

[AnotherGoldenTeapot]

The thing to realise is that you're usually only really interested in confirming that the end product water has a TDS of zero.

A meter to measure zero dosn't need to be calibrated EVER - the maximum measurement error at zero is tiny i.e. the meter will always read a tiny value near zero whether it is calibrated or not.

For other measurements a realitve measure is good enough e.g. measure the source water, then confirm the water after the RO membrane is no more than about 5% of the original value (the % reduction is what you want to test - the exact number isn't important). Then check the the water after the DI resin reads zero, or something very close.

 

[jonb]

Thanks for all the helpful info! I'm thrilled that my RO/DI is clearly doing its job.

By the way - if you've ever dared to taste your product water, you won't be making any coffee or tea with it

 

[tangir1]

I agree that relative measurement is all we needed in most cases, but I disagree with you on the zero and calibration.

>... A meter to measure zero dosn't need to be calibrated EVER -

For almost all the TDS meter that I encountered in this hobby, the TDS accuracy is usually no better than 2% full scale. I will be very interested in a TDS meter that can do better than that for less than $400. Now, keep in mind that 2% of full scale is +/- 40ppm if the range of the meter is 0->1999. or +/- 20 ppm if the meter give you 3 digits. When you see "0", it can be as much as 20ppm. Now, of course, before some of you freak out and throw it away, it is a lot better than what claimed by the manufacturer's spec, but if you always keep this in mind, you will not say that the error at "0" is tiny rather it is calibrated or not.