Do these TDS reading sound right?

I have a typical home RO/DI system that I got about 5 years ago, and just started using it for my aquarium needs. I wanted to test the effectiveness of the DI beads instead of guessing when they need to be changed. So I bought a dual in line TDS meter and hooked it up so the in measurement is just before the water enters the DI beads, and the out is as it comes out the DI canister.

Now when I look at the readings of the meter, when no water is flowing, the IN reads about 5ppm, and the out is 0ppm. If the unit is producing water, the IN ppm drops from a reading of about 5 to 1-0ppm (the out still stays at 0ppm). I thought the IN would read a higher value? My concern is that if the IN is reading almost the same as the OUT, then how can I tell if the DI beads are used up?

I have this RO system which has;

5 micron SED
GEC
1 micron CCB
100GPD ro membrane

So I guess what's happening is that the water coming out of the ro membrane is about 1-5ppm TDS. I always thought this would typically be higher than this?

 

So, the "IN" reading is after the RO and before the DI?

If so, then you're getting good rejection with your RO. I get 0 PPM on the output of my RO and I don't use a DI.

I would say if you're using a DI, and the IN is after the RO and the OUT is after the DI, then your DI is spent when the OUT number goes above Zero.

 

no, thats about spot on.. its great when stuff works, isnt it! ;D

 

The TDS of the water exiting the RO membrane is dependent upon the TDS of the water entering your house. I believe that in general, your numbers are right on. A TDS of less than 10 exiting the RO is acceptable, but if it's higher than 6 or so, it could be lower. Finally, the TDS of the water exiting the DI should be zero in all cases. When it's not, it's time to change the DI media.

Perhaps AzDesertRat, or anyone else who's familiar with what these numbers should be in all conditions, can chime in.

EDIT:

Thanks bama and gabba. You said exactly what I was trying to say. We typed at the same time.

 

Ya that's the way I have it set up. The IN is after the RO membrane and just before going into the DI unit. The OUT is reading the water as it comes out of the complete system.

So I wonder how I tell when the DI is spent if the IN reading is pretty much the same as the OUT reading. I do have the color DI beads that change color when they are used up, but I thought the TDS meter would tell you that instead.

 

Guess were all typing at the same time!

I assume what happens is that the ppm OUT of the DI beads will increase once they are spent since they are now releasing TDS back into the water instead of absorbing them.

 

If I'm not mistaken, the DI media doesn't release TDS, they'll just fail to remove the remaining TDS not picked up by the RO membrane.

EDIT:

Thanks for the correction, AzDesertRat. flyboy320 was right on the money.

 

I think the beads will discolor also.

 

You are hooked up correctly and your readings are good.
In order to know how good though you need to know your incoming or tap water TDS. Thats the drawback with the dual inlines, they are a dedicated meter meaning they only measure two points, depend on flow past the probe to read accurately and are not quite as accurate as a good handheld due to not being temperature compensated.
I imagine your tap TDS is extremely low to begin with if you are getting a reading of 0-1 with RO only, probably a TDS of 50 or less going in. The national average is around 250 so yours is probably much lower when you consider a membrane is a 96-98% device meaning at 250 in you should see between 5 and 10, you are seeing 1.

Although I own a couple dual inlines I prefer my COM-100 handheld for accuracy, ease of use and portability. I can measure TDS from the tap, RO only, RO/DI, my ATO storage, the LFS, the water store etc all with one meter. The inlines do not read actual water temperature so are not as accurate. If you were to cut one of the probes apart you would find a temp probe inside the fatter part of the probe inside the little rectangular opening you see. It is reading air temperature not water temperature and how often are those the same? It can be significantly off, especially if the water is very cold.

Regardless, it sound like your RO/DI is performing very well for you.

Now to answer Reefsparkys last comment. DI does release contaminants and in very high concentrations even before it is exhausted. This is why you should always replace DI resin at the very first signs of anything other than 0 TDS. Remember DI resins work with cation and anion electrical charges which attract ions. Weakly ionized substances like nitrates, phosphates and silicates arte hard to attract so you need high quality resin in a vertical configuration with lots of contact time, and do not register well on a hobbyist grade TDS meter at low levels so can be missed. When DI resin starts to wear out, or is saturated if you will, it begind to release these wwakly ionized substances in big gulps, much higher than the levels were to begin with in the tap water since it has been storing and concentrating these substances for hundreds of gallons. This is also why most color changing resins ar a poor indicator of condition, they usually are releasing substances before they fully turn color.

 

Thanks for the correction, AzDesertRat!