RO/DI - TDS help needed

Need some thoughts on what's going on.......


75 gpd RO/DI unit. Membrane is old - about 4 1/2 yrs (but I only make about 18 gal/week of water). Cartridges and DI resin changed in June last year. I've got a dual inline TDS meter. Been getting a TDS reading of about 4-5 after the membrane and 0 after the DI unit.


In response to one of the parameters on my water test, I decided to change out the membrane. FIlmtec 75 gpd membrane replaced. Flushed it for an hour. Made water over the week-end. The TDS reading after the membrane was now 10, and before I finished making enough water, the TDS meter was also reading 10 out of the DI unit.


So, I dumped all that water and changed the DI resin (color-changing resin).


The water I made today was getting a TDS reading after the membrane of 8-10 and 0 after the DI unit.


I did a test of the water after the membrane - eliminating the DI unit - and the TDS reading was about 6.


Then unhooked my TDS meter and just set the tip in the make-up water - and it is giving me a TDS reading of 7-8. ????????


I then tested the TDS meter on my other water container which was filled after the RO membrane was replaced, but before the DI resin was replace. Its giving me a TDS reading of 4-5 ???????????


So my questions are: 1) Why would the TDS reading be higher out of the membrane when I have the DI unit attached as opposed to when its not attached.


2) Can I use the inline TDS meter to test the water in my reservoir? If so, why would I get a 0 reading when its measuring the water as it is coming directly out of the DI unit, but yet my container of water is giving me a 4 -8 TDS reading? I don't know which one should be trusted???





HELP!

 

Hmmmm...... Interesting thoughts.... The inline TDS meter was replaced less than a year ago.


As for dissolved solids in the membrane case/holder - I rinsed that when I replaced the membrane - shouldn't that have cleaned it out.


Yes - I've got a flush kit and I run it before and after each use......


I guess I'll call AWI and BRS today and see what other suggestions they have.

 

I sent the following message to Air, Water, Ice this morning:



I currently use the inline TDS meter. I recently changed out my DI resin and membrane (membrane was 4 yrs old) as my TDS reading had gone from 0 to 10 after the DI resin. After changing both components, my TDS reading was about 7-8 TDS after the membrane and 0 after the DI unit.



Before throwing away the last water I made BEFORE changing the components, I unhooked my inline TDS meter and simply put the electrodes in the make-up water. It read a TDS of 10 so I threw out the water.



AFTER changing the components and getting a TDS reading of 0 after the DI unit, I decided to take another test of the the water in the water container, using the inline TDS meter. Again I just put the electrodes in the water container. The TDS meter still read 7-8 TDS. Why would this be??? Which can I believe to be the accurate reading??? The TDS meter was replaced within the last year.


Here is the Response I received from them. It is basically the same response I received from BRS.


There are two TDS electrodes in your Dual TDS Meter ... It could be that one is not giving a correct reading .... Use both electrodes on the same water to test ... Make sure you clean the dry the electrodes off in between each reading ... The best place to get the water for testing is to make RODI water for say 15 minutes then Capture some RODI water coming directly from the unit ( not from a storage bucket ... this will insure no TDS creep .. ... Both meter probes should show ZERO TDS ...



Water that sits in any container that is not vacuum sealed will pick up a small amount of TDS from the air around it ... The longer it sits ... the higher the TDS ...



The Initial TDS from any RODI system will have some TDS as it is making water from the stale water in the system this is called TDS CREEP ...



Let us know if you need further info ...





Interesting response. I didn't know that TDS would climb the longer that the water sat........

 

Does anyone clean their containers to reduce the contaminants??? If TDS starts to build up after setting, then having 1-2 coming out after the DI doesn't seem so bad.........