My rodi tds says that it’s 23 coming in and 0 coming out. 23 seems pretty low. Is it possible for the rod tds to be bad or is the water in my area just that clean? I’ve seen other YouTube channels where the water was in the 50’s or higher coming in to their rodi and they needed several stages to get to 0.
Trust RODI TDS?
- Thread starter jman930
- Start date
it's possible. In Gwinnett where i live, i get about ~20 out of the tap water.
This area has pretty low TDS. I'm on Dekalb County water and the water coming in is between 30-38 TDS. Most of out water comes from reservoirs and is fairly "soft" which means it isn't loaded with minerals. My parents live in IL, they have well water and the TDS out of the tap is 150+. In Arizona they can see TDS upwards of 300+.
The standard TDS meters we have on our RODI units aren't the most exacting instruments. RO membranes have a "rejection rate" and it's typically around 95%. That means with 100 TDS, it will remove 95 TDS and leave you with 5 TDS on the other side. The membrane I use has a 98% rejection rate and with 100 TDS, it would take out 98 TDS leaving me with 2 TDS water. When the water coming in has a super low TDS level, it'll produce water in the 1-2 TDS range and the meters we use have trouble picking up low TDS because they only display in 1 TDS increments.*
I personally monitor the water coming out of the RO membrane for 2 things. First, so I know that water has a low TDS (obviously). Second, I use it to show me when the RO membrane is starting to degrade. If the water has been "0" for a long time and I see if go up to "1" then I know it's time to swap the membrane out.
*RO membranes are tested at a specific water pressure with a specific water temp. Pressure and temp change thing quite a bit but I'm hoping you get the idea. We've got some smart folks on this forum and if my understanding is incorrect, please set me straight. There's a lot that I don't understand but this is how TDS meters and rejection rate were explained to me when I reached out to Spectrapure.
The standard TDS meters we have on our RODI units aren't the most exacting instruments. RO membranes have a "rejection rate" and it's typically around 95%. That means with 100 TDS, it will remove 95 TDS and leave you with 5 TDS on the other side. The membrane I use has a 98% rejection rate and with 100 TDS, it would take out 98 TDS leaving me with 2 TDS water. When the water coming in has a super low TDS level, it'll produce water in the 1-2 TDS range and the meters we use have trouble picking up low TDS because they only display in 1 TDS increments.*
I personally monitor the water coming out of the RO membrane for 2 things. First, so I know that water has a low TDS (obviously). Second, I use it to show me when the RO membrane is starting to degrade. If the water has been "0" for a long time and I see if go up to "1" then I know it's time to swap the membrane out.
*RO membranes are tested at a specific water pressure with a specific water temp. Pressure and temp change thing quite a bit but I'm hoping you get the idea. We've got some smart folks on this forum and if my understanding is incorrect, please set me straight. There's a lot that I don't understand but this is how TDS meters and rejection rate were explained to me when I reached out to Spectrapure.
If its the in inline TDS meter, isn't it before and after the DI (deionization) stage? Or do you it set up differently?
My tds is in line so it has one for the inlet and outlet. Not sure if I answer your question.
Usually inline TDS are installed after the sediment, carbon, & membrane filtration. Meaning, the “in” is actually after all that filtration and right before the DI. You will have to look. So if you are getting TDS in the 20s after sediment,carbon, and membrane, thats a bit high. Again, that is the typical setup for inline TDS.
Yeah, so 20s right before the DI is a bit high. How old is your membrane? Also, how often do you change your sediment filter?
I change sediment filter every 2-3 months, carbon once a year, DI about 18 months, and membrane 3 years.
I just reread the location you asked about for the location of the in-line tds. It’s located off the water in line (red line on the brs 4 stage) just before it goes into the sediment filter.
I’ll get a picture.
I’ll get a picture.
you placed the TDS in the wrong place.
you typically want to place the TDS before the DI canister and then one after the DI (to measure the product water).
having said that, as i stated before certain Gwinette area is blessed to have water with relatively low TDS out from the faucet. therefore it's not a bad idea to know what your incoming TDS when coming straight from the source. However it does not indicate the performance if your membranes. For better measurement, i would put the two TDS probes before and after DI canister.
you typically want to place the TDS before the DI canister and then one after the DI (to measure the product water).
having said that, as i stated before certain Gwinette area is blessed to have water with relatively low TDS out from the faucet. therefore it's not a bad idea to know what your incoming TDS when coming straight from the source. However it does not indicate the performance if your membranes. For better measurement, i would put the two TDS probes before and after DI canister.
Oh mine came like this but it was part of a buyout. Had no idea it was in the wrong spot.
no worries. all good learning experience. but low TDS coming into your RODI usually is good for the life of the membrane or interval before replacement. so cheers!!!
These fellows are correct. I actually do test the water coming in directly from the supply, similar to how yours is set up currently. But I use 2 different TDS meters. In your case you'd want to test the water just like they said, directly out of the RO membrane (going into the DI resin) and directly after the resin (going in to the reservoir). In your case you should see 1-2 coming out of the RO membrane (depending on temp, pressure, RO membrane age) and when that start to climb, get a new RO membrane. After the DI resin, it should read 0 and if it starts coming up, replace the DI resin.
I've never exhausted the DI resin in my unit. I always change it out once every 2 years. I use a "mixed bed" DI resin and that removes Cation (positive charged) and Anion (negative charged) ions. It's important to have both since that'll get he most stuff out of the water. However, they tend to be exhausted at different rates depending on the condition of the water going in. There's probably some way to test when either one is exhausted but I'm not that smart and I just replace the DI resin every two years and haven't had any problems. Bigger systems (I'm looking at you @Adam) that require a ton of RODI water will have unit with separate chambers for cation and anion resins.
I've never exhausted the DI resin in my unit. I always change it out once every 2 years. I use a "mixed bed" DI resin and that removes Cation (positive charged) and Anion (negative charged) ions. It's important to have both since that'll get he most stuff out of the water. However, they tend to be exhausted at different rates depending on the condition of the water going in. There's probably some way to test when either one is exhausted but I'm not that smart and I just replace the DI resin every two years and haven't had any problems. Bigger systems (I'm looking at you @Adam) that require a ton of RODI water will have unit with separate chambers for cation and anion resins.
I’ll do as suggested and pick up another in line meter. Lots of good info here. Glad I asked!
Also, to everyone's point. We are pretty lucky to have soft water and low TDS here in GA for the most part. I'm in Gwinnett County and I get around 30-40 TDS from tap.
Dang it, here in Bibb county I’m getting 73 
I am also in Gwinnett, I have never seen more than 26 TDS on the supply side.
