Strange TDS Reading on my RO/DI

rubikcube · 04/07/2007, 08:43 PM

So I have a strange issue with my RO/DI. For a brief history read this thread.

http://www.reefcentral.com/forums/sh...readid=1065260

Long story short: After getting a replacement RO membrane for my unit, I decided to settle for the 65 ppm TDS I was reading. This after 650-700 in.

So after AZDesertRat telling me I shouldn't settle for that, I figured I'd tinker around with it. I figured maybe the TDS meter was lying to me(I know it's crazy). So here is what it looked like before:

On my tap, I read 650-700 ppm. On the out line(which you can see is in between the membrane and the pressure tank, I read 65 ppm. You can see that I never actually read the RO TDS while I was making DI water. I never really thought it made a difference where I put the probe. So I moved it to this configuration:

This is where it gets strange. When I'm making DI (with the valve on the pressure tank turned off), I read 14 ppm on the TDS meter. When I open the pressure tank valve and let the RO water flow into the tank, the TDS reading is 65!

I don't understand this at all. The meter is in the same place for each of the readings(tank on and tank off).

Buckeye Hydro · 04/08/2007, 06:01 AM

The readings of 14 and 65 are after the DI?

Not sure if was just a shortcut for you when making the diagrams above, but you should not have pressurized RO water from the tank flowing through the DI. That pressurized water should go to the faucet - not through the DI.

Russ @ BFS

Icefire · 04/08/2007, 06:28 AM

yeah, too much flow in the DI due to pressure.

You'll need a check valve I guess or the storage tank inline with GAC

Colin · 04/08/2007, 07:26 AM

I have pressurized water going through my DI and it's fine. Of course, it's a dual DI with standard 10" cartridges.

Rubik, here's what's probably going on. You're getting TDS creep when the unit first switches on and fills the tank. This is exacerbated if you are only taking small amounts out and the unit switches on and off a lot. I get the same effect with my unit, but since my tap TDS is only about 175 it's not as noticeable. After the tank empties your TDS will drop back down (post membrane).

The solution is to either shut off the tank when you're making DI water, or live with the problem and go through DI at a higher rate. It may be worth it if you need smaller amounts of water really quickly or if you regularly empty the tank (minimizes the problem).

HTH,

--Colin

rubikcube · 04/08/2007, 07:47 AM

Actually the tank is off when I'm making DI and the water always measures 0 after the DI stage when the tank is off. The readings I'm talking about are always before the DI. To put it another way, the TDS is high when the tank is on, but low when the tank is off.

Colin · 04/08/2007, 08:01 AM

Yep, creep water in the tank. I think this is normal. Maybe a permeate pump would help?

--Colin

rubikcube · 04/08/2007, 08:46 AM

I monitored the TDS reading from empty to full going into the tank and it goes from 65 to about 50, but not below that. I didn't think a permeate pump would help me since my pressure is 58 psi.

rubikcube · 04/08/2007, 08:55 AM

Wait, do you mean that I may need a pump because the pressure of the tank is working against the tap water pressure?

Colin · 04/08/2007, 09:24 AM

If I understand correctly, the permeate pump negates the back pressure from the tank on the membrane, allowing it to work at full efficiency.

--Colin

04/07/2007, 08:43 PM	#1
rubikcube Registered Member Join Date: Dec 2006 Location: Lubbock, TX Posts: 49	Strange TDS Reading on my RO/DI So I have a strange issue with my RO/DI. For a brief history read this thread. http://www.reefcentral.com/forums/sh...readid=1065260 Long story short: After getting a replacement RO membrane for my unit, I decided to settle for the 65 ppm TDS I was reading. This after 650-700 in. So after AZDesertRat telling me I shouldn't settle for that, I figured I'd tinker around with it. I figured maybe the TDS meter was lying to me(I know it's crazy). So here is what it looked like before: On my tap, I read 650-700 ppm. On the out line(which you can see is in between the membrane and the pressure tank, I read 65 ppm. You can see that I never actually read the RO TDS while I was making DI water. I never really thought it made a difference where I put the probe. So I moved it to this configuration: This is where it gets strange. When I'm making DI (with the valve on the pressure tank turned off), I read 14 ppm on the TDS meter. When I open the pressure tank valve and let the RO water flow into the tank, the TDS reading is 65! I don't understand this at all. The meter is in the same place for each of the readings(tank on and tank off).

04/08/2007, 06:28 AM	#3
Icefire Seasoned reefer Join Date: Jan 2007 Location: In Quebec, Canada Posts: 3,653	yeah, too much flow in the DI due to pressure. You'll need a check valve I guess or the storage tank inline with GAC __________________ Temp 80F, PH 8.5-8.0, Alk 6.8-7.2, Ca 430, Mg 1700, NO3 0-0.25, PO4 0.04, 34.4 PPT Lights @ 100% all others, 35% White Current Tank Info: 60g Cube, 120lbs live rocks, Hydra52 2x120W, 2 MP40+ 1 MP10 @ 70%, Phosban 550 GFO+Carbon, 200W Heater, SWC Skimmer, Kalk ATO, 150 gpd RODI

04/08/2007, 06:01 AM	#2
Buckeye Hydro .Registered Member Join Date: Sep 2006 Location: Cincinnati, Ohio Posts: 4,857	The readings of 14 and 65 are after the DI? Not sure if was just a shortcut for you when making the diagrams above, but you should not have pressurized RO water from the tank flowing through the DI. That pressurized water should go to the faucet - not through the DI. Russ @ BFS

04/08/2007, 07:26 AM	#4
Colin Registered Member Join Date: May 2000 Location: Oklahoma City Posts: 848	I have pressurized water going through my DI and it's fine. Of course, it's a dual DI with standard 10" cartridges. Rubik, here's what's probably going on. You're getting TDS creep when the unit first switches on and fills the tank. This is exacerbated if you are only taking small amounts out and the unit switches on and off a lot. I get the same effect with my unit, but since my tap TDS is only about 175 it's not as noticeable. After the tank empties your TDS will drop back down (post membrane). The solution is to either shut off the tank when you're making DI water, or live with the problem and go through DI at a higher rate. It may be worth it if you need smaller amounts of water really quickly or if you regularly empty the tank (minimizes the problem). HTH, --Colin

04/08/2007, 07:47 AM	#5
rubikcube Registered Member Join Date: Dec 2006 Location: Lubbock, TX Posts: 49	Actually the tank is off when I'm making DI and the water always measures 0 after the DI stage when the tank is off. The readings I'm talking about are always before the DI. To put it another way, the TDS is high when the tank is on, but low when the tank is off.

04/08/2007, 08:01 AM	#6
Colin Registered Member Join Date: May 2000 Location: Oklahoma City Posts: 848	Yep, creep water in the tank. I think this is normal. Maybe a permeate pump would help? --Colin

04/08/2007, 08:46 AM	#7
rubikcube Registered Member Join Date: Dec 2006 Location: Lubbock, TX Posts: 49	I monitored the TDS reading from empty to full going into the tank and it goes from 65 to about 50, but not below that. I didn't think a permeate pump would help me since my pressure is 58 psi.

04/08/2007, 08:55 AM	#8
rubikcube Registered Member Join Date: Dec 2006 Location: Lubbock, TX Posts: 49	Wait, do you mean that I may need a pump because the pressure of the tank is working against the tap water pressure?

04/08/2007, 09:24 AM	#9
Colin Registered Member Join Date: May 2000 Location: Oklahoma City Posts: 848	If I understand correctly, the permeate pump negates the back pressure from the tank on the membrane, allowing it to work at full efficiency. --Colin