RO DI Where should TDS be Measured??

marc111 · 05/12/2010, 08:50 PM

I just got a new Spectrapure RODI unit with a dual TDS meter.

As delivered the TDS meter measures at the input to the DI cartridge (ie output of the RO membrane) and the output of the DI cartridge (pure water out).

Is this the best way to use the meter or would it be better to move the 1st measurement to the actual faucet input water?

How do most people use their TDS meters?

Thanks,
Mark

sjm817 · 05/12/2010, 08:56 PM

That is correct. Before the DI lets you know that the membrane is working properly. After the DI shows the final output and when the DI needs to be changed.

marc111 · 05/13/2010, 08:18 AM

Thanks. So a low level at the membrane output lets you know the membrane is OK but one is left in the dark on how good or bad the incoming water supply is. I guess this makes sense. it just leaves me curious on the incoming.

The question came up because I have seen lots of people post their incoming levels and then the outgoing level. This left the question in my mind of Whether people were using these differently than Spectrapure had delivered the measurement points.

Thanks,
Mark

sjm817 · 05/13/2010, 08:25 AM

The incoming level is good to know, but nothing in the RO/DI unit has any effect on that, so its not that important to measure. You want to see that after the RO, most of the cleanup is done (~96 - 98%), and that the DI is taking care of the rest with a 0 TDS out.

deflux · 05/13/2010, 03:13 PM

If you're curious to know, and the TDS uses quick-connect, you could toss one on the incoming momentarily to see what it is, and then return it to its rightful home.

Also, if you know the avg incoming TDS, and the efficiency at which your RO membrane is rated, you can ensure it is performing efficiently, and then also know when it's going downhill.
Of course you can tell that without knowing the incoming TDS if, for example, the outgoing today is 10ppm and starts to rise over time. Depends on how obsessed you are with knowing the actual numbers.

marc111 · 05/13/2010, 04:22 PM

Thanks guys I appreciate the feedback. I think it has disconnects and I will try it to establish a benchmark.

PurdueWaterGuy · 05/13/2010, 09:42 PM

You really want to watch all three points. If you see a spike in TDS coming out of the RO, it might be because of a spike coming in from the well or the utility. If your city water somes from surface sources the TDS can drift all over the place, causing the TDS in the RO permeate to vary as well.

The big boys with commercial systems track TDS in and out of the RO each shift to track membrane health.

And no, I don't sell instrumentation for a living. :-(

05/12/2010, 08:50 PM	#1
marc111 Registered Member Join Date: Aug 2009 Posts: 198	RO DI Where should TDS be Measured?? I just got a new Spectrapure RODI unit with a dual TDS meter. As delivered the TDS meter measures at the input to the DI cartridge (ie output of the RO membrane) and the output of the DI cartridge (pure water out). Is this the best way to use the meter or would it be better to move the 1st measurement to the actual faucet input water? How do most people use their TDS meters? Thanks, Mark

05/12/2010, 08:56 PM	#2
sjm817 On Yer left! Join Date: Sep 2004 Location: Northern VA Posts: 18,777	That is correct. Before the DI lets you know that the membrane is working properly. After the DI shows the final output and when the DI needs to be changed. __________________ - Scott

05/13/2010, 08:25 AM	#4
sjm817 On Yer left! Join Date: Sep 2004 Location: Northern VA Posts: 18,777	The incoming level is good to know, but nothing in the RO/DI unit has any effect on that, so its not that important to measure. You want to see that after the RO, most of the cleanup is done (~96 - 98%), and that the DI is taking care of the rest with a 0 TDS out. __________________ - Scott

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05/13/2010, 08:18 AM	#3
marc111 Registered Member Join Date: Aug 2009 Posts: 198	Thanks. So a low level at the membrane output lets you know the membrane is OK but one is left in the dark on how good or bad the incoming water supply is. I guess this makes sense. it just leaves me curious on the incoming. The question came up because I have seen lots of people post their incoming levels and then the outgoing level. This left the question in my mind of Whether people were using these differently than Spectrapure had delivered the measurement points. Thanks, Mark

05/13/2010, 03:13 PM	#5
deflux Registered Member Join Date: Oct 2009 Location: Boston, MA Posts: 64	If you're curious to know, and the TDS uses quick-connect, you could toss one on the incoming momentarily to see what it is, and then return it to its rightful home. Also, if you know the avg incoming TDS, and the efficiency at which your RO membrane is rated, you can ensure it is performing efficiently, and then also know when it's going downhill. Of course you can tell that without knowing the incoming TDS if, for example, the outgoing today is 10ppm and starts to rise over time. Depends on how obsessed you are with knowing the actual numbers.

05/13/2010, 04:22 PM	#6
marc111 Registered Member Join Date: Aug 2009 Posts: 198	Thanks guys I appreciate the feedback. I think it has disconnects and I will try it to establish a benchmark.

05/13/2010, 09:42 PM	#7
PurdueWaterGuy Registered Member Join Date: May 2010 Location: Lafayette, IN Posts: 196	You really want to watch all three points. If you see a spike in TDS coming out of the RO, it might be because of a spike coming in from the well or the utility. If your city water somes from surface sources the TDS can drift all over the place, causing the TDS in the RO permeate to vary as well. The big boys with commercial systems track TDS in and out of the RO each shift to track membrane health. And no, I don't sell instrumentation for a living. :-(