TDS Reading - Top Off Water

[FranktheTankTx]

Help me here -

Today I was going to refill my top off acrylic reservoir when I decided to take a TDS reading from my reservoir. 264!!! I was like - Holy $@!#

My RO/DI unit DOES NOT hook up directly - I manually connect it to the reservoir and fill it up, then disconnect. So I rinse out my reservoir and start fresh. I take a TDS reading from my RO/DI unit and it reads 0.

I then begin filling up my reservoir and half way thru take a TDS reading - 0.

I then plug the ATO pump back in and let it run. After 30 - 60 minutes I took a TDS reading of the reservoir - 61ppm. I don't get it!! So I quickly tested the water coming out of the RO/DI unit and it's 0. How can I get a TDS reading of 61ppm when the water going in is 0.

Then I started thinking - ok, when the pump shuts off does it create a slight vacuum that sucks saltwater back into the reservoir? Thus slowly giving me a TDS reading.

Otherwise - I'm baffled! What could it be?

 

[flyyyguy]

unless you pulled the product water you were testing directly from the DI output line, into a PERFECTLY clean glass, that tds measurement from your hoklding container means nothing. A tiny bit of residual salt will give you a huge false reading. Doesnt mean the waater isnt perfect.

I switch from one 55 gallon drum to another to make up water and to collect new ro/di water. If you test my container of newly made ro/di water it would read several hundred as well. The water is zero coming out of the DI and that is all that matters.

 

[FranktheTankTx]

Why is that?

This is exactly what I am talking about - how can water be 0 out of the DI line - but 61 in the container?

Makes no damn sense to me - unless it is creating a very slight back siphon after the pump shuts off.

Everytime I test water out of the DI - it reads 0ppm. And the acrylic reservoir reads 0ppm until I turn the ATO pump on. Then give it a few minutes and you'll start reading TDS. It has to be a small back siphon pulling saltwater back into the line and reservoir.

So - obvoiusly this is nothing to worry about?

 

[MileHighFish]

when it siphoned back did it put saltwater back in?

 

[MileHighFish]

does a TDS meter read correctly with salinity in the water?

 

[Flipper62]

<a href=showthread.php?s=&postid=13654239#post13654239 target=_blank>Originally posted</a> by g stone
does a TDS meter read correctly with salinity in the water?

Once you put salt in the water, the TDS will be wacked out. The TDS water needs to be checked right from the DI unit. You can get an inline meter to put in the line after the DI filter.

I use a COM-100 portable meter. This way I can just stick it in the container & check it.

By the way, Most meters will read 0.0 even if the TDS is 0.6 or 0.7. The COM-100 will read the true reading.....And a True RO/DI reading is 0.3

 

[FranktheTankTx]

<a href=showthread.php?s=&postid=13654239#post13654239 target=_blank>Originally posted</a> by g stone
does a TDS meter read correctly with salinity in the water?

Okay - It looks like I went to a $@!# load of trouble for nothing by cleaning out my reservoir and refilling.

I just put 5 or 6 drops of saltwater in a clean glass and filled it with RO/DI water. TDS reading = 192ppm.

Rinsed out the glass and filled it with RO/DI water. TDS reading = 0ppm.

So when the ATO pump shuts off - it sucks a small amount of saltwater into the line which gradually reaches the acrylic reservoir and gives a falty reading.

Which in turns causes me to go into a panic mode and spend an hour rinsing everything and cleaning it in order to get fresh RO/DI water. Thanks to the Powers that Be.

 

[crvz]

<a href=showthread.php?s=&postid=13654299#post13654299 target=_blank>Originally posted</a> by FranktheTankTx
I just put 5 or 6 drops of saltwater in a clean glass and filled it with RO/DI water. TDS reading = 192ppm.

Good test, something that should show you how little it takes to screw up the TDS. Glad you've sufficiently calmed down. happy reefing!

 

[AZDesertRat]

Put a check valve in the ATO line so nothing siphons back or mount the reservoir above the sump with an air gap.

 

[BigJay]

What pump are you using for the ATO?

How much does it back-siphon?

 

[Dave & Monica]

I haven't had this happen but know your pain was a lesson for many, good thread and info.

 

[FranktheTankTx]

I'm using a Via Aqua power head - it's very small and came with my CPR HOB AquaFuge2... but I bought an extra one which I am using for the Refugium.

I don't see any reason to do anything really - I don't see any harm being done here.

 

[hypernesia]

Is your resevoir completely sealed or is it open to the air. If its open you will always get CO2 in your water from gas exchange which will raise the TDS reading in the resevoir. The only thing that matters is that you have 0 tds at the DI. My water at the DI reads 0 but I have two 32gal trash containers filled with RO water and the TDS is 60 or so( nothing to worry about)

 

[FranktheTankTx]

It's almost air tight - but custom made with 2 removable lids. It has 4 slots for tubing & power cords to come out of which are almost sealed by the lids. Here's a pic - though it's kind of far away.

Like I said - it appears this is nothing to worry about.

 

[hypernesia]

Nice design, Chaulk it up to another what the he@# this hobby is great isn't it!