Tds ?????

[TandN]

Ok Does it make a diffrence if you collect a RO/DI water sample into a new never used plastic cup and dip your TDS meter in that ?

I ask because thats what I did recently. I first tested like I always did (run water from tubing right into the meter) and it read "0"

Then I collected some water into a new plastic cup and placed the meter into the cup and it read "11"


I right away changed my filters. Which in all honestly are not all that old and did not look that bad.

[nakadoc]

Yup I noticed the same thing also. What I did is when ever I got done with one of my water storage containers I would rinse them with the clean ro/di water and wipe them dry with paper towels. This would get all the desolved solids out and off the storage containers I guess?

[Billybeau1]

The dissolved solids you are seeing should not be a concern.

Your ro/di took out all the ugly stuff (hopefully) if it is functioning properly. If your post di reads 0 tds, you are likely fine.

I don't clean out my ro/di storage container except maybe once a year. :)

[TandN]

lol Im not talkin about a storage container at all. I have small hard plastic drinking glasses I buy to use for feeding the fish. I checked my ro/di water coming out of my top off by putting the tds meter under the ro/di flow coming into the tank and it reads "0". I then after that filled a brand new cup up with ro/di water from the same stream ad placed the tds meter into the cup and it read "11"

[AZDesertRat]

Use a squeaky clean clear drinking water glass. No water spots or soap residue and always triple rinse it with DI water after each use and store it upside down so nothing gets in it. TDS meters are sensitive and what you are probably seeing are the mold release agents coating the plastic cup. Lots of new plastics have a oily film when new.

[TandN]

ok, the cup is not a reusable one eiher there the hard plastic cocktail cups

[Tranquility]

USE IN LINE TDS METER ...

[TandN]

I used to use them but stop truting them after a mishap with a brand new one

[AZDesertRat]

Inline meters are not as accurate since they are not temperature compensated. Thye can be significantly off especially in the low ranges where you want accuracy.

[monty]

to get the best reading, let the rodi unit run for about 15 minutes before testing it...the cup shouldnt matter. im not sure what type of meter you have, but if it has the ability to read levels while being actually installed inline, you will get really accurate readings because there is uniform flow over the meters and no air. Spectrapure sells them for about $30, they work awesome

[TandN]

http://www.airwaterice.com/product/HMTDS3/TDS3_Meter_with_Carrying_Case.html


this is the one I use

[Billybeau1]

You should listen to A.J.

After all, this is the year of the Rat. :D

[rbursek]

I make up 60gal RODI at a time. inline reads 0 hand head 3, after a week covered container read 65, talk to Jim at the FilterGuys, pure water picks up ions out of the air or stuff from the storage container, not really TDS's,unless cat liter box near by, and if I am wrong Dr or/and BB chime in here.
Bob

[rbursek]

Water is a powerful thing, everything is measured by it, just think of a 24 year old virgin in white walking into a bar on Saturday night, everything is attracted to her, well not all if she had bad teeth LOL, but that is what happens to pure RO/DI water. It is very erosive, will kill a Hanna meter probe if not rinsed in regaler water after testing.
Bob

[kzooreefer]

Typically an RO membrane alone will reduce the TDS down to about 20 ppm and the DI cartrdige will then remove about another 10 ppm (can initally be 100% when new). So a reading of 11 ppm, up from 0 ppm, indicates either that the resin is just getting to a noraml reading or it might be time to replace the DI cartridge. But you could still have some capacity left, if it has gone over 20 ppm it is probably spent and needs to be replaced. DI cartrdiges only have a life of 100 - 400 gallons depending upon the amount of resin in them. So running a filter for 15 minutes as someone suggested just wastes water and uses up your DI cartridge quicker. Waste of money and water. And I store my TDS probe in DI water without any problems ( have for 15 years), it will not corrode the electrodes.

[kzooreefer]

The only thing water picks up from the air is CO2 and 02 and they don't read as TDS, neither do particles like dust, those are suspened solids or TSS a totally different thing. An increase of TDS from a container also isn't because of leaching from the plastic. It is caused by picking up CO2 from the air which over time is converted to HCO3 (bicarbonate) which a TDS meter will read.

[AZDesertRat]

There is nothing typical about an RO membrane so you cannot put numbers to it like 20 ppm or 10 ppm. What you can say is RO membranes typically reduce the incoming TDS, whatever that number may be, by 90-98%. If the incoming TDS is 200 you can expect to see a RO only TDS of about 4 or 5 ppm with a good membrane. If its 100 you should see 2 or 3. If its 1000 you should see 20 etc. You can't just put a TDS mumber to it without knowing the tap water numbers.
Any good full sized vertical 20 oz DI cartridge should reduce that to 0 indicated TDS for a length of time. The better the RO membrane is working and the better the quality of the resin the longer it will produce 0 TDS.

[Bri Guy]

What container could you use that was "one time use" clean 0 readings everytime?

Would sandwich bags work? or would it have to be something sterile to be uniform?


OT A.J. how big is your sulcata? (there sweet)

[kzooreefer]

Yes you can use general numbers when making estimates and 20 ppm is accepted in this industry as what is produced by the TFC membranes used in these small filters. Also a standard 9.75 inch DI cartridge can remove about 7000 ppm of TDS (a good grade resin could do 17,000 so it varies), again just a general number but good enough for making an estimate on capacity. So if you take 20 ppm out of an RO unit and divide 7000 by it you get 350 gallons before you need to replace the DI cartridge (maybe 700 gallons with a good resin). These are just estimates and what someone sees will be different depending upon their water quality, the type of resin and membrane used.

So, as I said earlier, if your RO/DI unit is producing 11 ppm TDS water it may be time to replace the DI Cartridge not the membrane.

[AZDesertRat]

Not true. How can you throw a number of 20 out when you have no idea what their tap water is like? You can't do it. You can say the membrane should remove 96-98% when new.
My tap water TDS is 835. If I used your number or 20 thats pretty good removal. If my tap water TDS was 250 like the national average i would be highly disappointed with a RO TDS of 20, thats only 92% rejection which is horrible. In fact with my tap TDS of 835, I am getting an RO only TDS or 5.3 to 6.2 which is much better than average but not unheard of.

[kzooreefer]

I throw out the number 20 because that is the number used in the filter manufacturing industry. I'm only using it as a reference to show that the DI resin may need to be changed. Yes some membranes will get 1, 5, 10 or 15 and I could have just as easily used one of those numbers. But sorry 20 is what is used, it is like an average. You can use the number to do rough estimates of capacity. Nothing more. By the way the majority of TFC membranes tested at my facility do about 92% rejection taking, 300 ppm TDS down to 25 ppm. I've also had some only do 83% taking it down to 50 ppm. The majority of what remains is silica, phosphate and nitrate which TFC membranes have difficulty removing.

[AZDesertRat]

Wow those are some pretty poor performing membranes. I have found most of the shelf 50 or 75 GPD household type membrane to do more like 94-96%? Mine was wet tested and guaranteed to be better than 98% and has averaged 99.23% rejection for the last 18 months thats with a tap TDS of over 800, pressure at 64 psi, temperature between 56 and 64 degrees F, waste ratio at 4:1 and a product water of 5.3 to 6.2 TDS. We do have CO2 problems so normal mixed bed resins only lasted 150 gallons per 20 oz refill but when I switched to Spectrapure resins it went up to over 630 gallons per cartridge.

[kzooreefer]

The high flow, 100-gpd, Dow Filmtec membranes are only rated at 90% rejection, you sacrifice rejection for flow. But what I've tested here were all low flow, 25-gpd, high rejection, 96%-98%, membranes and didn't get good results. Silica and nitrates still got through.

The Nalco Water Handbook (2nd ed.) based their data on RO membrane performance on a 90% rejection rate. Which back in the late 90's was pretty much standard. Nalco shows an 85% rejection of nitrates and 90% rejection of silicate from a TFC membrane (this is also where I get the 20 ppm TDS from). As we have fairly high silca in our groundwater here and nitrates fluctuate with the season due to farming I expect a higher effluent TDS.

As far as this thread goes, not knowing what the starting TDS was in this problem I just used the 20 ppm which is fairly standard. if I knew what the TDS was and what type of membrane it was I might use a different number to compute a DI resin capacity. I would also think the type of resin used is important as there is quite a range on those also. But I was just doing a quick and dirty computation, nothing to be quoted in a science journal.

[AZDesertRat]

Today 90% rejection is considered nano filtration and is not approved by NSF for drinking water. That's why the Dow 100 GPD carries a NSF certification "For Pool and Spa Use Only."
Membrane technology has come a long way in the last 10-20 years for sure.

[kzooreefer]

Actually Dow still sells them for home drinking water. NSF certification is nice to have but you don't need it. WQA and UL Laboratories also offer the same certification programs. We do witness testing for WQA and NSF so I'm very familar with most of the certification programs. Just about all of the filtration media and component suppliers have some type of NSF certification for their products, usually under Standard 61 Drinking Water System Components and Materials, as the end users (filter manufacturers and water treatment plants) like to see it but we don't need it to sell our products for drinking water treatment. In fact the industry is pretty much unregulated and you can sell anything you want.

http://www.dow.com/liquidseps/prod/tw30_1812100.htm