RO no DI = TDS?

[captexas]

Steve - a "mixed bed" DI resin is a mixture of anion and cation resins. There are also different grades of resin, some listed as medical grade, semiconductor grade, and nuclear grade. I have no idea what the real differences are between them.

[JimD]

Big difference! Nuclear grade is the only way to go. Most lfs's use medical grade resin, Take a tds meter with you next time you go and measure the RO/DI water they sell... lol! You may be suprized...

[NaCl_H2O]

Here is another question for the RO/DI crowd. Can your water pressure be too high? Just noticed on my Kent instructions that 85 psi is "Ideal". I have GREAT water pressure and the guage on my RO/DI unit reads 120+ :o I cut back the valve feeding the unit to drop it to 85 psi, and my output TDS dropped Since I have been running with 120+ psi, have I damaged the membrane? It is a 60gpd Kent membrane.

[Instar]

Over 120 is way over. You need a pressure regulator for your entire house if that reading is correct. Your plumbing and sink fawcet valves may fail you at too high a pressure. Good that you cut it back for your RO. The higher pressure, to a point at least, will give you better output but they do have a rating. The manufacturers make pressure pumps to boost pressure to get more production, but, over 120 seems excessive to me. You have to check the rating for your membrane and see what it is. Makes sense the TDS would drop if you lower pressure but a half open valve is not a perfect solution. Get a regulator if you want it right.

Joshua, all membranes are not created equal. Some allow nearly 5% of the solids to pass through, others allow kess than 2% at their specified temperature and pressure. Different companies may use the same brands, but, there are different performance specs for different rated membranes. Thats coming from Kent lit, so if others buy from one source, maybe they all buy only one grade? Theres a lot of sales hype over all this too and that throws off the real facts of the matter.
Since your prefilters don't keep size, thats probably not what I am interested in. Mine stay what they are always, wet or dry.

[NaCl_H2O]

www.H2Oinstruments.com
Ive been using Hanna tds meters for years, cheap but reliable.

Just in case someone is still trying to find that perfect gift for my birthday

http://www.h2oinstruments.com/Mercha...oduct_Code=C99

[BuckeyeHydro]

The following may be of some help in this discussion. Although it was prepared to answer the question: "When should I change my filters?" it also covers the issue of pre and post system tds levels as well.

A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the useable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or both of the filers is beginning to clog.

Also be cognizant of the chlorine capacity of the carbon block. The Matrikx+1 for example will remove >90% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons.

Regarding your RO membrane and DI resin, use your TDS meter to measure, record, and track the tds (expressed in parts per million) in three places:
1. Tap water
2. After the RO but before the DI
3. After the DI.

The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 ppm. Common readings are 100 to 400. So for sake of discussion, let's say your tap water reads 400. That means that for every million parts of water, you have 400 parts of dissolved solids.

How do we go about getting that TDS reading down to somewhere near zero?

If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block filter do very little to remove dissolved solids. So with your tap water at 400 ppm, you can measure the water at the "in" port on your RO housing and you'll see its still approximately 400 ppm.

The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 90% (i.e., they reject 90% of the dissolved solids in feed water). So the purified water coming from your 100 gpd membrane would be about 40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce less permeate, but have a higher rejection rate (98%).

If you measure the TDS in your system after the RO membrane, and before the DI housing, you'll be able to measure the rejection rate of your membrane. A declining rejection rate is a good way to tell when your membrane needs to be replaced.

After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the 40 ppm water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm to 3 ppm, 5 ppm, and higher, you know that your resin needs to be replaced.

Sometimes people complain that their DI resin didn't last very long. Usually the culprit is a malfunctioning RO membrane sending the DI resin "dirty" water. This will exhaust the resin quicker then would otherwise have been the case.