rodi pressure gauge

[ailachami]

the pressure gauge that came with my rodi is pretty crappy so I bought a new one from brs. from what I've read, the pressure should be at 50psi before the ro membrane to function properly? mine is at 38psi. I changed out all my filters and its still at 38. but my tds is reading 5-9ppm after ro membrane and 0 after di. is my ro membrane overworking? how do I raise the pressure?

 

[Mav]

Booster pump

 

[jahmic]

If that new gauge is reading properly...then it sounds like you might need a booster pump; BRS sells them as well.

Low pressure in your RO/DI will make the system produce less GPD than it's rated for...you also end up with a lower rejection rate which makes you burn through DI resin faster.

If you know what your RO membrane's rejection rate is, you can measure the TDS just before the membrane and compare that to the TDS of 5-9ppm after to see how efficiently it's working. If the difference in TDS matches the rejection rate, I'd be inclined to ignore the reading on that gauge and skip spending the money on a booster pump.

edit: here's a post made by Spectrapure on another forum...came across this when I was shopping for a system last year.

Another perspective on Booster Pumps for RODI systems

We often are asked the questions, “If you operate your RODI system at less than optimum tap water pressures, what is the downside? Is there a negative?”

To help answer these questions, I have provided data (below) that shows the actual results of various pressures applied to a typical RO membrane.


Membrane Pressure GPD Measured % Rejection % Increase
35 39.3 97.5
45 55.2 98.2 0.7
55 67.9 98.3 0.8
65 81.8 98.3 0.8
75 97.7 98.5 1.0
85 111.5 98.7 1.2

Note: 80.8 F, .932 TCF, 950ppm TDS, 1150 uS,
OPCF 7 psi; corrected psi 67


Generally speaking, production rate is directly proportional to operating pressure. It can be seen in this example that raising the pressure by the use of a booster pump from 35 pounds (a commonly found pressure in many homes and offices, especially as demand fluctuates during busy periods) to 75 pounds increases production rate by a whopping 2.5 times! This is the primary and most common reason for adding a booster pump to your system (either an RO or an RODI system)
you want to make more water faster! In cold-water locations, a booster pump can also increase the production rate, as production rate is proportional to water temperature.

Another reason for booster pumps that is often over looked, is that in hard water locations (many places in the country) % rejection improves significantly with the increase in pressure provided by a booster pump. Our data above shows a rejection performance increase from 97.5% to 98.5%, or about 1%. How important is 1%? The rule of thumb for DI resin capacity is for every 1% RO membrane rejection improvement you get a 25% increase in DI cartridge life (due to the reduced ionic load). Thus, with this particular RO membrane as an example (they do vary somewhat), you can see that operating an RODI system at higher pressures (75 psi) will gain a 25% increase in cartridge life over lower pressures (35 psi). In other words, with a 25% gain, every fifth DI cartridge is free! As DI cartridge changes are a significant cost of producing DI water over the life of your system, a 25% increase in cartridge life can add up fairly quickly.

Rereading that now makes me think twice about not getting a booster pump on my system. My pressure was 55 in the winter and only gets as high as 60 most of the time. I have to replace my DI resin every 3 months with only a couple nano tanks running.

 

[kmellon]

+1 on the booster. I have the 5 stage BRS with the additional 75gpd ro membrane and had a huge rejection problem with my in house pressure. Added the booster and now get around a little better than 1-1 RO-Waste. Prior to adding the booster, I was producing almost a 2-1 Waste-RO. If i remember correctly, my psi out of the line was around 38 or so also. BRS recommends somewhere around 70 psi for the dual RO membrane to work effectively.