Flowmeter size for homemade RO.

Planning on getting an early start to this years maple season here in Maine. I'm going to start with a homemade RO using plans I found online. It uses (4) 150 GPD membranes hooked in series with an Aquatec 8800 pump running around 70-80 PSI. I'm hoping to add flowmeters to the permeate and concentrate outlets to monitor the flow and adjust the system without having to run them into glasses and measure every time. Does anyone have a recommendation on how much flow I should expect / what size flowmeter would be appropriate. Thanks Attachment 20316

Flow meters on such a system would be a waste of money, IMHO. That being said, 10 gph rotameters would probably be about the right size. For what it's worth, you can run those membranes up to at least 120 psi without risking damage. That would nearly double your throughput, relative to 70 psi. Set the pressure to 120 and let the flow rates be what they may. You'll know when to stop based on the volume change in your feed tank..

I have the RB15 RO Bucket which has three 400 gpd membranes and I use 0.05-0.5 gpm flow meters I got on Amazon (https://www.amazon.com/gp/product/B00EZ3T1GM). These work well for my system which typically processes 15 gph of sap into 7.5 gph concentrate and 7.5 gph permeate. These meters have built in needle valves so you leave the one on the permeate meter wide open and control flow with the valve on the concentrate meter. This type of flow meter is calibrated for water (specific gravity of 1.0), and the concentrate will have a slightly higher s.g.. Therefor to achieve equal flow the concentrate meter reading needs to be slightly higher than the permeate meter reading since the higher density fluid will lift the indicator weight higher for a given flow.

Does anyone know when the 400 gpd membranes arrived on the scene? I don't remember seeing them when I made my first RO system back in 2007. I believe 100 gpd was the max at that time. The 400's must use a different canister than the 150's right?

Since flow meters used on an RO are scaled in gal/minute, whith a 400 gal/day you at most will get 16.667 gal per hour and .2777 gal/minute. That will be total, now divide that into concentrate and permeate, such a meter that could measure accurately might well cost more than all of the other components together.
I think you should just regulate the flows until they look to be 50/50 or maybe 60/40, with the 40% being concentrate, if that set up will do that.
This does not mean building your own RO is a waste of time, by all means build one, just forget the flow meters.

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Since flow meters used on an RO are scaled in gal/minute, whith a 400 gal/day you at most will get 16.667 gal per hour and .2777 gal/minute. That will be total, now divide that into concentrate and permeate, such a meter that could measure accurately might well cost more than all of the other components together.

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Did you not see my reply to the OP? The flow meters I use (and provided a link to) are not very expensive and are ideal for the flow rate in question. I use them and have found them very effective and accurate.

OP was looking to use four 150 gpd membranes in series. The outflow rate of concentrate will be very small. 6.25 gph (0.104 gpm) max for that membrane. Divide that by 2^4 since it's 4 in series. This puts outflow rate somewhere in the range of 0.00423 to 0.0065 gpm depending on actual membrane performance. Normally at 40 degrees or cooler you will see only about 65% - 75% of advertised flowrate.

(1) doesnt look like the flow meter posted comes in a range that low
(2) very low flowrate across final 2 membranes might be problematic and inefficient.
(3) I would recommend running 2 membranes in parallel first, then combining their concentrate and feeding into a 3rd in series with them. This gives you ability to quadruple sugar content on single pass and keep higher flowrate.
(4) honestly I would buy the aquatech 8855 for highest flow rate and use all 400 gpd membranes in the config I suggest above. Price isnt that different but you will notice big speed difference. Time is personally valuable and its crucial for keeping bacteria levels down before boiling.

I did see your reference, I just think 2 flow meters @ about $40 each excessive for such a small system for what it could possibly gain. I still see the flow meters as a waste of money on anything under a system that will process 75 GPM or more.

Thanks for the replies everyone!

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Originally Posted by DRoseum

OP was looking to use four 150 gpd membranes in series. The outflow rate of concentrate will be very small. 6.25 gph (0.104 gpm) max for that membrane. Divide that by 2^4 since it's 4 in series. This puts outflow rate somewhere in the range of 0.00423 to 0.0065 gpm depending on actual membrane performance. Normally at 40 degrees or cooler you will see only about 65% - 75% of advertised flowrate.

(1) doesnt look like the flow meter posted comes in a range that low
(2) very low flowrate across final 2 membranes might be problematic and inefficient.
(3) I would recommend running 2 membranes in parallel first, then combining their concentrate and feeding into a 3rd in series with them. This gives you ability to quadruple sugar content on single pass and keep higher flowrate.
(4) honestly I would buy the aquatech 8855 for highest flow rate and use all 400 gpd membranes in the config I suggest above. Price isnt that different but you will notice big speed difference. Time is personally valuable and its crucial for keeping bacteria levels down before boiling.

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Thank you for the reply. I'm very new at this and just trying to get all my ducks in a row before the season actually hits. If I understand you correctly you are recommending three 400GPD membranes. The first two are straight through and feed their concentrate into the 3rd membrane with the use of an Aquatech pump.
Again thanks for the reply!

Yep - I think you will be happier with the higher throughput rate of that configuration and it won't cost you much more.

Good article on the details of reverse osmosis:
https://puretecwater.com/reverse-osm...everse-osmosis

Has a good diagram of the 2 stage set-up I was referring to:
Attachment 20326

Tapped 36 in Summit County yesterday the 15 day forecast looks good.

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Originally Posted by jrgagne99

Does anyone know when the 400 gpd membranes arrived on the scene? I don't remember seeing them when I made my first RO system back in 2007. I believe 100 gpd was the max at that time. The 400's must use a different canister than the 150's right?

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Looks like the 400s use a 3 inch housing where the 150s use a 2 inch.

For what its worth, i use a system exactly like what the OP is showing. I run it at about 100 to 120 psi with 150 gpd membranes. My sap is usually about 1.5 to 1.75% and I can get it to almost 6% in one pass. Often during the week i will run the concentrate back into the holding tote and slowly get the whole batch to about 7.5 to 8%.