Minehart, you are correct, but with just 1 membrane you will not be able to go from 2% to 8% in one pass. You will need to back off on the pressure or the RO will shut down on high pressure.
Yes, that 125 by Ray Gingerich uses the same pumps (feed and high pressure) as his 250 and the pump will move the same amount of sap, the difference is that you will not be able to make the higher concentrate in 1 pass. That will work out just fine. While I've never run a 125 by Ray, my guess is that if you start at 2% sap you might get to maybe 5.5 or so in the first pass and then to 8% or very close in a second pass. That will not be a problem. You can either run a second intermediate tank or you can simply do like I do with my 250. If I start at 2% I run it to 7.5-8% in one pass into the head tank, then I draw from the evaporator feed line from the head tank, back thru the RO and pump the new higher concentrate back into the head tank. I feed my head tank near the top on one end and the evaporator feed line is at the opposite end of the head tank. One my second pass I just run the RO continuously but at a lower pressure. I run 1st pass at about 270-275 PSI and back off for 2nd pass to about 240-250 PSI for best results. When the head tank level gets low (I'm boiling at the same time) in about 60-75 minutes, I just open the sap feed valve the the RO as I close the recirculate valve.
On that 125 I suspect there are 2 needle valves, just like my 250. The smaller one is for recirculation. The more you open that valve the less total output you will have from the concentrate hose, but the higher the sugar % will be. I suspect with a 125 using the 250 pump you can likely have that valve open more that I do on my 250, thus getting to a higher sugar % out the concentrate. That will all just take practice. I suggest you get a good sap hydrometer that will read to 10% or even 12%. Use a long hydrometer, the short ones are harder to get an accurate reading, but even they are better than nothing.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.