Since replacing the 3/8" suction tubing & fittings on the liquidator with 1/2" I've been fighting vacuum leaks for several days. When I can get all the leaks stopped the flow will peg out the rotometer with the valve less than half open. The trouble is I can't keep the system leak free for more than about an hour before another leak develops.
The Teflon thread tape is just not cutting it. I finally replaced all the teflon tape with Blue Monster thread sealer. I really don't like using the stuff because it is so messy, but it will hold up under a vacuum. When I finished that I thought I had the problem licked until about 3 hours later I checked the system again and there was another leak -ARG . This time it was the $60 needle valve. I tightened up on the packing nut and the leak stopped for about 10 minutes then came back. So I go to Lowes and buy a 1/2" PVC ball valve and new fittings. Thought I would give the teflon tape one more try and sure enough there were no leaks until 15 minutes later. That's where I quit for the day and I'll start back tomorrow.
With even the smallest of vacuum leaks the flow meter will not go above 4.0 with the valve wide open. When all the leaks are stopped the meter will peg out and the hose will almost collapse when I close back on the flow valve. I'm aiming to maintain 5.5 ppm FC and so far the liquidator has done a good job even with all the leaks. If I can maintain the flow meter at just a little over 4.0 - say 4.25 the FC will stay very close to my target. It seems to stay between 5.0 and 6.0 consistently (at least for the past three days).
Now I just hope I can get my money back for the needle valve. For all the money I've spent on the liquidator itself, replacing the fittings and tubing, and the valves I could have almost paid for a peristaltic pump - and I wish I had gone that route to begin with.