1. From the time the VM valve is opened, I have to wait anywhere from 1 minute to 20 minutes for the product to come out. I found that if I momentarily suck on the VM's product take off (via an extension hose) I can get product out immediately. It seems a vacuum helps a lot in pulling vapors into the VM arm and once that has been initiated the vapor collapse sustains this vacuum.
2. My VM's output is unpredictable. It seems to go into an avalance mode once the vacuum is initiated. At one time I measured 3 liters per hour output rate so now I use temperature to tell me whether I have enough reflux going on. If I can hold the temp steady (+/- 0.1 deg C) then I can expect the purity to be steady as well. I get 94~95% purity by keeping the temperature steady.
3. It is very difficult to know what my vapor split ratio is, since one arm is vented to the atmosphere (reflux condenser) while the other arm sustains its own self-induced vacuum (vertical take-off & Liebig). We can not rely on aperture size since the pressures are not identical on the reflux condenser & vertical Liebig.
4. At one time I waited for the product arm to shut off by itself and I got tails coming out.
With all my difficulties stated above I think I am ready to construct another head and this is what I have in mind.
1. Dual condensers with both having its own vent to the atmosphere. This gives me equal pressure on both sides of the splitting tee. (condensers are the yellow-colored cylinders)
2. Sliding valve to control the vapor split. (red-colored cylinder)
3. Outputs from either condenser can be used for reflux or product take off.
4. Both condensers should be sized to knock-off 100% of the vapors all by itself.
The condensers can be oriented in a vertical position also, either pointing upwards or downwards but I'm not sure what effect that will have.
Advantages:
1. Predictable and repeatable RR setting
2. Instantaneous response (I think, please correct me if not)
3. No smearing
Comments/criticism/suggestions welcome. It is very possible that this has been discussed before but I have no way of searching for it since I don't know what it is called.
Inspiration for this head was supplied by Myles' & YHB's work.