Long response for a good question.
I have no tachometer to measure the rpm with. I am a rookie researcher but I can tell you that I set it as low as possible AND keep it running true and not wobbling. It is very slow and keaves a wake.
My thinking is this; the stir bar should be used only out of necessity to keep the majority of the sputters and pops frim jumoing up into the disty stream. Once I start to boil dry (a technique mostly frowned upon) then I slow the bar until it just barely balances on the convex dome built into the boiling flask. At the end I am counting on the stir bar to emulate a kugelrohr insofar as I use it to spread the oil into a thin film. A fast stir bar makes it hard to ease into a thin film mode and instead it will tend to sling the oil - not good. The stir bar as I use it is only set to keep the oil moving and not a touch more.
The total energy (enthalpy) of the system is best controlled by the heating mantle because that is its function. The gentle spreading of the oil film and moving the oil so that it tends to bust up the bubbles that form before they pop (throwing the next fraction up into the disty stream) is what I use the stir bar for. So setting the stir bar is a function of getting it to run smooth and not so much a set number. Frankly the set number ONLY tells us what the shaft speed is anyway - there is always slip with a magnetically coupled shaft with a load. So the RPM say of 1900 means the shaft is turning that fast but only the shaft connected to the motor is measured. The magnet itself is slowed by the gunk and slips as the magnetic field rotates. The faster you try to drive the shaft the slower the actual speed is of the magnet because it is pushing against a load. The thicker the extract gets during the boil then the greater the difference is between shaft rpm and magnet rpm. Monitoring shaft rpm then must take into account the greater forces needed to turn in the thicker tar and adjusted because the stir bar magnet is turning slower.
It is possible that as your extract thickens that the magnet rpm is slowing without your knowing it and therefor you see changing conditions IF you count on tye stir bar to input energy into the system. This is PURE SPECULATION but if your calculations rely on a fixed rpm then a unit incapable of measuring magnet rpm and adjusting for it will through off the math.
Sorry for the length but when I get stoned I tend to answer f4200 comments. I find I can think a little better this way lol. @Future4200 may have insight into this issue. @cyclopath, @Photon_noir, and a LOT of folks who post here hopefully are the most open minded and qualified folks to even talk at this theoretical level without demanding that my bedroom lab be dismantled and the “safe place” people come to take me to a safe place… I left out a LOT of good folks names here but my endorsement is valid for the site as a whole.
I have another disty run in me and I am out of med. I am going to go for a four pass, DCVC, LLE and BRTD (Beakers Room Temp Dewaxing) run including chlorophyl removal. I plan to slide show it. I am going for the highest purity I can bang out. I will monitor stir bar rpm if I can find a way and measure the actual speed with a tachometer that relies on flashes of light to measure RPM. It is a strobe light arrangement and you adjust the strob until the stir bar seems to freeze visually. From the rate of light flash pulses you can determine exact RPM. From that we can test how much slower a magnet becomes during a SPD run.
whew… time for some GASP!!! Kief!
Yup, Beaker is puffing kief.
years of smoking very high potency oil and
i decided to switch to a lighter blend. It makes me cough a bit but not like before. Add a little D9 isolate to make kief balls stick together and we have a winner!
PS in my rig 190°C is about 35°C higher than I run unless the extract is particularly tar like and thick. I set my mantle PID at around 150°C-165°C for a run at three to ten microns system pressure.