Originally, SPD systems came with a ¼’’ GL connection through which the vacuum would pass. Vacuum lines were used to connect each gl-14 barb to daisy-chain the system together. This worked perfectly with smaller systems like the 1L and 2L stills, but we discovered that the larger we scaled our boiling flask, the harder it became to sustain deep vacuum and evacuate the system during a run. There were other issues with this design; we soon discovered that certain types of material would “off-gas” and interfere with the distillation by disrupting the vacuum depth. Silicone and rubber lines were the primary culprits when we exceeded vacuum depths of 300 micron or 0.300 torr.
There were also other issues such as gaseous fractions condensing at the openings of the GL ports causing clogs and disrupting the distillation process. This created an odor issue with the final distillate – and not all the compounds were being cleanly separated as before. Subsequently, the connections were made wider and 3/8’’ gl-connections were employed to fix the issue. We also hanged the type of material we used for our vacuum lines to a more rigid polymer and this required us to grease and clamp each barb fitting to ensure no leaks would disrupt the run.
We noticed the vacuum depth increase exponentially when we removed the bottleneck between the pump and the cold trap – even more so after we removed all lines and allowed for a full wide-bore path from the point of distillation. This caused some small issues in the short-term, although we were now achieving ultra-deep vacuum, our pumps were being loaded up with so much vapor during the volatile/terpene fractions, a dual-pump solution had to be employed and more cold trapping power added. Since then the tech has been much improved upon allowing for a more efficient and effective process.
If you are choosing to buy an SPD set-up, we highly recommend a full-bore configuration for maximum versatility and fraction resolution.