Use tubing with better chemical compatibility for reagent pumps

[with-eight-hands]

Silicone has many chemical compatibility issues. PTFE (Teflon) is trickier to find and to work with, but PTFE tubing for peristaltic pumps does exist. For example, one can find listings https://www.aliexpress.us/item/3256801720045974.html (China) or https://adhesivedispensing.com/peristaltic-pump-tubing (UK) for ~3 USD / meter, which is really not bad. (When searching, it might be helpful to include a gauge number (e.g. "11G"), as that seems to help select for the tubing meant for pumps.) PTFE is fairly rigid, so most tubing for liquids or 3D printers are not suitable. Also worth noting is that the thin-wall peristaltic pump tubing only seems to be available with ID<3 mm, so flow rates will be lower for the same rotor speed.

I have ordered and tested some 11 G (2.41 mm ID, 3.01 mm OD) PTFE tubing, and it does indeed work with a simple pump design (more to be posted at #215 ). Unlike silicone, it is not stretchy, so pump designs that rely on the tubing being stretched into place may not work. Also, it does not spring back to its original shape. This image shows how flattened the tubing remains even after being removed from the pump.

I do wonder about durability, but in short-term tests it seems to work in spite of its lack of springiness, and it is sold for this application, so I think it's a reasonable choice. For the above tubing with 2*wall_thickness = 0.60 mm, I found that a nominal bearing-to-fixed-surface spacing of 0.70 mm was too big, and that even 0.55 mm might still be on the permissive side. Given that it is less squishy, it might never quite seal as well as silicone, but with a 0.55-mm spacing, I was not getting backflow with 10 cm water head pressure, so I think it is probably adequate.

Given the relative affordability of this tubing, I highly encourage anyone testing or designing pumps to start working with PTFE. Silicone is not suitable for many solvents and reagents, so for this thing to eventually be useful, I'd argue that it should accommodate PTFE tubing as a drop-in option from the start.