4 inputs determine the right tubing.

A distributor-facing pre-quote checklist. If the customer answers most of these at the first call, the second call is the quote.

1. 01
   How does the run move — static, routed once, or constant flex?

   Sets the tubing polymer. Constant flex (robot arms, end-effectors, hand-tool drops) → PU. Static routed runs with oil, coolant, fuel, or higher pressure → nylon. Low-pressure utility, drain, instrument- sense → PE. The wrong polymer on a flex run fatigue-cracks in months.
2. 02
   What does the line touch — oil, coolant, fuel, solvents, food, normal air?

   Drives the polymer chemistry. Oil / coolant / fuel → nylon, not PU, which swells and fails. Aggressive chemistry, high temperature, or UV → FEP/PTFE. Food-contact → FDA-grade PE or a certified line. PU is for clean general air.
3. 03
   What's the operating pressure and temperature in the run?

   PU's pressure derates fast above ~150°F — nylon (240-350 PSI) is the answer at heat and pressure. PE has the lowest ceilings; FEP/PTFE has a lower working-pressure rating than PU or nylon at comparable OD despite its temperature range. Get the actual service condition.
4. 04
   What's the OD, and is it metric or inch?

   Tubing OD has to match the fitting — 4 mm and 1/4" are not interchangeable. Confirm the OD measured with a caliper and the metric / inch call before pulling part numbers, and quote tubing by the spool / coil.