Tubing is the unsung element of any compressed air system. Everyone focuses on the valves and fittings, but get the tubing wrong and none of the rest performs as it should. Wrong material, wrong size, or wrong temperature rating, and you end up with leaks, kinking, pressure drop, or contamination that tracks back to a component that cost pennies to specify correctly.
This guide covers the key decisions: material, sizing, pressure and temperature ratings, colour conventions, and the most common mistakes worth avoiding.
Tubing Materials
Polyurethane (PU)
Polyurethane is the most widely used tubing material in modern industrial pneumatics. It’s flexible, lightweight, and highly kink-resistant — which makes it easy to route through tight spaces and around machine frames without additional support.
PU tubing handles typical working pressures well (generally rated to 10–12 bar at 20°C) and is available in a wide range of sizes and colours. It’s the default choice for most push-in fitting applications and works reliably with Parker Legris and John Guest fittings. The main limitation is temperature: PU softens at sustained temperatures above 60°C and can become brittle in very cold environments. Always check the rated temperature range for applications near heat sources.
Nylon (PA)
Nylon tubing is stiffer than PU but handles higher pressures and a wider temperature range. It’s common in higher-pressure pneumatic circuits, automotive and engineering applications, and wherever chemical resistance matters. PA tubing is less easy to route in tight bends — it prefers gentle curves — but it holds its shape better in long unsupported runs along machine frames.
Polyethylene (PE)
Polyethylene tubing is softer and more flexible than both PU and nylon. It’s typically used in low-pressure applications, vacuum systems, instrumentation, and food or beverage applications where FDA-compliant materials are required. PE carries lower pressure ratings than PU or nylon, so verify your working pressure before specifying it.
Sizing: OD vs ID — and Why It Matters
Pneumatic tubing is always specified by its outer diameter (OD), not its inner diameter. This is because push-in fittings grip the outside of the tube — so the OD is the critical dimension for a reliable connection.
Common sizes are 4mm, 6mm, 8mm, 10mm, 12mm, and 16mm OD. For a given OD, the wall thickness varies between manufacturers and materials. The inner bore affects flow rate: a 6mm OD tube with a 1mm wall has a 4mm bore, which is meaningfully different to one with a 0.75mm wall. For high-flow applications — cylinder feeds, large valve exhausts — use the largest bore that fits your fittings. For signal lines and pilot feeds, smaller bore is perfectly adequate.
Pressure and Temperature Ratings
Always check the manufacturer’s rated pressure at the actual working temperature — not just at ambient. Pressure ratings drop as temperature rises. Parker publish derating curves for their tubing: a tube rated at 10 bar at 20°C may be rated at only 6 bar at 60°C. The minimum bend radius also matters — tight bends can cause kinking or wall collapse, which restricts flow and creates a failure point. Route tubing with smooth curves or use elbow fittings at direction changes.
Colour Coding Conventions
Colour coding isn’t fully standardised across all manufacturers, but the conventions most widely used in UK industry are:
- Blue — compressed air (by far the most common)
- Black — general purpose; often nitrogen or non-air services
- Red — vacuum, or high-pressure in some systems
- Yellow — pilot signals or pushed air in certain control circuits
- Natural/clear — sight lines, level gauges, food-grade applications
Using consistent colour coding throughout an installation makes fault-finding and maintenance significantly faster. It’s worth establishing a convention and sticking to it from the start.
Common Mistakes to Avoid
Under-sizing the bore — the most common cause of slow cylinder response or poor valve actuation speed. If the system feels sluggish, check the flow path and tube bore before adjusting the regulator pressure.
Cutting the tube at an angle — a square, clean cut is essential for push-in fittings to seal properly. Use a proper tube cutter, not a knife or scissors. An angled cut will cause a slow leak that can be very difficult to find.
Routing past heat sources — PU tubing near welding equipment, hot manifolds, or heat guns will soften and may collapse or seal shut. Use a heat-resistant sleeve or switch to nylon tubing in hot zones.
Mixing tube OD sizes with non-matching fittings — always verify that tube and fitting are specified for the same OD. Parker and John Guest fittings are made to tight tolerances; off-specification tubing may not seal reliably and can cause intermittent leaks.
Browse Pneumatic Tubing at Captivair
Captivair Pneumatics stocks a comprehensive range of pneumatic tubing in polyurethane, nylon, and polyethylene — including the full Parker range in all standard sizes and colours. If you’re unsure which specification is right for your application, our team is happy to advise.
Call 01474 334537, email sales@captivair.co.uk, or browse online. Same-day despatch from our Gravesend warehouse.
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