The Cylinder That Lagged: Air Volume Between Valve and Actuator
A cylinder that was slow to start moving after the valve energized. There was a 0.3-second delay between the valve clicking and the piston moving. The customer thought the valve was slow. The valve was fine. The 3-meter tube between the valve and the cylinder was filling with air before pressure built enough to move the piston.
The delay calculation
When the valve energizes, air flows through the tube to the cylinder. The tube has volume. The air must fill the tube before pressure builds at the cylinder. For a 6 mm tube, 3 m long: V = π × 3² × 3000 = 84,823 mm³ = 0.085 L. At 6 bar, the free air volume is 0.085 × 6 = 0.51 L. At a valve flow of 100 Nl/min (1.67 L/s), the time to fill the tube is 0.51/1.67 = 0.3 seconds. That’s the lag.
The fix: move the valve closer
We moved the valve from the electrical panel (3 m away) to a valve terminal mounted 200 mm from the cylinder. The tube length dropped from 3 m to 0.2 m. The volume dropped to 0.006 L. The fill time dropped to 0.004 seconds. The lag disappeared. The cylinder moved immediately when the valve energized.
The options
| Setup | Tube length | Fill delay | Use for |
|---|---|---|---|
| Remote valve (panel) | 3 m | 0.3 s | Non-critical, slow cycles |
| Near valve (local mount) | 0.5 m | 0.05 s | Most applications |
| Valve terminal on cylinder | 0.05 m | <0.01 s | Fast cycles, high speed |
| Proportional valve (closed loop) | Local | 0.02 s | Precise motion control |
The tube size
A larger tube fills faster but has more volume. For fast cylinders, use a shorter tube, not a larger one. A 1 m run of 8 mm tube has about the same volume as a 3 m run of 6 mm tube. The larger tube doesn’t help if it’s long.
The application
For a cylinder that must start within 50 ms of the valve signal, mount the valve within 300 mm of the cylinder. For a cylinder that can wait 0.3 seconds, a remote valve is fine. The lag is a function of tube volume and valve flow, not valve quality. The valve that lagged wasn’t slow — it was 3 meters away from the cylinder it was controlling.
The lag rule: If a cylinder is slow to start after the valve energizes, it’s filling the tube. Move the valve closer to the cylinder. Use a valve terminal for fast cycles. The lag is air volume, not valve response. A 3 m tube always has a delay.
