One is a push. One is a delivery rate.
Almost everything downstream falls out of grasping this single distinction, so it's worth slowing down on.
You can have pressure with zero flow (a closed, pressurised tank), and you can have flow at falling pressure (a tank emptying out faster than it's refilled). Your gripper needs both held at once: the right pressure and enough flow to sustain it.
Potential, not movement.
Pressure is force spread over an area. One bar ≈ 100,000 newtons pressing on every square metre — roughly the weight of the whole atmosphere above your head. It is stored potential: a gauge reading 6 bar in a sealed vessel stays at 6 bar forever, with no air going anywhere.
For your venturi gripper, the supply pressure is a threshold. Below the rated pressure the nozzle can't generate proper suction and the part drops — no amount of extra flow fixes a pressure that's too low.
Movement, measured per minute.
Flow only exists when air is in motion. Your "240 L/min" means: while the gripper is running, it swallows 240 litres of air every minute. Crucially, that's almost always quoted as free air — the volume that air would occupy out in the open at normal atmospheric pressure, before it gets squeezed into your pipe.
Compressor specs use the same currency. A compressor's FAD (Free Air Delivery) is also in litres of free air per minute. So you can compare them directly: the gripper wants 240 L/min FAD → the compressor must be able to make at least 240 L/min FAD. Same units, fair fight.
Height is pressure. The tap is flow.
This is the intuition that makes the tank click. Picture a water tower on stilts feeding a tap.
The water level (height) is your pressure — it sets how hard water shoots out of the tap. The amount of water is the volume stored. The tap is your flow demand. Open the tap and water flows — but as the tank drains, the level drops, so the pressure drops. That's exactly the effect you spotted: draw air from a fixed tank and the bar falls.
You're right that a bigger tank stores more — it drops more slowly. But a tank, on its own, always drains eventually. To hold the level steady you need something refilling it as fast as the tap empties it. That refiller is the compressor.
If circuits are more your language.
The same maths shows up in electronics, and the mapping is exact enough to reason with:
| Pneumatics | Electrical equivalent | Behaviour |
|---|---|---|
| Pressure (bar) | Voltage (V) | the potential / push |
| Flow (L/min) | Current (A) | the rate of delivery |
| Storage tank | Capacitor | buffers spikes, then sags |
| Compressor | Power supply | sets sustainable current |
| Pipe / fitting restriction | Resistance | causes pressure drop under flow |
A capacitor can dump a big current spike instantly — but if you keep drawing, it's the power supply's continuous rating that decides whether the voltage holds. A bigger capacitor buys you a longer spike, not a bigger supply. The tank is your capacitor; the compressor is your supply.
Not "air" — usable air.
Here's the part that makes sizing concrete. A tank doesn't give you all its air; it only gives you the air sitting above your minimum working pressure. Squeeze air into a tank and the free-air volume it holds is roughly:
So the usable buffer — the air you can draw before pressure sags below what the gripper needs — is the slice between your compressor's cut-out pressure and the gripper's minimum:
Divide that usable air by the flow rate and you get your buffer time — how long the tank alone can feed the gripper if the compressor stopped. Play with it below.
Sizing sandbox.
Pull these from your real datasheets. Defaults are typical venturi-gripper figures — change the minimum pressure and consumption to match yours.
The tank changes the shape, never the average.
Your worry — "draw from a fixed tank and the bar drops, so it doesn't make sense" — is correct physics. Here's the resolution in one line:
A tank buffers peaks and rides out short bursts; it does not add capacity over time. For anything you run continuously, your compressor's FAD must meet or beat the average draw, or the tank slowly empties and the pressure collapses — no matter how big the tank is.
So two genuinely different cases:
Bursty / intermittent gripping
If the gripper only pulls air for a couple of seconds per pick and rests in between, your average demand is far below 240 L/min. Here a generous tank does real work — it covers each spike while a modest compressor lazily refills between picks. Tank size is your friend.
Continuous gripping
If the gripper draws steadily (continuous-flow venturi, long hold times), the average is ~240 L/min. The tank only smooths ripples and stops the compressor short-cycling; the compressor itself must deliver ≥ 240 L/min FAD at your working pressure. A huge tank just delays the inevitable sag.
"Buy a big tank and it won't matter how weak the pump is" is only true for bursts shorter than your buffer time. Run longer than that and you're back to needing real compressor capacity.
The shopping checklist.
- Working pressure required. Read the gripper/ejector datasheet — most venturi units want 4–6 bar, not 1. Your compressor's max (cut-out) pressure must clear this comfortably.
- Air consumption (FAD). The 240 L/min figure, confirmed as free air. This is the number to match against compressor FAD.
- Duty cycle. How much of the time is the gripper actually drawing? This is what separates "small compressor + big tank" from "big compressor." Measure or estimate it honestly.
- Compressor FAD ≥ average draw, with headroom. Size FAD to (consumption × duty), then add ~30% so it isn't running at 100% forever. Check FAD is quoted at your working pressure, not at zero.
- Tank sized to your bursts. Big enough that buffer time comfortably exceeds your longest continuous draw and to stop the compressor short-cycling — not as a substitute for FAD.
- Mind the plumbing. Undersized hose and fittings cause pressure drop under flow (the "resistance" in the analogy). A correctly sized supply can still starve the gripper through a thin line.
Pressure decides if the gripper works at all. Flow (FAD) decides if it keeps working. The tank only buys you time between the two.