How a Vacuum Seal Works on Porous Concrete, Brick and Stone
A suction cup sticks to glass and fails on concrete. A powered vacuum pad sticks to concrete because it never stops trying.
The seal is never perfect
Concrete, brick and stone are porous and textured at the scale of the sealing lip. Air leaks into the evacuated zone continuously, through surface pores and through the inevitable micro-channels at the lip interface.
A passive suction cup — the kind used for glass — contains a fixed, finite evacuation. Once air leaks in, the pressure difference decays and the cup releases. On a polished stone or glass the leak rate is near zero, so the cup holds. On concrete the leak rate is high enough that a passive cup holds for seconds, not minutes.
A powered pad solves this by evacuating continuously. A pump removes incoming air at least as fast as it leaks in, sustaining the pressure difference. The pad is not a sealed cup; it is a small, continuously evacuated chamber.
Leak rate, flow and the pump
Two numbers matter: the leak rate (how fast air enters) and the pump flow (how fast air is removed). Holding force is stable only while pump flow ≥ leak rate. If leak rate rises above what the pump can handle, pressure difference falls and so does holding force.
Leak rate rises with surface roughness and permeability. A dense, steel-trowelled slab leaks little; a dry, open-textured block leaks a lot. Dampening a dusty surface slightly can reduce leak rate by closing pores with a water film — a common field trick, though the seal should be tested, not assumed.
Pad compliance matters equally. A soft, conforming lip bridges texture and accommodates minor undulation. A hard lip seals only on flat, smooth surfaces. VACDRILL's pad is designed for construction tolerances, not laboratory glass.
Practical checks on site
Listen to the pump. On a good seal the tone steadies within a few seconds as pressure difference builds. A hunting, surging tone indicates the pump is chasing a large leak.
Watch the pad. The lip should compress and seat visibly. If the machine rocks or the lip gapes at an edge, reposition to a flatter zone or clean the surface. A wire brush and a wipe with a damp cloth often turn a marginal seal into a solid one.
Test before you drill. After anchoring, apply a moderate lateral load by hand. A well-sealed pad resists firmly. If it slides or peels with light force, it will not resist hammer recoil reliably.
Mind the edge. Sealing within 40–50 mm of an arris or joint invites a leak path through the edge. Keep the pad fully on the face material, away from cracks, joints and board edges.