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Pressure Differential Inside a Dry Bag

What pressure differential means inside a sealed bag

A pressure differential is simply the difference between the pressure of the air trapped inside a bag and the pressure of whatever is pressing on the outside. When those two are equal, the body is relaxed and the closure does very little work. The moment they diverge, the film either collapses inward or bulges outward, and every join, fold and weld line feels the resulting force.

Because a dry bag has no rigid frame, it cannot resist much differential by structure alone. Instead the geometry changes: volume shrinks under external pressure, or expands under internal pressure, until the two balance again. That flexibility is why roll-top bodies survive situations that would split a hard case, and also why the closure, rather than the fabric, ends up deciding the outcome in nearly every real failure.

Units are worth settling once, because the rest of this guide uses them throughout. One atmosphere is 101.3 kPa, one bar is 100 kPa, and one psi is about 6.9 kPa, so a differential of 25 kPa is roughly a quarter of a bar or just under four psi. Those numbers sound small against a car tyre, but spread across the large soft panels of a dry bag they move hundreds of newtons total force.

Hydrostatic pressure: the numbers at real depths

Water pressure increases linearly with depth according to the product of density, gravity and height. Fresh water at 1000 kg per cubic metre and standard gravity of 9.81 metres per second squared gives about 9.81 kPa for every metre; seawater runs roughly 2.5 percent higher because dissolved salt raises its density. In round terms, one metre of depth costs one tenth of an atmosphere.

This is why the standard IPX7 immersion test is a modest challenge in physical terms. Thirty minutes at one metre subjects the shell to roughly 111 kPa absolute, only about 10 percent above what it sees sitting on a table. Sales copy that implies survivability at depth is rarely backed by a protocol, and the grading language itself belongs to IEC 60529 rather than to any diving anecdote.

Boyle’s law and why a rolled bag shrinks underwater

For a fixed amount of gas held at a constant temperature, pressure multiplied by volume stays constant. Double the absolute pressure and the volume halves; raise it by ten percent and the volume falls by roughly nine percent. Every figure in the depth table above comes straight from this relationship, which is why even a lightly submerged bag changes shape visibly.

The practical consequence surprises people: bringing a bag deeper does not usually force water in at the seam, it tends to slacken the closure instead. As internal air volume falls, the body collapses inward, the three folds lose their compression against one another, and any fold that was already marginal becomes a flat contact rather than a seal. Deep leaks therefore nearly always begin at the mouth rather than along a weld.

The squeeze effect: ten percent volume against eleven percent pressure

Squeezing a bag with a forearm is the same physics running in the opposite direction. Compress the enclosed volume by ten percent and internal absolute pressure rises by about eleven percent, which is roughly 11 kPa, or eleven times the entire hydrostatic load at one metre of depth. This is why a bag that has just passed a thirty-minute tank test can still leak the moment somebody kneels on it.

Where does that compression go? If the roll-top holds, pressure rises and water is pushed away from every fold; if there is a weak point, that is where pressure escapes, and escaping air is frequently followed by incoming water once the load relaxes. Compressing a sealed bag inside a hatch, under a tie-down strap or beneath a passenger seat is therefore one of the most reliable ways to arrive with damp gear.

Temperature swings from 20 °C to 50 °C on a deck

Temperature is the second driver and it is frequently ignored. Gas volume scales with absolute temperature in kelvin, so a bag packed at 20 °C and left on a dark deck that reaches 50 °C sees its absolute temperature rise from 293 to 323 kelvin, an increase near ten percent. Given room to expand, three litres of trapped air becomes roughly 3.3 litres.

If the body cannot expand because it sits under a strap or is wedged into a hatch, the same energy appears as pressure instead of volume. That is another 10 kPa or so pushing outward on every seam, which is harmless alone but stacks on top of whatever altitude or squeeze load already exists. Black films in direct sun can exceed 60 °C, at which point the differential by itself is enough to ease open a tired closure.

Altitude and air freight: a quarter bar of outward push

Going up is where the push reverses, because ambient pressure falls while the air sealed inside stays where it was. Aircraft holds are commonly pressurised to the equivalent of roughly 2400 metres, or about 75 percent of sea-level pressure, leaving close to 25 kPa acting outward across the shell. Distributed over a 600 square centimetre panel face, that translates into hundreds of newtons total force.

A roll-top handles this gracefully because it vents. Once internal pressure exceeds the friction holding the folds together, air pushes past the first fold and escapes with a soft hiss, then the folds reseat at the new equilibrium. A genuinely airtight closure cannot do that, which is why some fully sealed rigid cases arrive with domed lids or popped gaskets after flight. The buoyancy side of that equation is worked through in the air and floating analysis.

How the roll-top relieves pressure on its own

A roll-top is best understood as a pressure-relief valve built from fabric. The three folds create a long tortuous path held together by nothing more than compression and friction, so any sustained internal pressure finds the route of least resistance and releases air outwards while refusing liquid water the same route inwards.

That asymmetry is the clever part. Air escaping needs only a few kilopascals above ambient, whereas water entering against the same path must overcome surface tension at each fold plus the compression of the fabric layers beneath the buckle. This is why a bag can hiss audibly as it is lifted from water and still have completely dry contents. The step-by-step mechanics are set out in the roll-top closure explainer.

Why hard items pressed against seams cause point leaks

Differential loads are uniform, but the objects transmitting them rarely are. A bottle cap, a stove rim or a shoe edge pressed against the wall by outside pressure concentrates the entire local load into a few square millimetres. The film itself usually survives; what fails is the boundary between the film and whatever sits behind it, whether that is a weld edge or simply a fold.

Once local pressure exceeds the small hydrostatic head the film can resist across that contact patch, water moves along the contact line by capillary action rather than by bulk flow. This is why point leaks are slow, intermittent and almost impossible to reproduce deliberately. Padding every rigid object with a soft layer removes the contact geometry that makes the mechanism work at all.

Air traps: when keeping some air helps and when it hurts

Trapped air is usually described as a fault, but it does two useful things. It provides a compressible cushion so external pressure changes are absorbed by a change in volume rather than by stressing the closure, and it adds temporary buoyancy if the bag ends up overboard. A body carrying roughly 15 to 20 percent of its volume as air behaves predictably across ordinary depth and temperature swings.

It turns into a liability beyond that range. Large internal volume means large swings with altitude and heat, buoyancy that fights any attempt to settle the bag into a hatch, and extra oxygen feeding whatever mildew has started to grow. The trade-off is worked through numerically in the air trap discussion, and the head-loss side of the same argument is covered in the hydrostatic head explainer.

Testing for differential failure at home

Every mechanism above can be reproduced without specialist equipment. A bath gives shallow depth, a warm car supplies temperature swing, a drive up a hill covers altitude, and a bathroom scale under a board applies a controlled squeeze load. Run them in sequence and you will locate your own bag threshold within about an hour of work.

Record what actually happens rather than simply whether the contents stayed dry. Note whether the bag hissed, how far the folds shifted, how the body felt on descent, and whether any single corner behaved differently from the rest. Repeatable detail is far more useful than a pass-fail result, and it tells you which habit to change before an expensive item depends on it.

Practical packing rules that follow from the physics

1. Leave roughly 15 to 20 percent of the body as air so external pressure has somewhere to go.
2. Pad rigid items with clothing so no single edge concentrates load against a weld or fold.
3. Press air out gently rather than forcing the bag into a drum-tight cylinder under a strap.
4. Keep heat-sensitive loads out of direct sun, since 20 °C to 50 °C adds about 10 percent volume.
5. Expect a roll-top to vent during air travel and to reseal afterwards; that is correct behaviour.
6. Avoid wedging a sealed body into a narrow hatch where it cannot change shape freely.
7. Recheck the closure after any flight, long climb or extended period in strong sun.

Notice what these rules have in common: none of them asks you to buy something new. They are adjustments to how the vessel is loaded, closed and transported, and they address the same differential arithmetic that produces the failures. The immersion testing guide adds repeatable protocols if you want evidence rather than observation.

A related habit pays off on multi-day trips. Mark the roll line you used at the start of the day with a small piece of removable tape, then check whether the closure has crept since you last handled it. A shift of even 5 mm usually indicates that the body has been through a pressure cycle, and it tells you to rebuild the folds before the bag meets real water rather than after.

How much differential a seam and closure actually tolerate

Two numbers bound the design space. A well-made RF weld typically exceeds 35 N per 25 mm in peel, which corresponds to internal pressures far above anything a normal bag will generate; the join is not usually the limiting factor. The closure is. Friction between three folds generally releases somewhere between 2 and 8 kPa above ambient, which is precisely why it vents rather than bursts.

That low release threshold has a benign side effect: it caps the maximum differential the body ever sees. Instead of building pressure until something tears, the vessel bleeds off excess and equalises. The failure modes you should actually worry about are therefore not explosive, they are slow: gradual fold slackening under load, and capillary ingress at a contact patch that has been pressed hard for hours.

Designing and specifying around pressure behaviour

For anyone developing product rather than packing it, the useful exercise is to define the differential envelope first: the depth, altitude range, temperature band and likely external load the bag must tolerate, then work backwards to closure geometry, film thickness and weld width. A body intended for airline baggage needs generous venting; one intended for towing behind a hull needs a closure that reseats reliably after repeated collapse.

Validation follows the same logic. ISO maintains coated-fabric and seam test methods that let you quote comparable figures, and most credible programmes combine immersion work with pressure decay rather than relying on either alone. Our own lines have built waterproof bodies since 2014 in an SGS-verified 4,950 m² facility running 7 lines and 149 machines, with roughly 200,000 pieces per month of capacity, sampling in 6 to 10 working days, bulk in 35 to 50 days, a 500-piece minimum and inspection to AQL 2.5. We document the full process from sampling to bulk for every programme, and comparable specifications across current bodies are listed with the product range.

One closing caution for anyone writing a specification: a vessel that vents is not leaking. Air escaping past a roll-top after you step off a plane or pull the bag out of cold water is the mechanism doing exactly what it was designed to do. Water entering through the same path is a different failure entirely, and conflating the two causes more unnecessary warranty claims than almost any other confusion in this category.