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Hydrostatic Head Explained for Dry Bags

Hydrostatic head in plain language

Imagine clamping a fabric over the open end of a vertical tube and slowly filling the tube with water. The growing column creates pressure against the specimen. The height at which defined droplets appear on the dry face is reported in millimetres of water, often written mmH2O. A result of 5,000 mm means the specimen resisted the pressure created by a five-metre water column under that procedure. It does not mean a bag can safely be taken five metres underwater.

The method isolates the barrier layer so materials can be compared before cutting and welding. It is especially useful for coated or laminated textiles, where tiny pores, uneven coating or flex damage may reduce resistance. For a continuous TPU film, the result can be very high until stretch, fixture limits or edge leakage intervenes. Hydrostatic head explained correctly is therefore a test result with boundaries, not a universal waterproof score. The water column test basics article focuses on apparatus and specimen preparation.

Converting millimetres of water into pressure

Pressure follows the relation p = rho g h, where rho is water density, g is gravitational acceleration and h is column height. At standard conditions, one millimetre of water is about 9.80665 pascals. Multiply a result in millimetres by 9.80665 to obtain pascals, divide by 1,000 for kilopascals, or divide by roughly 10,197 for bar. The conversion is linear: doubling the column doubles the pressure on the clamped specimen.

Temperature, water purity and local gravity make small differences, but rounding to 9.8 Pa per millimetre is adequate for product discussion. The more important issue is unit confusion. A report marked 10,000 Pa is only about 1,020 mmH2O, not 10,000 mmH2O. Likewise, 10 kPa is approximately 1,020 mmH2O. Always copy the unit with the number and retain the original report. Converting a headline while discarding its method creates false precision.

How a hydrostatic test is run

A conditioned specimen is clamped with a known exposed area and the water-facing side identified. Pressure then rises at a stated rate, either by increasing a physical water column or by using a controlled pressure system. The operator watches the opposite face for leakage. Some procedures stop at the third droplet; others define a pressure hold or record the first sign of penetration. Clamp leakage, water at a pre-existing fold and droplets at a deliberate needle hole must be handled according to the selected method.

Rate matters because polymers and textiles respond over time. A fast ramp can produce a higher peak before water finds a tortuous pore, while a slow hold may reveal seepage at lower pressure. Specimen size, number of pieces, conditioning humidity and whether the coated face touches water also change comparability. ISO and ASTM International publish relevant textile test frameworks. A useful report identifies the exact edition and deviations rather than citing an organisation alone.

Why the fabric number is not a bag rating

A dry bag is a system with at least four possible water paths: through the panel, along a weld, around an attachment and through the roll-top. Hydrostatic head measures the first path on a flat sample. It normally says nothing about a buckle pulling the mouth out of alignment, a webbing anchor crossing a seam or grit trapped in a fold. Those discontinuities fail at much lower pressures than an intact TPU barrier and are the reason a giant fabric number cannot stand alone on a product page.

Pressure distribution differs too. In the test fixture, the clamped area receives an even load. A submerged bag deforms, trapped air pushes outward, contents create hard points and the closure may sit at a different depth from the base. A person squeezing a 10L bag underwater can briefly add more local pressure than static depth alone. Finished-product evaluation should follow the exposure scenarios in rain protection versus submersion, not a depth claim calculated from fabric head.

Hydrostatic head versus IPX ratings

Hydrostatic head and ingress protection answer different questions. The former challenges a material specimen with pressure and reports a water-column result. IP testing evaluates an enclosure under defined water exposure. In IEC 60529, IPX5 and IPX6 concern jets, IPX7 concerns temporary immersion, and IPX8 uses conditions agreed for the product. A dry bag may use high-head material yet have no IP rating, or it may pass an immersion protocol without the underlying report being expressed as mmH2O.

Do not convert one directly into the other. A 10,000 mm textile result does not equal IPX7, and an IPX7 bag report does not reveal fabric head. The IEC defines the IP framework, while the IPX rating guide explains what each exposure means for roll-tops. A sound specification can require both: a material minimum for batch control and a closed-bag immersion result for user-facing performance.

Reading results for coated and laminated fabrics

A coating sits on or partly within a textile, while a laminate bonds a distinct film to it. Coated cloth may show local thin spots at yarn intersections, so results often scatter more across specimens. A TPU laminate can deliver a more continuous barrier, but bond defects, contamination and flexing can still create channels. Report minimum and individual values, not only an average. If five pieces return 12,400, 12,100, 11,900, 3,200 and 12,300 mm, the low outlier demands investigation.

Face orientation should match expected exposure or be tested both ways when the construction may be reversed in use. A textile face against water can absorb liquid before the barrier is challenged; a film face may show the barrier response directly. After initial testing, fold or abrade another set and repeat the method. The laminated versus coated fabric guide explains construction differences, and waterproof rating degradation covers why pristine values decline.

The role of seams, welds and attachments

Welds can be evaluated with local pressure, peel tests, burst methods and complete-bag checks, but they are not represented by a panel-only water column. A narrow weld may look neat and still contain a channel where dust crossed the electrode. An overheated weld can thin the material at its edge, creating a stress line that opens after repeated rolling. Stitching through a barrier introduces holes unless a separate sealing process closes them. Each attachment should be mapped as a potential water path.

Use an air screen before wet testing. Close the bag, apply low controlled internal pressure and brush a dilute soap solution over welds while keeping it away from areas that cannot be rinsed. Bubbles locate channels quickly. The bag then needs a clean water exposure because air and water do not behave identically. Detailed joining variables are covered in welded versus stitched seams. Never inflate a roll-top to high pressure; the closure is not a pressure-vessel fitting.

Why pressure rises quickly with depth and squeezing

Water adds about 9.8 kPa for every metre of depth. At one metre, the external pressure above atmosphere is approximately 9.8 kPa, equivalent to a 1,000 mm water column. That seems far below many fabric results, yet the bag closure is flexible and may be loaded unevenly. If air is trapped, descending compresses the bag and changes folds. If the user squeezes it between a body and a kayak, internal air can push at the weakest opening.

Dynamic events further complicate the comparison. A wave slap, falling bag or high-speed jet creates short local peaks rather than a uniform static head. A bag towed behind a boat sees turbulence and closure loading not represented by gentle immersion. Specify the actual scenario: thirty minutes stationary at one metre, repeated jets, a 1.2 m drop into water, or deck splash for four hours. The pressure differential guide develops the trapped-air calculation in greater detail.

Reasonable thresholds for different product claims

There is no universal millimetre threshold that automatically makes a dry bag suitable for every activity. A rain organiser needs a continuous barrier and reliable closure under low pressure. A paddle bag needs welded seams, jet resistance and short-immersion evidence. A professional equipment case may need agreed depth, duration and post-conditioning tests. Set a material threshold high enough to catch coating faults, then let complete-product tests determine the claim. Avoid specifying a larger number simply because a competing page prints one.

The table below is a planning framework, not a declaration that water-column values grant those uses. It pairs a suggested material-screening range with the more important product evidence. Values should be adjusted after pilot testing and risk review. Electronics, medical contents and cold-water safety justify redundancy even when the bag passes. A small inner pouch, moisture indicator or hard case may be more valuable than raising an already high fabric result.

Ageing, flexing and contamination before retesting

A new flat panel provides a baseline. Service performance requires conditioning that represents use: repeated folds at the mouth, abrasion on the base, heat exposure in a vehicle, cold bending, UV where relevant and contact with salt or cleaning solution. The sequence matters. A panel may retain head after UV but fail when flexed afterward because the surface has become less extensible. Keep an unconditioned control set so a reduction can be attributed to ageing rather than normal specimen scatter.

Contamination at the test fixture can create false failures. Salt crystals, detergent residue and sand should be documented, not silently wiped away, when the aim is a used-state assessment. Conversely, incoming material tests should use clean conditioned samples. Report both absolute result and retention: a drop from 15,000 to 9,000 mm is 60% retention even if both numbers exceed a chosen minimum. That retention trend helps compare constructions more honestly than a single fresh-material peak.

Common ways water-column claims mislead buyers

The first red flag is a number with no unit. The second is a unit with no method. Other warning signs include presenting a pressure limit as a safe diving depth, quoting the best specimen rather than the minimum, testing only the polymer face while selling a different composite, or displaying a laboratory value after adding stitched attachments. A claim such as tested to 20,000 mm should lead to the report, specimen identity, conditioning and endpoint. If those cannot be matched, treat it as promotional language.

Another error is assuming more is always useful. Once a reliable barrier comfortably exceeds the product need, closure design, weld consistency and abrasion life become better places to spend weight and test budget. A 30,000 mm panel with an awkward two-roll mouth is a worse dry bag than a lower-head panel in a well-tested three-roll system. Use the dry bag specification guide to compare like with like and retain original reports for traceability.

• Reject a hydrostatic number that omits its unit and named method.
• Check whether the tested specimen matches the production material code.
• Ask for all replicate values, including the lowest result.
• Separate flat-panel evidence from seam, closure and attachment evidence.
• Never translate millimetres of water directly into a safe-use depth.

A complete verification ladder for a dry bag

Start with material identity, thickness, mass and hydrostatic head on multiple roll positions. Next validate weld peel and continuity on process strips. Screen each closed bag or a defined sample with low-pressure air, then expose finished units to rain, jets or immersion matching the intended claim. Repeat the exposure after closure cycling and abrasion. Finally, run a loaded field trial to reveal handling problems such as a buckle hidden under a strap or a rigid object pushing into a base corner.

Document sample count and acceptance criteria before testing. For production, AQL 2.5 can govern visual and dimensional inspection, while safety-critical leak checks may need a stricter plan or 100% screening depending on the product brief. Record water temperature, duration, depth, load and permitted ingress in grams. The quality-control guide connects these stages so a material certificate cannot be accidentally treated as complete product approval.

1. Confirm material code, construction, thickness and specimen orientation.
2. Measure hydrostatic head on agreed positions and retain all values.
3. Validate weld settings with peel, visual and channel checks.
4. Test the closed bag under the intended rain, jet or immersion exposure.
5. Repeat after flex, abrasion and relevant temperature conditioning.
6. Run a loaded field trial and document leakage location rather than only pass or fail.

How to write hydrostatic head into a purchase brief

State the exact material code and construction, minimum result, test method and edition, pressure rate, specimen face, conditioning, number of specimens and failure criterion. Add a rule for lot sampling and what happens after a failure. Then write separate requirements for weld strength, air screening and complete-bag exposure. The brief should say whether zero visible droplets are allowed, whether absorbent paper is used inside and how much mass gain counts as ingress. Ambiguity disappears when every result has a unit and endpoint.

Production since 2014 and an SGS-verified 4,950 m2 facility with 137 staff, 7 lines and 149 machines support controlled waterproof-bag programmes. Typical commercial planning uses a 500-piece MOQ, 6-10 working days for sampling, 35-50 days for bulk and FOB Xiamen where agreed. See process context then use the contact page to send the test method, target load and exposure conditions with the drawing.

Trust a hydrostatic result when it is traceable to the correct material, uses a named method, reports conditioning and replicates, and includes the lowest value. Use it to screen barrier consistency and compare ageing retention. Do not use it to promise that a roll-top can remain at an equivalent depth. The complete bag has a closure, seams, hardware and a user, all of which change the route and pressure of water.

After reading the number, ask one question: what finished-product test follows it? A credible answer names depth or jet condition, duration, closure rolls, internal load, sample count, permitted ingress and post-conditioning state. That sequence turns hydrostatic head from an isolated headline into useful engineering evidence. If the use is ordinary rain, avoid paying for an extreme material figure while neglecting ergonomics; if the use includes immersion, insist that the entire closed bag has been tested accordingly.