Pressure Testing Method

Definition and Goal of Pressure Testing Methods

Pressure testing methods are standardized tests in which seals, hose assemblies, or fluid-power assemblies are pressurized with a test medium (liquid or gas) up to a defined test pressure. The test evaluates whether the test object stays tight, withstands mechanically, and functions reliably under pressure. In sealing technology, the focus usually lies on tightness, because even small leakages can impair the function of units, valves, or pipework.

The test goal follows from the application: in an acceptance test, the test object must demonstrate that it shows no unacceptable leakage at the specified pressure when assembled. In strength verification, the test checks whether housings, fittings, hose couplings, or sealing gaps survive without permanent deformation. For functional safety and service life, pressure cycles are additionally considered, because seals and hose assemblies often fail through fatigue, settling, and material relaxation.

Many pressure tests are considered non-destructive testing (NDT), because the test object can be used afterwards. An important exception is the burst pressure test, where loading is deliberately taken up to failure.

Pressure Levels: Operating Pressure, Proof Pressure, Burst Pressure

So that specifications remain unambiguous, pressure levels are clearly separated. The operating pressure is the maximum pressure that occurs continuously or recurrently in intended use. The proof pressure lies above the maximum operating pressure and serves as safety and quality proof, without damaging the test object. The burst pressure is the pressure at which the test object fails; the burst pressure test is therefore destructive and is frequently used as a type test or as a spot check.

Method Types: Hydrostatic, Pneumatic, Burst, and Impulse Test

Which method type is chosen depends on the test goal, on the allowable medium, and on safety requirements. In practice, hydrostatic testing is frequent, because liquids are barely compressible and therefore release less energy abruptly in case of damage. Pneumatic tests are technically sensible when liquids are unsuitable — for example, due to corrosion, residual moisture, or strict cleanliness requirements. They demand more protective measures, however, because gases store significantly more energy at the same pressure.

Burst and impulse tests complement the acceptance test. The burst test answers the question of what static safety reserve a component has. The impulse test shows how sealing systems and hose assemblies behave under repeated pressure cycles — that is, under conditions that frequently cause fatigue in the field.

Hydrostatic Pressure Test (Liquid)

In the hydrostatic pressure test, the test object is filled with a liquid (often water or hydraulic fluid), vented, and then brought up to test pressure in a controlled way. A hold time follows during which leakage, crack formation, or unacceptable deformation are assessed. For seals, proper venting is important, because trapped gas bubbles can distort the measurement signal and cause pressure fluctuations.

The method is frequently preferred because the stored energy is low. As a result, the risk in case of failure decreases and protective measures become simpler — yet the test remains safety-relevant when large volumes or high pressures are involved.

Pneumatic Pressure Test (Gas)

In the pneumatic pressure test, a gas such as air or nitrogen is used. This is advantageous when components must stay dry, when liquids could attack materials, or when very small leakages are to be detected sensitively. In sealing technology, this method is therefore occasionally used for sensitive assemblies, for cleanliness requirements, or for leak detection.

The disadvantage is higher hazard: because of the compressibility of the gas, more energy is stored and can be released abruptly in case of damage. Therefore, controlled pressure build-up, safety distances, and suitable shielding are particularly important, as is a clear definition of the test sequence.

Burst and Impulse Test: Type Test vs. Service Life

The burst pressure test raises pressure up to failure and provides values for safety reserve and design quality. It is mostly used for design release or as a spot check, because the test object is destroyed afterwards. For hose assemblies, fracture pattern and failure location also provide important hints regarding reinforcement quality, fitting installation, and local notch effects.

The impulse test works with defined pressure cycles between two pressure levels over many cycles. As a result, fatigue behavior is assessed — which, for seals, is influenced by micro-movements, settling, and temperature cycles. For hydraulic hose assemblies, this is particularly relevant, because pulsating pressures are typical in operation and failures are often cycle-driven.

Test Procedure and Evaluation Criteria (Leakage, Hold Time, Operating Conditions)

A robust test procedure begins with the correctly assembled test object, because assembly errors at seals (twisting, wrong preload, damaged sealing edge) would otherwise wrongly appear as component faults. The system is then filled and vented, pressure is built up in a controlled way and held for a defined time. During the hold time, the evaluation is carried out; pressure is then released in a controlled way, and the result is documented.

For interpretation, operating conditions are decisive. Temperature changes affect pressure, because medium and components expand or contract. In addition, materials in sealing systems show elastic deformation and time-dependent relaxation; both can cause an apparent pressure drop without real leakage being present. Therefore, a stabilization phase is frequently scheduled before the measurement time starts.

Documentation contents are similar in many specifications: test pressure, medium, temperature, hold time, measurement method, leakage criterion, test-object condition (e.g., dry/wetted), and a clear assessment.

Leakage Detection: Visual vs. Pressure Decay

In visual detection, leakage is recognized directly as escaping medium, often at defined observation points such as sealing joints, connections, or hose couplings. This is simple, yet it can hit limits with very small leakages or unfavorable wetting.

The pressure decay method measures the pressure change over a defined time. After pressure build-up, the test object is typically isolated from the pressure source; then Δp is recorded over the measurement time. A stabilization or settling time reduces measurement errors caused by temperature equalization and material expansion. The result is evaluated as either an allowable pressure drop or a derived leakage rate, depending on the specification.

Selection Guide: Suitable Test, Standards Reference, Safety Aspects

The choice of method starts with the question of what is to be proven: tightness, strength, or service life. For tightness and acceptance, the hydrostatic pressure test is often sensible when the medium is allowable and the system can be safely vented. For dry or particularly clean assemblies, a pneumatic test is considered, although safety measures can strongly shape the test setup. When the design reserve must be substantiated, a burst pressure test is appropriate. When operational reality is dominated by pressure cycles, the impulse test delivers the relevant fatigue insights.

In sealing technology, the operating conditions of the installation situation are also decisive. A seal that is tight in the test laboratory can behave differently in the field due to different surfaces, different preload, or different temperature ranges. Therefore, test pressure, hold time, temperature, assembly condition, and allowable leakage should be defined unambiguously beforehand, and they should match the later application. The specific standards reference depends on the component group — for example, on regulations for hose assemblies, fittings, or pressure equipment; the corresponding test parameters are then derived from the respective rule sets and product specifications.

When there is uncertainty about medium choice, pressure level, or safety concept, a short, specialized technical consultation is sensible, so that test setup and informational value match.