Hydrostatic Pressure Testing of Borosilicate Glass

Hydrostatic pressure testing of borosilicate glass measures how well laboratory glassware, pharmaceutical containers, sight glasses, and sealed lighting parts resist internal hydraulic pressure. Testing follows ASTM C147 and ISO 7458. It records burst pressure and hold time, and confirms design margins and batch quality before the articles enter service.

Principle

Internal hydraulic pressure creates circumferential (hoop) stress in the wall of a closed vessel. For thin walls, the stress is approximated by σ = p·d/2t. Here p is internal pressure, d is mean diameter, and t is wall thickness. Glass fails in tension, so hoop stress governs the strength of most cylindrical articles. Surface flaws control strength, so results scatter and replicate specimens are required. Two loading modes exist. In the first, pressure rises at a uniform rate until rupture, and the failure pressure is recorded. In the second, a fixed pressure is held for a defined time as a proof test. Water serves as the pressure medium because its low compressibility delivers smooth, controlled loading without stored elastic energy. Pneumatic loading, by contrast, stores compressive energy and carries higher risk at rupture. Hydrostatic testing of hollow borosilicate ware also differs from Flat glass testing, which applies bending or impact loads to planar panes.

Sample

Preparation and Sealing Requirements
Specimens are cleaned, dried, and inspected visually before mounting. Articles showing cracks, stones, or chips are excluded and recorded, because such defects dominate the measured result. Conditioning at controlled laboratory temperature removes thermal gradients that could alter strength. Sealing is the critical step. For containers, a pressure-resistant closure with a resilient gasket and clamping plates seals the finish. For tubular articles, end fittings rated above the expected test pressure are used. The seal must not introduce local stress concentrations. The gasket material must also remain stable across the full pressure range. The vessel is then filled completely with de-aired water. Entrained air compresses under load. This distorts both the applied pressure curve and the energy released at rupture. After assembly, a low preliminary pressure check confirms seal tightness before the formal sequence begins.

Internal

Pressure Test — ASTM C147 / ISO 7458
ASTM C147 covers hydrostatic pressure testing of glass. It includes both rate-controlled testing to rupture and constant-pressure holding. ISO 7458 addresses glass containers and defines two procedures in the same pattern: a constant rate of pressure increase to destruction, and a fixed pressure maintained for a set time. In a rate-controlled run, the sealed specimen is connected to a hydraulic test unit fitted with a calibrated pressure transducer and a recording device. Pressure rises at the rate stated in the standard or product specification until rupture, and the maximum value is logged. In the hold procedure, pressure is raised to the specified proof level and held for the defined duration. The specimen is then examined for rupture, leakage, or permanent set. Equipment calibration status, pressure rate, and medium temperature are documented in the test record. The laboratory selects the procedure according to the controlling specification, since the two modes answer different questions. Rupture pressure characterizes strength, while the hold test verifies fitness for a defined service pressure.

Measured

Parameters — Burst Pressure & Hold Time
Burst pressure is the maximum hydraulic pressure a specimen withstands before rupture. It is reported in kilopascals, bar, or megapascals. Where wall thickness has been measured, the value can be converted to hoop stress for comparison across different article sizes. Individual values, the arithmetic mean, and the scatter of the replicate group are all reported. A single result carries little statistical meaning for glass. Failure location and fracture origin are noted, as breaks near the seal may reflect mounting stress rather than material strength. Hold time is the duration the specimen sustains the specified proof pressure without rupture or leakage. Results are recorded on a pass or fail basis per specimen. For batch release, the combination of group burst data and proof-pressure pass rate characterizes the pressure resistance of the lot. Readings from the calibrated transducer, not pump gauge estimates, form the accepted data set.

Co-Testing

Parameters — Thermal Shock & Leak Rate
Borosilicate glass frequently operates where pressure and temperature changes coincide. Thermal shock is therefore tested alongside pressure resistance. Thermal shock methods such as ISO 718 subject containers to a defined temperature difference by transfer between hot and cold water baths, followed by inspection for cracks. Running a pressure hold test after thermal cycling reveals interaction effects that neither test alone exposes. Leak rate is the second companion parameter. It applies to sealed articles and assemblies that must retain pressure over time. Pressure-decay leak testing measures the drop in internal pressure across a defined hold period. The leak rate follows from that decay together with the internal volume. Tracer-gas methods exist for finer leak detection where the application requires it. Recording thermal shock endurance and leak rate next to burst data gives a compact picture of whether a borosilicate article survives the combined pressure, thermal, and sealing demands of its duty cycle.

Acceptance

Criteria and Standards Compliance
Acceptance criteria are defined in the applicable product or material specification. They typically include a minimum rupture pressure, a proof pressure that every specimen must sustain for the stated hold time, and absence of leakage or visible cracking. Compliance work requires that the test follow the declared standard revision. The conditioning environment, pressure rate, equipment calibration, and any deviations must be documented. The test report identifies the specimen description, sampling plan, procedure applied, individual and mean results, and failure observations. A lot is accepted when all specimens pass the proof-pressure hold, or when group burst data meet the specified minimum, depending on the specification. Failed lots trigger investigation of fracture origin, commonly by fractographic examination, to separate material flaws from handling damage or sealing artifacts. Results from an accredited laboratory carry traceable calibration records. Customers and regulators can therefore verify that the acceptance decision rests on controlled, documented measurement.