Kristina Putz from Thyracont examines the importance of pressure rise testing for leak rate determination in process vacuum systems
Pressure rise testing is widely used for assessing vacuum integrity, yet in many production environments leak rate determination remains either impractical or inconsistently applied.
Established methods such as helium leak detection offer high sensitivity but are often limited in routine quality assurance due to equipment complexity, required expertise, and testing effort.
For applications where the total permissible leak rate is the relevant acceptance criterion rather than localisation of individual leaks, integral methods provide a practical alternative. Pressure rise measurement enables a quantitative assessment of the total.
MEASUREMENT PRINCIPLE
Pressure rise testing is based on a simple procedure. After evacuating a system to a defined base pressure, it is isolated from the pumping system, and the pressure increase over time
is recorded.
EQUATION TO GO IN HERE
In this relation, Q represents the total leak rate, V the system volume, and dp/dt the rate of pressure increase. The method provides an integral value for all gas sources without requiring localisation of individual leaks.
This makes it suitable for acceptance testing, where compliance with a defined maximum leak rate is more relevant than identifying leak positions.
APPLICATION IN THERMAL SEPERATION SYSTEMS
A typical application is the manufacture of short path distillation systems used for separating thermally sensitive substances. In such systems, vacuum conditions directly influence evaporation behavior, separation efficiency, and product quality.
Laboratory and pilot systems operate with volumes from a few litres up to several tens of litres, while industrial systems can reach several cubic metres. Across all scales, defined limits for ultimate pressure and permissible leak rates must be maintained to ensure stable operation.
A representative case from thermal separation equipment manufacturing shows how pressure rise testing is applied in practice across different system sizes. Laboratory and pilot systems with volumes between 1 and 50 litres are typically tested against a maximum permissible leak rate of 0.1 mbar·l/s and an ultimate pressure requirement of 0.001 mbar.
After evacuation, the system is isolated and the pressure rise is monitored over approximately 30 minutes. This duration provides sufficient data to characterise the total gas load under defined conditions. Data is re\zcorded and used for quality assurance documentation.
For industrial thin film distillation systems with chamber volumes up to 50 m3, the same principle is applied. At this scale, even moderate leakage can significantly affect achievable base pressure and increase pumping load, directly impacting process stability and energy consumption.
TECHNICAL CONSIDERATIONS
Accurate interpretation requires control of several factors.
Temperature stability is essential, as small fluctuations can induce pressure changes comparable to leak-related signals. Measurements should therefore be performed under stable thermal conditions after sufficient equilibration.
A key challenge is the influence of outgassing on the measured pressure rise. Both outgassing and leakage contribute to the overall gas load, and their effects cannot be reliably separated in most practical systems. Extended measurement periods and temperature-stabilised conditions can help improve reproducibility and reduce uncertainty. However, pressure rise testing inherently provides an integral measurement of the total gas load rather than a differentiated leak rate.
Sensor selection is also critical. The measurement requires reliable pressure detection over a wide range, from atmospheric pressure during pump-down to fine vacuum conditions down to 0.001 mbar.
LIMITATIONS OF THE METHOD
Pressure rise testing does not allow for localisation of leaks and is therefore not suitable when precise defect identification is required.
Sensitivity is limited by system volume, measurement time, and background outgassing. Very small leaks may be difficult to distinguish from residual desorption effects, particularly in large or recently serviced systems.
The method also depends on stable boundary conditions. Temperature variations or residual process media can influence results and must be controlled during testing.
CONCLUSION
Pressure rise measurement provides a robust and accessible method for determining integral leak rates in vacuum systems. Its simplicity and minimal equipment requirements make it suitable for routine quality assurance, acceptance testing, and maintenance procedures.
While the method does not replace high-sensitivity or local leak detection methods, it offers a practical solution wherever total gas load is the relevant parameter. When applied with proper control of influencing factors and limitations, pressure rise testing enables reliable assessment of vacuum integrity across a wide range of process applications.
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