TechArchive

Original article date: October 1997

The cost to industry of leaks, detection, measurement, prevention and correction is massive. In current production control situations, there is a tendency to look for trends, in addition to the application of simple pass or fail criteria, leading to the prediction of process failure, which is more cost-effective than rejection or sorting. Such analysis or control can only work if the measurements taken are accurate and reliable.

Sophisticated electronics and computerised systems have revolutionised the testing of many components, but leak testing has remained relatively crude, and often misunderstood.

All sorts of components need to be leak checked, from pharmaceutical drug aspirators to hair curler gas cartridges, to refrigeration systems. To take a car for example, radiators are an obvious example, but heat exchangers, air-conditioning systems, and vehicle suspension units are also extensively tested throughout the industry.

Reasons for leak testing include:

• to check the security of seals and welds
• to find small holes that would leak toxic or harmful substances into the environment
• to avoid waste on the production line when a leaky component is assembled and later found to be defective at a later stage in production or worse in the field.

Is the leak testing exercise just about detection, or is some measurement involved as well? Where should the leak test be applied? To answer both these questions, a prerequisite is an appropriate product specification.

There are a series of considerations which will influence the final choice of the specification

applied, these tend to range from an extreme quality standard to the most economic standard

that will be tolerated. Cost will always be a major factor.

For instance, when there is a choice between a specification where a component will never,

ever drip oil, or one where there is a risk that one drop of oil only might fall from very few

pieces but at a significantly lower cost, which standard is it more likely that will used?

Very often we are confronted with statements which do riot represent a definitive standard -

“no leaks” or “no bubbles”. On the other hand, a specification might state the test medium, the pressure differential, the permitted loss relative to duration and temperature. It might involve a description of the test conditions to be created, with criteria to be used for visual judgments.

A distinction should be drawn between leak detection and leak measurement. Broadly, the difference is between identifying the location of the leak and the measurement of the rate of leakage. Considerable assistance is provided in Tables 1 and 2, provided by TQC, and Fig 1, provided by Ion Science.

Ion Science also boasts a new tracer gas technique called Phionia which claims to be more sensitive than pressure decay, but with lower capital and maintenance costs than helium mass spectrometry. This is proving to be of considerable assistance to machine builders who are building leak detection systems for SHED leak testing, a USA test which arises out of renewed efforts to properly account for emissions from on-road motor vehicles. It is suggested that current methods of testing (water bath or pressure decay methods) will not be stringent enough to meet these tests. Helium is fast becoming an industrial standard, due to its inertness and small molecular size. In many applications, leak detection is possible down to a leak size of 1 x 10<-8>ml/s, 300 times more sensitive than pressure decay leak testing. If testing takes place under vacuum, even greater sensitivity can be achieved.

However, mass spectrometry instrumentation can be relatively expensive and is difficult to use in heavy industrial applications. Phionia (Photo Ionisation Leak Detection) offers similar sensitivity whilst being more adaptable to the rigours of production line use and perhaps half the cost. In addition, it uses a non-flammable hydrocarbon fuel mixture as a tracer, which is more indicative than helium of potential fuel vapour emissions.

Phionia operates on a Photo Ionisation (PID) principle of irradiating a test stream of

gas with UV light. Volatile organics are fragmented by the light into ions, which are stripped

out by electrodes having a high potential difference applied between them. The small

electric current moving between the electrodes is measured by advanced software controlled

electronics and is displayed as a leak rate. There are no moving parts and no expensive

vacuum equipment, and so less maintenance is required.

Meanwhile, Edwards has introduced a new line of helium mass spectrometer devices designed for fast, accurate dry leak detection in a wide range of applications, including semiconductor cleanrooms. The 600D features an oil-free diaphragm/molecular drag pump for applications which require 100% dry leak detection. A version designated the Spectron 600DS is available in a stainless steel enclosure with HEPA filters designed to meet the most stringent cleanroom standards.

Pressure decay systems are offered, amongst others, by Furness Controls. The company’s latest instrument is the FCS355 Six Sequence Box, which is used in conjunction with the standard leak detector to increase production throughput for the production line testing of components. The system, as its name implies, allows up to six components to be tested consecutively. Furness’ equipment include a serial communication link to permit data recording, analysis and reporting facilities in PC software.

Rietschle is introducing a sensitive ultrasonic vacuum leak detector which can sniff out the ultrasonic noise of a vacuum leak from several feet away. The AC 6600 is a portable instrument powered by a 9V battery. Said to be suitable for small leaks in vacuum pumps, on electronic production lines and in test laboratories, if the leak is large, or if another ultrasonic source is affecting the unit, it can be selected by covering the probe with a fingertip and pressing the reset button. Although the probe may respond to other ultrasonic sounds, the beep rate remains constant, unlike the vacuum leak, where the rate increases as the probe approaches the leak.

J W Froehlich has developed a new leak detection system which reduces the time required to test components having a large internal volume. According to the company, the new device is based on a mass flow measurement system combined with solid state electronics, so the measurement time for a component having a volume of 25 litres will be about 1.5s, compared to about 25s for a conventional pressure decay system using air. The new technology is being used to produce certified flow calibrators which are used for setting and checking flow rates for production applications.

With catalytic-type sensors for detecting flammable hydrocarbon gases, such as ammonia or butane, Zellweger Analytics can provide solutions to comply with standards and legislation relating to leak detection in modern cold stores, refrigeration plants or air conditioning systems EN378, the European safety standard for refrigerants, and COSHH regulations both lay down specific requirements for the use of gas detection systems and occupational exposure levels.

The SG 16 AP will detect a leakage of combustible or flammable gas, including ammonia. The alarm level for ammonia, often set below 1% v/v, is often outside the capability or reliability of standard catalytic gas detectors, but this sensor provides an alarm capability of 0.5% v/v.

For toxic level detection of HC gases and ammonia, Zellweger Analytics offers electrochemical cell sensors which suffer a chemical reaction when the target gas is present. An electrical signal is generated by the sensor which is proportional to the amount of gas present.

BOX COPY

Simple questions and point to consider before purchasing leak detection equipment

• Do we calibrate or validate it?
• Is it pressure ramp controlled?
• Is the transducer tolerant of catastrophic test failure
• Are the valves solenoid or pilot operated?
• What is the test valve displacement factor?
• How do we prove it’s the tooling, not the instrument?
• How do we achieve optimum stabilisation?
• The test specification must be both specific and appropriate to the product in question
• The test technique must be compatible with the test specification and the characteristics of the product
• The instrument must be appropriate for the specific test application
• The tooling used to present the product for test must be suited to the purpose

TABLES

Table 1 Fundamentals of leak location

(source: Brian Wilkie, TQC)

The common feature of leak location methods is that they are almost always operator-dependent, require strict supervision and are often very messy! All these methods may indicate a leak condition, but will not accurately quantify the degree of leakage (see Table 2).

• Observation of bubble streams – components are filled with pressurised gas and immersed in a liquid. Usually air underwater, but also might be nitrogen under acetone

• A variation of the bubble stream method – sealed components submerged in a liquid within an enclave and vacuum created at the liquid surface.

• Chemical trace – chemicals are added to the working media within an assembly prior to some form of functional test. If they leak onto the surface of the component, they can be readily seen when viewed frequently using ultraviolet light.

• Chemical penetration – chemical sprayed onto one side of a component which eventually by capillary action emerges at the opposite surface.

• Gas sniffing – components or assemblies are either filled or injected with an easily identifiable gas, creating a pressure differential. Helium, hydrogen and sulfur hexafluoride (SF6) are the most common of these, an operator searches for concentrations of gas at the surface of the test piece. Helium is the most common of these gases used. SF6 is generally avoided wherever possible, because of its toxicity, but is mandatory for some electrical equipment.

• Ultrasonic testing – large gas flows, because they are noisy, can be found using an ultrasonic detector. This method is reasonable for finding very large leaks, but not recommended for the fine leaks in a production environment.

Sources of misleading indications and other problems can include:

• low test pressures and capillary action sealing
• high pressure situations and component damage
• contaminated liquids
• preferential leeching
• rogue surface tensions

Table 2 Fundamentals of leak measurement

(source: Brian Wilkie, TQC)

There are three commonly used fundamental techniques relating to leak measurement: differential pressure, continuous flow and gas trace.

Pressure differential

• Absolute measurement – direct pressure change measurement, with a transducer or gauge, of conditions included within the test piece.

• Reference comparison – this test technique involves the location of the test and reference volumes either side of a transducer diaphragm. Ideally, these volumes should have similar pneumatic characteristics. They are arranged to be as stable as possible and at the same pressure. Subsequent time-related changes in the pressure within the test volume are used to measure the value of any leakage.

• High pressure comparison – a differential transducer is used indirectly via an arranged volume and an ambient pressure reference.

• Differential pressure component dosing – interrogation of sealed components using twin tandem pressurisation and reference volumes and the sequential application of a differential pressure transducer. The significance of this test sequence is the identification of pieces with gross leaks.

• Inter-stream testing – simultaneous testing of two adjacent volumes using two test pressures and three interrelated differential pressure transducers.

Continuous flow

• Controlled pressure – essentially, the creation of a prescribed pressure within the test piece, thereafter measurement of the gas flow needed to maintain that pressure using a flow measurement transducer.

• Free flow – involves the introduction of a flow measurement device into the gas stream.

Gas trace

• Helium – requires the enclosure of the test piece within an adequately sealed chamber and variously evacuating the chamber and/or test piece. After charging one or the other with helium, the gas loss from the respective volume is monitored with a mass spectrometer.

• Hydrogen – as for helium, but also with a technique which uses a sensor which is able to detect and measure levels of hydrogen in a continuous ambient or low pressure gas flow. This allows the use of low grade enclosures and ducting. The test gas is a mixture of 95% nitrogen and 5% hydrogen, which is non-explosive and relatively inexpensive, being extensively used in industrial welding processes.

• Gas bombing – exposure of a sealed component to a gas-charged environment with subsequent transfer to a test chamber which is monitored by gas sensing instrumentation. If this method is used, it is essential that the possibility of a gross leak condition be eliminated before fine leak measurement is attempted.

Various halogens can be used in each of these techniques, but tend to be avoided if possible. In addition to toxicity, they are heavy gases which tend to accumulate very readily in low recesses. As a result, there contamination occurs, the test area is difficult clear, this being made worse by the tendency of these gases to adhere to surfaces. All gases will stick to tooling surface to some degree, most particularly when flooded from a leaking component. However, hydrogen and helium being lighter, they are more easily removed or dispersed by extraction, flushing or natural diffusion.

TQC predominantly uses the following:

• Nolek leak measurement instruments in air flow and differential air pressure/vacuum test situations
• Alcatel mass spectrometer units for helium detection applications
• Sensistor instruments for hydrogen detection scenes

October 1997