Helium Leak Testing
Helium leak testing is the most sensitive method to detect leaks, before they reach the critical stage, available to industry today.
Helium is the lightest inert gas. It is non-toxic, non-hazardous, non-destructive, and it easily penetrates small leaks. As helium has one of the smallest molecular size, it is able to pass through leak openings before they become large enough to leak other hazardous materials such as chlorine.
Helium leak testing can accurately detect and measure leaks which are 1000 times smaller than could be located by conventional non-destructive testing procedures such as liquid penetrant, acoustic (ultrasonic), and air bubble testing (vacuum box). The equipment can be calibrated to detect a leak which would leak the equivalent of one drop of water per month.
APPLICATION AS A LEAK DETECTOR
A Mass Spectrometer Leak Detector(MSLD) consists of a spectrometer tube, the electronics to operate it and interpret its output, and a high vacuum system to maintain proper vacuum. Means are also provided for connecting a test object, and a roughing vacuum pump. A system of valves is provided to evacuate the test object for connection to the spectrometer tube. If the subject item is a sealed object containing helium, the MSLD can be equipped with a vacuum chamber to contain the test object and thereby measure the leak rate of the sealed object.
THE REASON FOR VACUUM
The purpose of the vacuum system is to support operation of the spectrometer tube. Helium molecules entering through a leak individually reach the spectrometer tube in a few milliseconds. Helium molecules, as well as molecules of other gases are continuously removed by the vacuum systems diffusing pump.
If helium is continuously applied to a leak, the concentration in the spectrometer tube will rise sharply at first. Then it will reach equilibrium when it is being pumped out at the same rate as it is entering. When helium is completely removed from the leak the input will drop to zero while the residual helium is pumped out of the system. A leak is indicated by a rise in output signal of the spectrometer tube.
THE SPECTROMETER TUBE
In the spectrometer tube, gas molecules are ionized (given a positive electrical charge) by bombarding them with electrons from a hot tungsten filament. Ions formed are accelerated into a magnetic field where the mass (helium) ions are deflected 90 degrees. Only helium ions reach the collector. Heavier ions are deflected too little to be collected; lighter ions are deflected too much to be collected.
An extremely stable electrometer provides an electron current to the collector which exactly neutralizes the current produced by the collection of helium ions. The resulting feedback current is presented on the leak meter. Since this current is directly proportional to the number of ions striking the collector per unit time, the rate meter indicates the concentration of helium in the vacuum system at any time. Any helium entering the system causes an increased concentration of helium within the spectrometer tube and therefore increases the response for the leak rate meter.
In addition to the electrometer, the electronics also provide suitable voltages to operate the spectrometer tube and control the instrumentation of the vacuum system. Test pieces are generally pumped down by a separate mechanical vacuum pump before they are connected to the spectrometer tube. This prevents over-loading the vacuum pumping system.
FACTS ABOUT LEAK RATES
VISUALIZING LEAKS IN EVERYDAY TERMS
10-5 std cc/sec = approximately 1 cc/day
10-7 std cc/sec = approximately 3 cc/year
AUDIBLE OR VISUAL DETECTION BY OBSERVER
a) bubbles rising in water = 10-4 std cc/second.
b) audible leaks = 10-1 std cc/second .
CALIBRATED HELIUM LEAK STANDARDS
Using special manufacturing techniques, calibrated leak standards are made to accurately leak at a given rate (10-6 std cc/sec to 10-7std cc/sec etc.) to enable the mass spectrometer to be calibrated to a known leak rate. The leak detector is calibrated prior to the commencement of testing each unit or system.
ACCEPTANCE STANDARD
Prior to testing any unit or system an acceptance standard must be established. All units will leak (or bleed) so it is important to establish the acceptable rate of leakage which will allow the unit to operate safely. Most product leaks are 10-6 std cc/sec or larger.
SUMMARY
Helium leak testing has the versatility and ability to detect minute leaks in a variety of applications throughout industry. The leak testing equipment is portable, making field testing in remote areas a relatively simple matter.
Using approved helium leak testing procedures, adapted to individual needs, leaks can be accurately located and measured. If leaks are located these can be repaired and retested by the same method while still on site.
Helium leak testing is a worldwide recognized method of non- destructive. Using approved ASME standards, testing can be performed to the latest specifications and guidelines.
Helium leak testing can help prolong the life of vessels, and piping systems, ensure the safety of personnel and decrease downtime due to lost production thereby reducing overall costs.