Nowadays, different ways to test for leaks are usually facilitated by special leak detection devices that could detect lower leak rates than methods that do not utilize special equipment. Most leak detectors are utilized with the aid of test gases—a medium other than the one used in normal operation.
Helium is commonly used as test gas for leak detection. This gas could be detected using a mass spectrometer. Leak detectors may, for instance, be utilized in the field of medical technology through cardiac pacemakers where the interior components of the device need to be protected against the ingress of bodily fluids during normal operation. In this example, it is evident that the varying flow properties of the test and the working media need to be taken into consideration.
Halogen Leak Detectors
Halogen Diode Principle
Gaseous chemical compounds whose molecules contain chlorine or fluorine, such as refrigerants R12, R22 and R134a, will influence the emissions of alkali ions from a surface impregnated with a mixture of KOH and Iron (III) hydroxide and maintained at 800 °C to 900 °C by an external Pt heater. The released ions then flow to a cathode where the ion current is measured and amplified using the halogen diode principle. This effect is so significant that partial pressures for halogens could be measured down to 1 • 10-7 mbar.
Such devices were utilized in the past to test for leaks and in accordance with the vacuum method; however, because of problems associated with CFCs, more sniffer units are being built and used today. For all devices, the attainable detection limit is about 1 • 10-6 mbar•l/s. Equipment that operates using the halogen diode could also detect SF6. Additionally, these sniffer units could determine the source of refrigerants: whether they are escaping from a refrigeration unit or from an SF6 type switch box that is filled with arc suppression gas.
Infrared Principle (HLD 5000)
The physical property of molecules, particularly their ability to absorb infrared radiation, is greatly utilized by the HLD 5000. Taken in by the sniffer line, the test gas flows through a cuvette that is exposed to infrared radiation. This radiation is absorbed by infrared-active gases or refrigerants that are inside the test gas. As an effect, the primary infrared signal is modified. This modified infrared signal is detected by a sensor, processed, and then displayed. The detection limit lies at around 5 • 10-5 mbar • l/s. Due to continuous measuring of the ambient air, the background level of the test gas is automatically considered when calculating the measuring value.
Leak Detectors with Mass Spectrometers (MS)
Using mass spectrometers to test for gas leaks is considered the most sensitive leak detection method, making it widely-used in the industry. Mass spectrometer leak detectors were developed for the purpose of quantitatively measuring leak rates in a range extending across many powers of ten, with the lower limit lying around 1 • 10-12 mbar • l/s. The MS leak detector could also quantitatively measure permeation or inherent gas flow through solids.
In principle, mass spectrometry could be used to detect all types of gas. Out of all available options, helium was found to be the most practical test gas. This is because helium possesses the following qualities:
- unequivocally detectable with a mass spectrometer
- chemically inert
- present in normal air in a concentration of only 5 ppm (equal to 5 • 10-4 volume %)
- economical and practical
In availing mass spectrometer leak detectors for commercial use, two types could be availed: the quadrupole mass spectrometer or the 180° sector field mass spectrometer. The latter is recommended for use due to its economic and simple design.
The ion source, separation system, and ion trap are three fundamental assemblies that all mass spectrometers must possess. To ensure efficient performance, ions must be able to travel along the path from the ion source and through the separation system to the ion trap to the greatest possible extent without colliding with gas molecules. This path totals to about 15 cm for all types of spectrometers. As such, a medium-free path length at least 60 cm, corresponding to pressure of about 1 • 10-4 mbar, is required. With these requirements, it is evident that a mass spectrometer will operate only in high vacuum. Among modern leak detectors, turbomolecular pumps are typically used in creating high vacuum. Associated with individual component groups are electrical and electronic supply systems or software that are required. These systems are facilitated through a microprocessor and allow for the greatest possible automation degree in the operating sequence that includes all adjustment and calibration routines and measured value display.
The Operating Principle of a Leak Detector with Mass Spectrometer
The figure illustrates an explanation on the operating principle of a mass spectrometer leak detector. The figure presents the most common configuration for leak detection using the test gas spray method at a vacuum component.
Operating principle of a leak detector with mass spectrometer (main flow leak detector)
When gas enters the component through a leak, it is pumped through the interior of the leak detector to the outlet, enabling the gas to leave the detector again. Assuming, that the leak detector is properly sealed, the gas flow q is always the same at any point between the inlet and the outlet of the leak detector. The following applies directly at the pumping port of the vacuum pump:
q = p • S
p = Inlet pressure directly at the pumping port of the vacuum pump in mbar
S = Pumping speed of the vacuum pump directly at the pumping port of the vacuum pump in l/s
At any other position x, the following applies while taking the line losses into account:
qx = q = px • Sx
px = pressure at position x in mbar
Sx = pumping speed of the vacuum pump at position x in l/s (Sx < S!)
This equation applies to all gases which are pumped by the vacuum pump and for the test gas TG (e.g. TG = helium). At the mass spectrometer (x = MS), the following applies:
qMS, TG = qTG = pMS, TG • SMS, TG = qL
pMS, TG = partial test gas pressure at mass spectrometer in mbar
SMS, TG = pumping speed of the vacuum pump for the test gas at mass spectrometer in l/s
In this case, the test gas flow qTG equals the lake rate qL being sought. In the case of the equation, the partial test gas pressure pMS, TG is present at the mass spectrometer. The measuring value for pMS, TG is provided by the mass spectrometer which must be set to the mass M of the test gas (e.g. M = 4 for TG = helium).
The value of SMS, TG is an experimentally-determined constant for each leak detector. The value for pMS, TG provided by the mass spectrometer is multiplied by the value SMS, TG which is stored in the microprocessor of the leak detector. The result of this multiplication is then displayed as leak rate qL.
Detection Limit, Background, Gas Storage in Oil (Gas Ballast), Floating Zero-Point Suppression
The natural background for the test gas that is to be detected is dictated by the smallest detectable leak rate. Despite the inlet at the leak detector being closed, the test gas would still be able to enter the mass spectrometer. It would also be detected if the electronic means are sufficient to do the process. The detection level of the leak detector is generated by the background signal in the MS. Normally, a high-vacuum pump used to evacuate the mass spectrometer would comprise a turbomolecular pump and oil-sealed rotary vane pump. Similar to any type of liquid, the oil in the vane pump is capable of dissolving gases until equilibrium is reached between the gases dissolved in the oil and the gas outside of it. Such equilibrium state represents the detection limit for the leak detector when the pump is warmed up; however, it is also possible for test gas to enter the leak detector in other ways apart from the inlet. The improper installation or inept handling of the test gas could allow it to enter the interior of the leak detector through the airing or gas ballast valve. Inevitably, this would result in a higher test gas concentration in the oil and elastomer seals. As an effect, the background signal increases. In conclusion, an increased amount of test gas present in the oil yields a higher background signal of the leak detector.
Nowadays, there is a common way of installing leak detectors where the gas ballast valve and airing valve are connected to fresh air. If possible, the outlet of the leak detector should be routed outside the room where the leak test takes place. By opening the gas ballast valve and introducing gas that is free from test gas, the increased background signal could be lowered again. This would make helium, which is stored in the oil, to flush out. Because this only affects part of the oil present in the pump body, the flushing procedure will have to be continued until the entire oil supply of the pump has been recirculated several times. The whole process would usually take 20 to 30 minutes.
Vacuum diagram of a counterflow leak detector
Dry leak detectors are leak detectors without oil-sealed vacuum pumps. In using this type of leak detector, the problem of gas storage in the oil does not exist; however, the leak detector must still be flushed with gas which is free of test gas because test gas particles may accumulate in such device over time.
A way to simplify the use of leak detectors, users may opt to use the floating zero-point suppression feature thavx is integrated into the automatic operating concepts of all Leybold leak detectors. Through such mechanism, the background level measured after the inlet valve has been closed will be stored and be automatically deducted from subsequent measurements when the valve is opened again. Only at a relatively high threshold level will the display panel show a corresponding warning.
Leybold leak detectors offer the capability for manual zero point shifting that is independent from the floating zero-point suppression. In such feature, the display for the leak detector at the particular moment will be reset to zero so that only rises in the leak rate from that point on would be shown; however, the manual zero point shifting mechanism only serves to facilitate the evaluation of a display and not influence its accuracy.
The figure below shows the zero-point suppression feature of leak detectors.
Example of zero-point suppression
Chart on the left: The signal is clearly larger than the background.
Center chart: The background has risen considerably; the signal can hardly be discerned.
Chart on the right: The background is suppressed electrically; the signal can be clearly identified again.
Calibrating Leak Detectors; Calibration Leaks
A calibration leak is defined as a leak whose leak rate at a certain temperature and under specific conditions is precisely known. When a leak detector display with an attached calibration or test leak is adjusted, a leak detector calibration is being done. The leak rate is usually provided on the calibration certificate or documented on a label attached to the calibration leak.
In vacuum operations or spray technique, two types of calibration exist—internal calibration and external calibration.
This information has been sourced, reviewed and adapted from materials provided by Leybold GmbH.
For more information on this source, please visit Leybold GmbH.