Poor system evacuation can cause many problems because it does not dehydrate the system properly. Any moisture remaining in the system can be devastating. Combinations of moisture and most refrigerants create acidic properties that can quickly saturate the system destroying the whole system from the inside out. More importantly, efficient system evacuation can indicate the presence of leaks. Today, eradication of leaks are not only important to the serviceman and client, but especially important to the environment.
1. The lowest leak rates can only be measured by employing a vacuum method, whereby the following applies. The lower the leak rate, the higher the requirements for vacuum evaluation methods.
2. If possible the test objects should be tested under the same conditions that will be used in their final
application, i.e. parts for vacuum operation should be tested according to the vacuum method and parts for overpressure operation should be tested using the overpressure method although pressure may not give the best leak results.
3. Some of the following information refers to the term “Outgassing”. This is a term used to account for the phenomena created under a medium to high vacuum when gas molecules are released from the surfaces or through different materials used in the construction of closed systems. It can also refer to the release of gases from other materials inside the closed system.
4. The tables at the conclusion of this article have been taken from the Leak Detection section of the current Leybold vacuum equipment catalogue. Javac Pty Ltd in Australia and Freeze Dry Systems Ltd in New Zealand are the premium Leybold agents in these countries.
There are two main groups of leak detection methods and are based on pressure or vacuum. For both methods there are special instruments available from Javac New Zealand.
Leak Testing Based on Overpressure Methods
(Overpressure within the test object)
The equipment to be tested is pressurized with a gas, search gas, or a search gas mixture. The pressure ratio between inside and outside is over 1:1. Between the two methods there exist many variations depending on the particular application.
Pressure Drop Method. The test object is filled with a gas (for example air or nitrogen) until the testing pressure is reached. Precision gauges are used to detect a possible pressure drop during the testing period. This method is simple to implement, it is suitable for the determination of gross leaks and can be improved upon by using differential pressure gauges.
Local Leak Detection with a soap solution. By applying soap solutions or similar, (typically sprayed or painted on) leaks can be located by watching bubbles forming in the leak area.
Leak Detection using a fluorescentdye in the system. An ultra-violet dye circulating with the compressor oil permeates through the leak and is able to be detected using an ultra-violet light.
Local Leak Detection with Electronic Leak Detectors – Sniffing. The test object is filled with the search gas or the search gas/air/nitrogen mixture to which the leak detector is sensitive. The leak detector is equipped with a sniffer probe, whereby there is a low pressure at the probe tip. If the sniffer tip passes suspicious points on the test object the search gas coming out of the leak is sucked in and transferred to the detection system of the leak detector. After conversion into electrical signals these are displayed optically and acoustically by the leak detector.
Typical hand held sniffer type leak detectors available from Javac New Zealand range in sensitivity from 11 grams per annum (Tekmate) to 3 grams per annum (D-Tek) leak rate. This method can also be used for pressurised leak detection using helium as a trace gas and a sniffer version helium leak detector. Helium leak detectors available from Javac New Zealand are capable of both vacuum and pressurised sniffer types.
Note: Except where used as a trace gas, fluorocarbon refrigerant must not be put into a system for the purposes of pressure related leak testing.
Local Leak Detection with Electronic Leak Detectors – Ultrasonic Sound
Hand held leak detectors using the detection of pressure escaping through the leak source by listening for gas leaking. These leak detectors are very efficient but are limited by the need for absolute quiet during the detection procedure.
Leak Testing Based on Vacuum Methods
(Vacuum inside the test object)
Background. The equipment to be tested is evacuated. The pressure ratio between inside and outside is 0:1.
Evacuate the refrigeration system using a vacuum pump. A vacuum pump cannot achieve a vacuum better than the Saturated Vapour Pressure of any moisture (water) in the refrigeration system or vacuum pump. This is because water that may have been drawn into the pump evaporates from the oil on the suction cycle and then re-condenses back into the oil during the compression cycle of the pump. If we reduce the partial pressure of the water vapour during the pump’s compression cycle with a measured and controlled amount of non-condensable gas (air) by opening the gas ballast valve, the water vapour will not reach its SVP during compression and will therefore be discharged from the pump.
Ultimate vacuum will not be instantaneous. It is relative to pump capacity and system size. If it appears the rotary pump is not achieving high vacuum, check the following. Is the oil level correct when pumping. Are all fittings, hoses and mechanical joints tight and valves shut. If no improvement is achieved, check the vacuum pump with a known good McLeod gauge or electronic gauge by removing the pump from system, connect the vacuum gauge to a suction fitting positively sealed. Run the vacuum pump. A McLeod gauge should indicate a vacuum of between 50 and 1 micron. An electronic gauge will show approximately 250 to 20 micron after five minutes, depending on the type of pump.
Pressure Rise Method. System must be evacuated to less than 1.0 Millibar absolute. After the system has been evacuated the vacuum pump should be isolated from the system. As a guide, with constant ambient conditions, the vacuum should not rise more than 0.13 Millibar (or 100 Microns) in one hour. A greater rate of rise may indicate a leak or the presence of moisture. Absolute vacuums must be measured using accurate measuring equipment selected for the specific application.
With this method it is only possible to determine the total leak rate. After the test object is evacuated with a vacuum pump or a vacuum pump system, a valve is used to isolate the test object from the vacuum pump. The pressure will then rise as a function of time. Pressure rise can exist due to outgassing from the surfaces of the test object. This pressure rise tends to tail off in the direction of a saturation level. If in such a case the time allowed for monitoring the pressure rise is too short, a leak will be indicated which in reality does not exist. If one waits long enough for the pressure to rise,the outgassing process can then be disregarded, so that the leak rate can be determined from the known volume of the test object and the measured pressure rise over a fixed rise time. In practice, where outgassing and leak rate are added together, the detectable leak rate depends on the volume of the test object, the obtained ultimate pressure and the outgassing from the test object. In connection with a very large test object, this method is time consuming if extremely low leak rates are to be determined in the fine and rough vacuum range. In practice, we have found a recorder to be useful in these circumstances.
Helium Based Local Leak Detection using a Mass Spectrometer
The test object is evacuated by a vacuum pump (auxiliary pump) until the pressure is low enough for a helium leak detector to operate. When using a helium leak detector, its own pump system will take care of further evacuation. Suspicious spots on the test object will then be sprayed with a fine jet of helium search gas. Search gas entering through leaks into the test object is pumped out by the vacuum pump and it is converted by the mass spectrometer leak detector into an electrical signal which is then displayed. This permits rapid detection and determination of the size of even the smallest leaks.
Leak Detection using helium leak detectors
Please note that helium under an adequate vacuum (or pressure) will pass through everything and all materials. Therefore, helium flow standards have been created to reliably predict at what rate a genuine leak is present. A helium leak detector permits not just the localization of leaks, but more importantly the quantitative determination of the leak rate, i.e. the gas flow through the leak. Such a leak detector is therefore a helium flow meter. In practice the leak detector performs this task by firstly evacuating the part which is to be tested, so that gas from the outside may enter through an existing leak due to the pressure difference present. If only helium is brought in front of the leak (for example by using a spray gun) this helium flows through the leak and is pumped out by the leak detector. The helium partial pressure present in the leak detector is measured by a sector mass spectrometer and is displayed as a leak rate. This is usually given in terms of volume flow of the helium.
The two most important features of a leak detector are its measurement range (detection limits) and its
response time. The measurement range is limited by the lowest and the highest detectable leak rate.
In practical applications, especially in the localization of leaks the response time is of great significance. This is the time it takes from spraying the test object with helium until a measured value is displayed by the leak detector. The response time of the electronic signal conditioning circuitry is an important factor in the overall response time. In the case of leak detectors the response time of the electronic circuitry is well below 1 second. The volume flow rate for helium at the point of the test object is of decisive significance to leak detection on components which are pumped down solely by the leak detector. This volume flow rate provided by the leak detector takes care of the helium entering through a leak and it ensures quick detection by the leak detector.
Whether a component or a system is leak-tight depends on the application and the leak rate that is acceptable. Absolutely leak-tight components and systems do not exist. A component is considered technically leak-tight if its leak rate remains below a value defined for a particular component.
In order to provide a quantitative measure, the term “leak rate” with the symbol “qL” was introduced and in vacuum technology mbar x l x s-1 is used as the unit for leak rates. A leak rate of 1 mbar x l x s-1 exists in a closed vessel having a volume of 1 liter when the pressure increases by 1 mbar within one second, or in case of an overpressure it decreases by 1 mbar within one second. The wide range of leak rates from several 100 mbar x l x s-1 to below 10-11 mbar x l x s-1 as they occur in practice necessitates the use of different leak detection principles and hence leak detectors. Besides the determination of the total leak tightness, it is usually important to locate the leak, quickly and precisely, in order to seal it.
The following rule of thumb for quantitative characterization of test vacuum equipment may be applied:
Total leak rate < 10-6 mbar·l·s-1: Equipment is very tight
Total leak rate 10-5 mbar·l·s-1: Equipment is sufficiently tight
Total leak rate > 10-4 mbar·l·s-1: Equipment is leaky
Table. Comparison of leak detection methods:
Table; Comparison of bubble test method (immersion or pressurise technique) with helium leak detector