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 General: The recommended use of a sniffer leak detection system is for leak location only. The recommended methods and procedures contained in this document are designed to optimize the performance of any sniffer type instrument. This goal is accomplished by dynamic on-line calibration minimizing the many variables encountered in sniffer leak detection applications. No sniffer leak detection system can be directly calibrated for making quantitative measurements. This conclusion is based on the following scientific facts: 1. The location of the sniffer probe is operator dependent. The reading depends upon the duration, angle and distance of the probe from the actual leak source. 2. Air currents - Air currents are always present. The amount of airflow around the leak directly affects the ratio of helium to air to be detected at the mass spectrometer. 3. Type of Leak - There are many types of leaks including orifice, capillary, crack permeation and many variations of the leak configuration. The way the gas penetrates the different leak configurations determines the flow rate and the machine's ability to detect a given leak. Note: For the above reasons, no company can claim precise calibration and quantitative measurements using a sniffer type instrument. 4. Background Helium background in the air is the most critical limiting factor pertaining to sensitivity. The more helium background present, the lower the sensitivity of the leak detector. Helium background generally causes instability on the leak rate meter or a gradual rise in the reading observed. Testing should be performed in a background free environment to minimize the effects of a changing helium to air ratio and the resulting instability. Ways to optimize the performance of the sniffer leak test system: 1. Be sure the replaceable filter tip on sniffer probe is changed daily in factory environment. 2. Background - Provide a special booth with a positive pressure or change of atmosphere to minimize helium background. Never house leaky units in an enclosed area employing a re-circulating air conditioning system. Leakers should always be kept outside the leak test booth. 3. Dynamic calibration - The best way to test the machine's ability to see a known leak rate is to use a dynamic test. The dynamic test is based on moving the sniffer probe across a calibrated leak to measure the approximate meter deflection. This movement should be practiced by each individual operator. The method of movement and speed of the individual operator will determine the amount of deflection observed. 4. Calibration using the dynamic method - The probe is inserted in a known calibrated leak or simulated test setup and left in place until a steady reading is observed. The sensitivity on the leak detector is adjusted to read the calibrated leak. Accumulation Technique: helium can be accumulated in a known volume placed around the suspected leak area. For example, a rubber cap of known volume can be placed over the top of a fire extinguisher to check for a suspected leak in the valve area. The cap should be designed with a precise volume and two holes. One hole will be for the sniffer probe to be inserted during the test. The other hole is a vent hole to avoid pressure change in the volume during the sniffing process. This technique works well for test pieces allowing for easy capture of helium gas in a known volume. The volume must be kept as small as possible. Each test must be precisely time to ensure an equal accumulation period during each test. 5. Safety factor - It is best to allow a factor of ten- safety factor to account for the inaccuracies that occur using the sniffing leak detection method. For example, if your leak rate rejection setpoint is 10-5 cc/sec, test on the 10-6 cc/sec leak rate range. 6. Range of operation - Leak testing should take place on the same range as the dynamic calibration. Autoranging should not be used when measuring a leak. Most sniffing leak detectors are not accurate range-to-range. 7. Use Of Electronic Enhancements for Sniffing- A. Auto tuning - Auto tuning is nice for vacuum test applications. Unfortunately, there is no relationship between tuning to static calibrated leak and the actual reading of a sniffer probe. If auto tuning is used, precise calibration will suffer. It is recommended that the probe be manually calibrated to an actual external sniffer style leak. Some newer leak detectors will allow for dynamic calibration of a sniffer probe using auto tuning. B. Auto zero - The use of auto zero should be avoided in sniffing applications. If the operator attempts to "zero out" very large leaks, this results in desensitizing the machine to the point where small leaks can not be detected. The zero point should not be the reference point. The best practice is to move the zero up to approximately ten (10) percent of full scale. Because sniffing is operator dependent, the operator can see changes in the zero whether positive or negative. Many systems using auto zero can not distinguish how far the zero may be negative. Again, this will cause the sensitivity of the machine to be decreased. The operator needs to know the precise location of the zero and the stability before making a determination as to the approximate leak size. |
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