ASTM E908-98 (R2018) pdf free download.Standard Practice for Calibrating Gaseous Reference Leaks.
1.1 This practice covers procedures for calibrating leak artifacts of a specified gas. that may be used for determining the response of leak detectors, or in other situations where a known small flow of gas is required. The purpose of this practice is to establish calibration without reference to other calibrated leaks in as straightforward a manner as possible using the likeliest available equipment. While the uncertainties associated with these procedures will most likely be greater than those obtained via traceable calibration chains (on the order of 10 ci). these procedures allow independent means of establishing or verifying the leakage rate from leak artifacts of questionable history, or when traceable leak artifacts are not available.
1.2 Two types of leaks are considered:
1 .2.1 Type I—Pressure to vacuum.
1 .2.2 Type II—Pressure to atmosphere.
1.3 Three calibration methods are described under each type of reference leak:
1 .3. 1 Method A—Accumulation comparison, using a known volume of gas at specified conditions of temperature and pressure as a reference.
1.3.2 Method B-—Accumulation comparison, using a leak artifact calibrated using Method A.
1.3.3 Method c—Displacement of a liquid slug. by the leak, in capillary tube of known dimensions.
1.4 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and arc not considered standard.
4. Interferences
4. 1 Type I Leaks, atmosphere to vacuum, Methods A and 13:
4. 1.1 For the purposes of this section. it will be assumed that the gas is helium and the detector is the mass spectrometer tuned for helium.
Note 1—Other gases or detectors, or both, can be used with little ditrerencc in procedures or interferences.
4. 1.2 Pressure Rise—There will inevitably be some pressure rise in a closed evacuated chamber, due to oulgassing and small leaks. This may cause a decrease in ioniiation eflicieney in (he spectrometer tube and thus a steadily declining signal as indicated in Fig.1. However, this effect should be quite constant from run to run, and so largely cancel out in final result.
4.1.3 Helium Signal Rise—There will usually be a noticeable increase in helium signal when the chamber is closed, due to outgassing and in-leakage from the atmosphere as indicated in Fig. I. Again, this will be a constant which mostly cancels
4.1.4 Spectrometer Sensitivity Drift—This will he noticed as variations in zero and in reading levels with (he same helium input. With properly tuned and maintained systems operating at least one decade below maximum sensitivity, this should be a minor eflect.
4.1.5 Leaks—All detectable valve leaks and leaks from the atmosphere should be repaired.
4.1.6 Baronwiric Variations—(Not applicable to sealed reservoir units.)ASTM E908 pdf download.