ASTM E908-98(R2018) pdf free download

ASTM E908-98(R2018) pdf free download.Standard Practice for Calibrating Gaseous Reference Leaks
1. Scope
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 %), 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 ofa 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 are not considered standard. 1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appro- priate safety, health, and environmental practices and deter- mine the applicability ofregulatory limitations prior to use. 1.6 This international standard was developed in accor- dance with internationally recognized principles on standard- ization established in the Decision on Principles for the Development of International Standards, Guides and Recom- mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
3. Summary of Practice
3.1 Method A—Accumulation comparison, using a known volume of tracer gas: 3.1.1 This method uses a closed chamber of nonreactive material having a means of removing all tracer gas and a connection to the tracer sensor. 3.1.2 A small, known quantity of tracer gas is discharged into the chamber and the response recorded for a period oftime in which it is anticipated the unknown leak will require to reach the same concentration.3.1.5 It will be observed that chamber volume and sensor linearity are not factors in this equation. However, the chamber volume must be selected to give a concentration within the sensor range. Also, this concentration must also be achieved by the unknown leak discharging into the chamber in a reasonable length oftime and must be appropriate so as not to significantly affect the equilibrium flow rate from the leak. This is particu- larly true of permeation leaks. 3.2 Method B—Accumulation comparison using a reference leak as calibrated in Method A, 3.1: 3.2.1 This method is a means of extending the primary calibration by a factor of up to 10, by comparing with previously-calibrated leak artifacts for longer periods of time. For example, a 5 × 10 −12 mol/s leak that calibrated in Method A at 300 s can be used for 30 s to calibrate a 5 × 10 −13 mol/s leak. 3.2.2 When this method is used, it should be realized that the total possible error will be at least doubled. 3.3 Method C—Direct measurement of leak rate by timing the movement of a liquid slug in a capillary tube of known dimensions:3.3.1 The tube is closely coupled to the leak, and has a vent/fill valve to allow gas filling or positioning of the slug, or both, which is then driven by the leakage of the gas.