ASTM D5305 Test Method for Determination of Ethyl Mercaptan in LP-Gas Vapor
6. Apparatus
6.1 Pump - A manually-operated vacuum pump, capable of drawing 100 mL per stroke of sample through the detector tube with an accuracy of +/- 2.0 mL.
6.2 Detector Tubes - Sealed tubes, made of glass with break-off tips sized to fit the orifice of the pump used (tubes and pumps from different manufacturers shall not be interchanged). The tube used shall be appropriate for the determination of ethyl mercaptan and shall produce a distinct color change when exposed to a sample of LP-gas containing ethyl mercaptan. Any substance known to interfere shall be listed in instructions accompanying the tubes (see 5.2). A calibration scale or other markings referenced to a scale shall be etched directly on the tube to allow direct interpretation of ethyl mercaptan concentration.
6.2.1 Detector tubes should be calibrated for a tube temperature of approximately 20°C and normal atmospheric pressure. Shelf life of the detector tubes shall be a minimum of two years when stored according to the manufacturer's recommendations.
6.2.2 Detector tubes and pumps form an integrally designed unit, that are to be used as a unit. Each manufacturer calibrates detector tubes to match the flow characteristics of its pump, and the use of one brand of tube with another brand of pump will give unreliable results.
6.3 Gas Sampling Chamber - Any container of a material that is not reactive with mercaptan and that provides for access of the detector tube into a uniform flow of sample gas at atmospheric pressure and isolated from the surrounding atmosphere.
6.3.1 A suitable container may be devised from a half-litre polyethylene bottle (see Fig. 1). A 6 mm outside diameter polyethylene tubing sealed into the bottle and discharging near the bottom of the bottle provides for flow into the sampling container. A 12 mm hole cut into the cap of the bottle provides both access for the detector tube and a vent for the excess gas flow.
6.3.2 Other possible inert materials for the gas sampling chamber and tubing are nylon, polytetrafluoroethylene (PTFE), chlorinated or fluorinated polyethylene and chlorosulfonated polyethylene.
6.4 Needle Valve and Tubing - A stainless steel needle valve that can be adjusted to control the flow of gas into the gas sampling chamber. Although a stainless steel needle valve is preferred, a pressure regulator may be used in lieu of a needle valve to control the flow of gas into the gas sampling chamber. Polyethylene or PTFE-fluorocarbon tubing may be used to connect the needle valve or pressure regulator to the gas sampling chamber.
7. Sampling the LP-Gas Vapor Phase
7.1 Select a sampling point that provides access to a representative sample ofLP-gas vapor from the container to be sampled. (Warning - When selecting a sample point, consider the safety aspects of the release of LPG vapor.)
7.1.1 Open the source valve (Valve A in Fig. 1) and briefly blow down vigorously to clear foreign material from the source valve and connecting nipple. Close the source valve.
7.1.1.1 Excess venting can result in a lower concentration of ethyl mercaptan in the vapor phase.
7.1.2 Install the control valve (Valve B in Fig. 1) or pressure regulator on the outlet of the source valve. Connect outlet of the control valve to the gas sampling chamber using the shortest length practicable of suitable tubing.
7.1.3 Open the source valve and then the control valve to obtain a slight positive flow through the gas sampling chamber, venting to atmosphere through the tube access and vent (Vent C in Fig. 1).
7.1.3.1 Purged gas shall be vented at a suitable rate so that pressure does not build up in the sampling chamber and increase the flow rate through the detector tube.
7.1.3.2 Conversely, the positive flow of LP gas vapor shall be sufficient so that operation of the detector tube pump does not pull ambient air into the gas sampling chamber which would dilute the LP-gas vapor.
7.1.4 Purge the gas sampling chamber for at least 3 min to displace air.
7.1.5 Maintain flow of LP-gas during the test procedure in Section 9.
8. Preparation of Apparatus
8.1 Before sampling, all sampling equipment should be thoroughly clean and dry.
8.2 Immediately before each series of tests, test the pump for leak-free operation in accordance with manufacturer's instructions. A loss in vacuum on the pump within 60 s indicates a leak. If a leak occurs, follow the pump manufacturer's instructions for resealing the pump and retest. If the pump vacuum cannot be maintained, do not use the pump for testing.
