Purge Gas Supply Line

Introduction

This is a maintenance procedure for purging a gas supply line to the CEGS. This procedure must be performed for each process gas during installation. It should be repeated whenever there is any reason to suspect the purity of the gas reaching the CEGS. For example, whenever the gas cylinder is changed or if the gas supply tubing has been opened to atmosphere.

This procedure assumes the user is familiar with the CEGS and its operation.

A note about materials

For optimal performance, the CEGS requires very pure process gases, commonly "research grade" or ultra-high-purity gases. Typically, these gases will be 99.995 to 99.9999% pure, and they are often delivered with certificates of analysis showing the measured amounts of important impurities.

Ideally, the tubes or "lines" that convey gases to the CEGS are stainless steel or copper, and the fittings use metal-to-metal seals throughout. However, for expedience or convenience, plastic lines may be used instead of metal. Due to the much higher permeation and leak rates inherent to plastic gas lines and their fittings, they must be purged on a regular basis, perhaps even daily, to ensure that the gases arriving at the CEGS meet the purity requirements of the process.

For the lowest blank levels and whenever processing old or small samples, Aeon strongly recommends high-purity stainless steel tubing for all gas lines. ("Low-level" samples are > 35,000 yr old, or contain < 100 µg C.)

Procedure

1. Close the manual supply valve at the top of the gas cylinder or on the wall where the gas enters the lab. This valve is located upstream of the pressure regulator. Some labs also have a manual supply valve, or low-pressure valve, just downstream of the pressure regulator as well. If so, leave the low-pressure valve opened.
2. Isolate the Vacuum Manifold.

   You can either (A) right-click on the VacuumSystem and choose 'Isolate Vacuum Manifold', or

   (B) right-click on the VM pipes and choose 'Isolate'. Both options do the same thing.

3. Make sure the VacuumSystem is 'Isolated'.

   Hover over the VacuumSystem area to check the VacuumSystem state.

   Double-click the VacuumSystem to toggle between Evacuate and Isolate mode.

4. Open the gas supply flow valve, if there is one.

   If a flow valve is present, it is upstream of the supply shutoff valve (next step). The flow valve icon has bent lines inside to show that it restricts the flow.

   Flow valves can be opened and closed by double-clicking, just like regular valves.

5. Open gas supply shutoff valve. This is different from the manual supply valve. The gas supply shutoff valve is the one in the CEGS user interface that connects the gas line to the vacuum manifold (VM). It is typically named "vH2_VM" for hydrogen, "vO2_VM" for oxygen, etc. Opening this valve will pressurize the VM.
6. Double-click to set the VacuumSystem to Evacuate mode.
7. Wait for the pressure to reach ≤ 1e-2 Torr. If the pressure fails to reach this value within a minute or two, something is wrong. Either the manual supply valve is not completely closed, or there is a leak somewhere in the supply line. Do not try to continue without correcting the problem.
8. Isolate the VacuumSystem.
9. Open the manual supply valve.
10. Close the gas supply flow valve, if there is one.
11. Read the step after this one before doing this step, so you know what to do ahead of time. When you are ready, double-click to set the VacuumSystem to Evacuate mode. It will be loud at first. This is ok.
12. Does the gas supply have a flow valve?
    • If not, then close the gas supply shutoff valve two seconds after setting the VacuumSystem to Evacuate.
    • If the gas supply does have a flow valve, wait for the pressure to fall to about 10 Torr, and then close the gas supply shutoff valve.