Introduction: Venting a Vacuum Chamber
So you need to vent a vacuum chamber? Whether your reason behind doing so is to replace a sample, take out an instrument, or fix a leak this Instructable will lay out the necessary steps behind this process. Vacuum chambers can be found in most research labs requiring an isolated system. Electron microscopes, material evaporators (as is the case in Figure 1), and plasma research all need vacuum chambers to acquire quality results.
Venting must be a gradual process to avoid complications such as damaging of equipment, especially from high vacuum (10-8 torr). Following these steps in order is imperative in order to not contaminate the quality of the vacuum chamber.
An example vacuum chamber is included to provide guidance, but your specific setup may vary. It may vary in the number of vacuum pumps, accompanying instruments, and other ways, but these instructions serve as a general guidline as to how to properly vent any vacuum chamber.
Atmospheric oxygen is very reactive and if too much of it enters the chamber during periods of being in atmosphere this can lead to poor conditions. Because of this nitrogen, a cheap and nonreactive molecule, will be used through much of this process as the substance to vent the chamber. Gasseous nitrogen may be used to vent vacuum pumps, but it is best to only vent the actual chamber with liquid nitrogen.
The entire process involves much wait time for filaments to cool and internal pressure to reach equilibrium with the atmosphere. As such, set aside about a full day into your plans for the venting process. Again, most of this time you need not actively be performing tasks during this time. In terms of the time it takes for you, the actions needed should only take around 45 minutes to an hour and a half net to complete.
A brief schedule of the steps are as follows:
- Turn off all filaments (This takes 5-10 minutes to do, but around twelve hours to let the filaments really cool down).
- Acquire liquid nitrogen (This takes 5-10 minutes).
- Closing the pump valves (This takes 2 minutes).
- Venting the pumps (This takes 15 minutes for each pump).
- Venting the chamber with liquid nitrogen (this takes 5-10 minutes to start, but around 3 hours to let the pressure equalize).
- Determining if the chamber is full vented (this can be done in seconds)
- Resetting the valves (This takes 5-10 minutes).
READ ALL INSTRUCTIONS IN FULL BEFORE ATTEMPTING TO VENT THE CHAMBER
Step 1: Turning OFF All Filaments
Before you begin, take care that all filaments have been set OFF for a sufficient time (overnight or half of the day). This includes all evaporators, ion gauges, and ion pumps. Without doing so, hot filaments can oxidize (react with oxygen in the air) and be damaged. Figures 2 and 3 show examples of filament controllers that must be turned off for 12 hours before venting.
Step 2: Acquire Liquid Nitrogen
- Filled Liquid Nitrogen Dewar: Liquid nitrogen will be the main material used to vent the chamber. It is cleaner than gasseous nitrogen alone, and is used to ensure only pure nitrogen enters the chamber as it approaches atmospheric pressures.
- Most universities have a laboratory equipment dispensary where liquid nitrogen can be found. If not, consult commercial sources.
Step 3: Close the Pump Valves
At this point, any vacuum pumps still on must be closed off from the chamber (these are NOT filament based, and so are able to be turned off in the presence of atmospheric conditions). Many pumps require oil and other dirty contaminants that would be a nightmare if they got inside the vacuum chamber.
To ensure no suction of these contaminants into the chamber, close off the valves the pumps should be connected to. When this is done, the chamber is totally isolated, with no gasses going in or out. The next goal will be to properly power down the pumps themselves.
Step 4: Venting the Pumps
DO NOT JUST TURN OFF THE PUMPS
The vacuum pumps can be contaminated just as the chamber can, and since they are connected to the chamber their contamination could lead to damaging the chamber itself. As such, it is important to take care in this step to vent the pumps with gaseous nitrogen.
For this step, gaseous nitrogen will suffice in keeping the pumps themselves clean from atmospheric contaminants. With nitrogen gas flowing, attach the source hose to the outlet on your pump, and then switch off the pump. This step may take around 15 minutes. Once the pump is completely off, repeat the process for any other pumps.
Step 5: Venting the Chamber With Liquid Nitrogen
DO NOT PUT ANY BODY PART INTO THE LIQUID NITROGEN DEWAR
LIQUID NITROGEN MAY CAUSE FREEZER BURN ON DIRECT CONTACT WITH SKIN
Now is the time to vent the actual chamber. Depending on the size of the chamber, this may take anywhere from one (small chambers) to three hours (large chambers).
- Submerge the liquid nitrogen connection line into the liquid nitrogen.
- While in a position where you can hear exchange of liquid nitrogen (this sounds like light 'gasps' coming from the dewar), slowly rotate the liquid nitrogen intake valve counterclockwise to allow the uptake of liquid nitrogen to occur. DO NOT TURN ABRUPTLY OR TOO FAR. Go slow here, and stop when you hear the 'gasps' about once per second.
- Once a steady rate of one 'gasp' per second is reached, the venting will proceed for a long time with or without you. If this is your first time, stay and monitor the rate until the chamber is fully vented. If you are comfortable with the procedure, you can go do something else for the next few hours.
Step 6: Determining Whether the Chamber Is Fully Vented
If your chamber has an openable window such as the one above, then the chamber will be fully vented once this window is able to be opened with limited force.
If your chamber does not have such a window, the best way to determine the progress of the vent is to listen for the 'gasps.' If they are still happening, then the chamber needs more time to vent. If there are no sounds, rotate the liquid nitrogen valve counterclockwise to make sure liquid nitrogen can freely enter the chamber. If after this step there si still no sound, the chamber is vented.
Step 7: Resetting the Valves
At this point, the chamber is vented. However, it is beneficial here to reset the valves to be ready for the next pump down. Doing so now will ensure no mess up occurs later when you attempt to pump down back into vacuum.
To do so, simply rotate the all valves opposite what they are. In the case here, you need to open the two valves to the pumps and close the valve to the liquid nitrogen intake valve.
Once this is done, the chamber is completely vented and you can do your necessary work to the chamber without damaging its integrity.
Step 8: Supplemental Information
The steps above provide a general, quick use guide toward venting a vacuum chamber. However, if more information is needed the following links should serve as excellent sources of information.
- This website is a retail dealer of vacuum chambers, and contains ample information as to the different types of vacuum chambers and their operation.
This website is the same dealer as before, only this page pertains to vacuum pumps and their usage. Operation manuals for specific types of pumps can be found here.
This is a pdf of the proper use and safety information of liquid nitrogen. As a general rule, do not let liquid nitrogen make contact with you or leave the dewar. This link offers in depth knowledge of liquid nitrogen safety.