Measurement of Outgassing Rates of Steels

The overall goal of this procedure is to measure the outgassing rate of hydrogen from vacuum chambers using the rate of pressure rise method. This method can help answer key questions in the vacuum measurement field, such as measurement of the outgassing rate of very low outgassing materials. The main advantage of this technique is that the outgassing rate can be systematically measured even from extremely low outgassing materials, or relatively small samples.

Demonstrating the procedure will be Se-Hyun Kim, a grad student from my laboratory. First, assemble the vacuum components of the experimental apparatus sequentially, using copper gaskets from the pump side to the sample side. Using a wrench, tighten all the flange joints face to face with a copper gasket, except the joint between the sample chamber and the chamber isolation valve.

Using a level meter, adjust the spinning rotor gauge, or SRG flange assembly, and the sample chamber, so that the axis of the SRG head is vertical. Tighten the flange joint between the sample chamber and the chamber isolation valve face to face, while maintaining the SRG flange's level. Next, connect the roughing pump and the helium leak detector or HLD with isolating valves to the clamp flange port of the exhaust end of the turbomolecular pump, or TMP.

To perform a leak test, turn on the HLD. Once the detector is ready, open the HLD valve, and close the roughing pump valve. Pump down the setup using the HLD, waiting for approximately 30 minutes to pump out the residual helium gas.

Ensure that the helium level is within the minimum detectable limit of the HLD. Spray helium gas through the leak test groove on the flanges. Then, measure any change in the helium level ensuring that the chamber is leak tight.

If not leaks are detected, stop the leak test, and vent the vacuum system. Then, open the roughing pump valve and close the HLD valve. Pump down the vacuum system by switching on the TMP and the roughing pump at the same time.

For bake-out, remove the SRG head from the flange assembly, wrap the vacuum components between the SRG flange assembly and the inlet flange of the TMP and band heaters. Following this, check that there is no electrical short circuit between the heaters and the vacuum parts using a handheld multimeter. Next, connect the heaters to the extension cable, then to their respective heater controllers.

Afterwards, wrap the chamber in aluminum foil. Now, raise the temperature of the chamber to 150 degrees celsius, at a ramp rate of one to two degrees celsius per minute. Maintain the temperature of the residual gas analyzer, or RGA electronics, under 50 degrees celsius, using a cooling fan.

At this point, de-gas each of the RGA filaments by electron bombardment for at least five minutes. Measure the RGA spectrum from a one to 50 mass to charge ratio, to ensure that the water peak is less than one half of the hydrogen peak. Allow the system to cool down to room temperature at a ramp rate of one to two degrees celsius per minute.

Check for leaks referring to the RGA spectrum measured during the cooldown. Next, analyze the residual gas in the sample chamber and measure the RGA spectrum. After closing the chamber isolation valve, measure the RGA spectrum again.

Verify that the sum of all impurity gasses, such as water, carbon monoxide, and carbon dioxide is below five percent. Following this, assemble the SRG head on the SRG flange assembly. Using a level meter for reference, ensure that the axis of the SRG head is within plus or minus two degrees max.

Start the SRG, and wait for stabilization of the residual drag, which usually takes a few hours. Enter the proper input parameters such as gas, temperature, and measurement interval. While waiting for the signal to stabilize, stabilize the temperature of the sample by switching on the chiller to run cool water through the system.

Then, set the fluid temperature to 15 degrees celsius. Following this, start the heater controller for the sample, and set the target temperature to 24 degrees celsius. After closing the oven door, wait for the temperature to stabilize within plus or minus 0.1 degrees celsius.

Now, verify that the variation of the offset value of the signal is within plus or minus one times 10 to the minus nine Pascal per second. Check the signal level provided by the SRG controller, which should be at least minus 10 decibels, and ideally between zero and six decibels. Next, check the damping level provided by the SRG controller which should be between minus 35 and minus 60 decibels, and is normally satisfied in the system using the TMP and scroll pump, that is laid on a rubber pad.

Gently close the all metal angle valve to start the pressure build up being careful not to subject the SRG to mechanical shock. After closing the door of the oven, acquire pressure data for eight to 24 hours using a computer. Pre-check the measured data to verify that the variation in temperature is within plus or minus 0.1 degrees celsius after stabilization, and that the pressure rise is linear with 10 percent error.

Continue to measure data until the pressure rise becomes linear within 10 percent error for at least 16 hours. Finally, turn off all equipment. The residual gas after the bake-out was mostly hydrogen, and the pressure rise was linear over a long period of time.

Thus, the reabsorption effect might be negligible, and the intrinsic outgassing rate for the steels tested in this study can be evaluated using the rate of pressure rise method. The measured outgassing rate for untreated STS 304 steel was consistent with reported values, and an approximately 22 fold reduction in outgassing was achieved with the medium temperature heat pre-treatment. While the outgassing rates for untreated mild steels were very low, the outgassing rates were second to the rates of stainless steels after intensive heat treatment.

In addition, the outgassing rate for mild steel decreased by only 66 percent following heat treatment, and no significant reduction in outgassing was observed. These measurements strongly suggest that the difference in outgassing between stainless steels and mild steels can be attributed to the differences in the steel making processes, and in particular, the secondary metallurgy processes during which impurity gasses are extracted. Once mastered, this technique can be done in five days, if it is performed properly.

While attempting this procedure, it is important to remember to check if there are leaks and to maintain the temperature of the system precisely. After watching this video, you should have a good understanding of how to build the outgassing rate measurement system, and how to measure the outgassing rate systematically, using the rate of pressure rise method.