Maintain High Purity Gases
The analysis system was designed to accurately measure the surface area of all types of materials. It is important that the gases (especially krypton) used for these measurements be of highest purity, especially when analyzing low surface area samples. Three ways to ensure high purity gases are to always maintain:
- thoroughly purged gas pressure regulators
- non-permeable gas lines
- leak-free connections
Impure gas is strongly indicated, for example, if a series of measurements on a low surface area material yields decreasing specific surface areas with decreasing quantities of sample. The analyzer uses very small amounts of helium; therefore any residual air in the regulator can distort results of subsequent analyses for quite some time.
Micromeritics offers the following suggestions to assist in maintaining high purity gases (particularly helium):
- Use metal gas lines only
- Remove trapped air from the regulator and gas lines
Use Metal Gas Lines
Always use metal gas lines which have been carefully cleaned of any oils and greases used in the manufacturing process. Do not use plastic or rubber gas lines. When these types of permeable, nonmetallic gas lines are used with helium, contaminants accumulate at a much faster rate. This causes errors in analysis results and can also contaminate a clean sample.
Remove Trapped Air
When connecting the regulator to the gas cylinder, air is unavoidably trapped on the high- and low-pressure sides of the regulator, as well as in the gas lines. Remove as much of this air as is possible before opening the gas cylinder valve. If this air is allowed to remain in the regulator, it will mix with the helium and cause inaccurate results in subsequent analyses. Or if the valve is open for any length of time, the air trapped on the high pressure side may diffuse back into the gas cylinder and contaminate its entire contents.
There are two methods for removing trapped air from the regulator lines: the Purge Method and the Evacuation Method.
Purge Method
This is the preferred method for removing trapped air.
- Go to Unit [n] > Enable Manual Control. Ensure a checkmark displays to the left of the menu item. If the analyzer schematic does not display, go to Unit [n] > Show Instrument Schematic.
If multiple instruments are installed, choose the correct Unit menu. |
- Close all valves by right clicking each valve, then click Close.
- Open the regulator shutoff valve.
- Open the gas cylinder valve briefly and allow the regulator to be charged with gas until the high-pressure gauge reads just over half the tank pressure, then quickly close the valve.
- Use the Pressure Control knob to set the output pressure (gas cylinder pressure gauge) to 15 psig.
- Loosen the fitting at the instrument helium inlet until the low pressure side drops to approximately 3 psig (0.02 MPa), then tighten the fitting.
- Repeat steps 4, 5, and 6 three times.
- Briefly open the gas cylinder valve, then use the Pressure Control knob to reset the regulator output pressure to 15 psig.
- After the pressure has stabilized (indicating there are no leaks), open the gas cylinder valve.
Evacuation Method
To use this method, the gas tank must be within 10 feet of the instrument. |
- Do one of the following:
If... | Then... |
---|---|
The regulator has not been filled with gas and the gas line is attached to the instrument: | Close the gas cylinder valve. |
Open the regulator shutoff valve. | |
The regulator is filled with gas: | Close the gas cylinder valve. |
Open the regulator shutoff valve. | |
Loosen the helium inlet fitting (or nut) on the instrument. | |
Allow all of the gas in the regulator to expel from the line (pressure reading will be zero). | |
Retighten the helium inlet fitting (or nut). |
- Go to Unit > Enable manual control (if the instrument schematic is not displayed, go to Unit > Show instrument schematic).
If multiple instruments are installed, ensure the correct Unit menu is selected. |
- Close all valves, then open valves 6, 7, and 10.
- Allow evacuation to continue for 20 minutes. This pulls a vacuum on the helium line to the gas cylinder. The manifold pressure transducer should fall close to zero.
Allow evacuation for a full 20 minutes. If evacuation time is too short, trapped air may remain in the lines. |
- Close valves 6, 7, and 10.
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