Important notes on French pressure cell use
A French pressure cell is a high grade, precision machined piece of metal designed to work at pressures exceeding 1,000 atmospheres - akin to an ultracentrifuge rotor. Cells are expensive and they are not hard to damage if allowed to corrode, or misused in a press generating tons of force. To avoid damage and maintain the life of the cell, the following precautions are necessary. Most people mistakenly assume, since it all looks so simple, that there is no finesse to using the devise. By the contrary, there are a considerable number of things to look out for.
- Never use any corrosive material (such as acid - even acetate solutions are acidic) in the cell.
- A small amount of high vacuum Dow Corning silicon grease may be applied to the O rings on the plunger and the bottom piece of the cell to make them fit smoothly into the cell. The grease only needs to be supplied if the pieces cannot be inserted and removed smoothly.
- Check to make sure the nylon ball in the valve stem is not excessively worn. If so, it must be replaced. With the value stem flat on a table, use scissors on the ball itself to dislodge it. Press the value stem into the press to seat the new ball before use. With a good ball, you only need to have the value barely finger-tight to maintain excellent pressure. If the cell won't maintain pressure, the ball needs to be replaced (sometimes you can just rotate the ball 180 degrees in its socket).
- The outlet stem only needs to be screwed onto the cell barely finger-tight. The "wrench slot" on the outlet tube is only as an aid to removing it in the event that somehow it goes on too tightly.
- Fill the cell in an inverted position. Allow the minimum amount of air in the cell possible. Otherwise, at the end of the run, the air will be expelled under extreme pressure, possibly splattering your sample over you and the surroundings. Be sure there is nothing like broken Pasteur pipette tips in your sample. If there is, the piston and cell closure will be damaged, the walls of the cylinder may be gouged, etc. as the press comes down.
- When you close the cell by adding the bottom piece, open the valve to allow air to be displaced out of the cell. Have the outlet tube on the cell and loosen it half a turn so it is pointing down in a collecting vessel. Any sample forced out by closing the cell will thus go into the vessel, not your eyes or onto the machine, etc. Close the valve before setting the cell upright (point the outlet towards the real bottom of the cell again, too). Make sure the bottom closure is pressed all the way into the cell. As you mount the cell in the press, keep your hand over the bottom of the cell to keep the bottom piece from slipping out. The other hand holds the cell itself. If the bottom piece slips out and is not completely seated in the cell when you start to apply pressure, you may bend it and possibly, if the cell surfaces are not 100% horizontal, shoot the cell out of the press!
- If you don't have the cell perfectly flat in the press (top and bottom surfaces), there is the danger of deforming the components and shooting the cell out at you. The bottom of the cell must be pushed all the way into the main part of the cell and the cell must never be filled with more than 40 ml of fluid at a time. If it is, the piston will not seat and may be bent sideways.
- Be sure to clasp the cell in the press. With a large cell, there is not much danger of the cell riding up off the bottom piece, but the top clasp is designed to prevent this from happening. If the cell rides up off the bottom piece, as the press comes all the way down, the cell will be deformed and possibly shot out of the press. Before starting the press, make sure the piston latch on the top of the cell is swung out of the way!
- Never exceed 20,000 psi. The cell says "pressure tested at 30,000 psi" on its face. That simply means that it was tested once for safety's sake at that pressure but the detailed instructions say your safety is not assured over 20,000 psi.
- Start with the pressure setting on the press turned all the way down. Don't start the press and presume that no one before you has fooled with the dials. Once you start a run, with the cell under pressure, you should not back off and then resume pressing without first releasing the pressure inside the cell. If you do when the cell is under immense pressure, the cell may ride off its bottom as described above, be improperly seated and deformed when the press comes all the way down.
- As long as you don't exceed the 20,000 psi upper limit, you can pressurize the cell several thousand extra psi above the pressure you want to maintain when you begin to open the valve.
- Put a Kimwipe over the mouth of your collecting vessel to prevent spraying of sample.
- Open the valve carefully. Fractions of an inch make a tremendous difference.
- Do not overtighten the valve in an attempt to maintain pressure. Excessive valve closing force can damage the valve seat and ruin the cell. When the ball gets worn, excessive tightening is required to maintain pressure. The nylon ball should be replaced rather than continuing to tighten the valve and possibly damaging the cell (some people like to live dangerously and deliberately deform the ball, feeling it gives them better control - don't do this with my cell).
- When you are done and your sample has stopped flowing out, open the valve completely to make sure there is no pressurized gas within the cell before stopping the press.
- Clean up the area around the press. The press itself will start to corrode rather badly if you leave spilled salt solutions around.
- When you are done, you must carefully rinse out the cell to prevent corrosion. The cell, just like an ultracentrifuge rotor, although in this case made of high grade stainless steel, will corrode if not thoroughly cleaned. Disassemble the cell and rinse all pieces with running water. First, use warm tap water and then running distilled water. Pay particular attention to rinsing water through the outlet stem and through the orifices of the cell itself. Cover the bottom of the cell and allow water to flow through the valve hole. Cover the bottom and the valve hole and allow water to flow through the outlet hole (the hole on the right as the body of the cell stands upright). Allow the cell to air dry on paper towels.
- Don't store the cell somewhere it might corrode, e.g. on a salt-stained shelf in a cold box or below a shelf where someone is keeping corrosive substances, etc. (the instructions that come with the cell say to store it with an oil coating for long term storage! I wouldn't think of doing this but the makers of the cell take corrosion seriously).
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