Dewar Precautions
Always handle glass dewars with care. Any product incorporating a vacuum is a potential safety hazard and should be treated with caution. If in doubt, contact your safety officer. |
When handling dewars containing liquefied gases or cryogenic liquids:
- Wear protective equipment:
- goggles or face shield
- an insulated or rubber apron
- insulated gloves
- When pouring liquefied gases from one container to another:
- cool the receiving container gradually to minimize thermal shock
- pour the liquified gas slowly to prevent splashing
- vent the receiving container to the atmosphere
For glass dewars
- Use a plastic stirring rod when stirring substances in a dewar containing liquefied gases (or other materials of extremely low temperature). Do not use a glass or metal stirring rod unless it has a protective coating.
- Do not handle heavy objects above the dewar. If unavoidable, place a protective cover over the dewar opening. If an object of sufficient weight is accidentally dropped into the dewar, shattering may occur.
- If the dewar has a protective mesh covering, do not remove it. This cover minimizes the risk of flying particles should the dewar be knocked over, dropped, or broken.
Mixing an IPA/LN2 Slurry
An isopropyl alcohol (IPA) /liquid nitrogen (LN2) slurry is used to maintain cold trap temperatures of approximately -80 °C. Improperly mixing an IPA/LN2 slurry could cause injury. If the mixture is not stirred continuously, gas may build up under the surface, causing the liquids to splash out of the dewar. |
- Chill a 600 mL dewar by rinsing it with LN2. Allow a small amount of LN2 to remain in the bottom of the dewar (approximately 1 cm deep).
- Stirring constantly, slowly add approximately 500 mL of IPA. For greatest safety, use a laboratory squirt bottle. Squirt the stream of IPA along the inside edge of the dewar, close to the top of the dewar, allowing the IPA to flow down the inside wall of the dewar. Do not stop stirring the mixture in the bottom of the dewar, even if stirring becomes difficult. As stirring and adding IPA continues, the mixture will loosen and become easier to stir.
- When all the IPA has been placed in the dewar, slowly begin pouring LN2 into the dewar. Approximately 1 liter is needed. Continue to stir the mixture as LN2 is added. As ice chunks form, break them up and stir them down into the mixture. Avoid splashing. Gently knock ice chunks away from the sides of the dewar and continue stirring. Add liquid nitrogen until the slurry is within 25 mm (1 in.) of the top of the dewar.
- Lift the stirrer out of the slurry and observe as the slurry drips into the dewar; the stirrer should be thickly coated with slurry.
The goal is to achieve a slurry that has a thick, syrupy consistency without large chunks. When the slurry is cold enough, small pieces of ice may be present (10 - 20% of the mixture). A little ice accumulation along the sides of the dewar is acceptable.
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