The Interdepartmental Equipment Facility
Dry Ice Maker
The Frigimat dry ice maker is used to prepare cylindrical blocks of dry ice, about 450 g each. The Frigimat has to be connected to an appropriate cylinder for supply of liquid CO2
Dry ice is frozen carbon dioxide. Its surface temperature is -78.5 degrees C so you can use it to transport biological samples in a frozen state. It is claimed to give more than twice the cooling energy per unit weight compared to regular ice. Dry ice also has the nice feature of sublimation. As it melts, it turns directly into carbon dioxide gas rather than a liquid, so there is no liquid mess left as you would have with regular ice.
How long does it keep?
It all depends on the quality of insulation. The sublimation rate of a block of dry ice can be between 0.5% and 20% of its weight per hour. You should have dry ice prepared as close as possible to the time that you leave, and put it in a good ice-box.
Precautions
- 1. Dry ice should be prepared in a well ventilated area. Carbon Dioxide becomes toxic at concentrations exceeding 5% in the air.
- 2. Do not handle dry ice with your bare hands; you'll get frost bites. Wear appropriate gloves.
- 3. Do not store dry ice in an air-tight container. Pressure will build up inside and cause the container to explode.
How Do you make Dry Ice?
You start by connecting a high-pressure syphon-type cylinder with liquid carbon dioxide, to the dry-ice chamber. You can tell that it is a syphon-type cylinder by the vertical black stripe painted on the cylinder. Make sure that the chamber is properly installed and the knobs on the back plate are firmly tightened. Now open the room windows, put on heavy gloves and open the valve on the dry ice chamber about one-third of the way. You should get a loud hiss as the pressurized carbon dioxide escapes.
The liquid expands and cools down as it escapes. Carbon dioxide cannot exist as liquid at room temperature, at pressures lower than about 30 bar, so about two thirds of it turns into gas and escapes into the atmosphere. The remaining one third is cooled down by the expansion and evaporation processes, all the way to the freezing point (-78.5C), and turns into solid CO2.
The solid snow flakes just created are packed tightly in the dry ice chamber, making a compact cylindrical cake of dry ice, 8.5 cm in diameter and 11 cm long. You can tell when the block is ready by a distinct change in the gas hiss and when snow flakes start coming out. The density of the ice block is about 0.77 g/mL, which is half of the density of crystalline solid carbon dioxide. Each cylinder of liquid carbon dioxide will make about six blocks of dry ice.
Disposal of Dry Ice
Do not dump it in a sink or toilet. The extreme cold can harm sink and toilet parts and pipes. Do not dump it in the garbage. Do not leave dry ice to evaporate in an unventilated room. The best thing to do is to leave it outside in a place that is out of reach of people, packed in its own container, allowing it to evaporate.