Stormy Weather
I woke up this morning to the sound of wind pushing against the cloth walls of the Jamesway. The likelihood of flying today was low. Although it was still relatively warm (around -9 degrees Fahrenheit), the wind made it feel like -33 degrees Fahrenheit. I was not surprised when I heard our plane was again delayed due to bad weather.
The clouds came in and the South Pole turned white and windy.
Sun Dogs
Luckily the storm passed and by mid-afternoon the skies were blue. Bob alerted Susan and me that there was a sundog outside. A sundog (known scientifically as parhelion) is a special phenomenon in the sky that creates a halo of refracted light around the sun. The sundog we saw had a beautiful rainbow around it with double rainbows around certain spots. Sundogs are caused by cold cirrus clouds, which create ice crystals that refract light like a prism.
Bob Melville and Michelle Brown stand in front of a sundog that formed around the sun.
Cryogenics Facility
Susan (our field guide) and I were invited to tour the cryogenics facility today. Flint Hamblin, a cryogenic technician, took us through the facility. Before entering the area, we had to place tags in a box by the door so people would know that we were inside the building. This is a safety precaution in case of an accident.
Flint moved three tags to the inside section before Susan, Flint and I went into the cryogenics facility.
There are two main gases that are compressed and stored at the cryogenics facility: helium and nitrogen.
Liquid nitrogen
Since air is made up of almost 80% nitrogen, Flint is able to cool air to get liquid nitrogen. A machine compresses the air to about 145 psi (pounds per square inch). The compressed air is cooled back to room temperature and then expanded. As the air expands, it cools and condenses as a liquid.
Flint shows us the liquid nitrogen at the cryogenics facility.
Liquid nitrogen can bring the temperature of instruments down to 77 degrees Kelvin (-320 degrees Fahrenheit!), allowing them to be calibrated. In the past, liquid nitrogen was also used to cool down the outer part of the telescopes at the South Pole. Today scientists use a more efficient system that involves liquid helium.
Liquid helium
Liquid helium is used to cool the BICEP 2 telescope to 4 degrees Kelvin. A small cryorefrigerator inside BICEP2 cools the light detector from 4 degrees Kelvin to 0.25 degrees Kelvin. The liquid helium at the cryogenics facility is stored in dewars. Dewars are containers that have two chambers: one that holds the liquid and the other that has a vacuum surrounding it.
A diagram of the dewar is drawn on the front of the giant liquid helium dewars.
The vacuum makes it difficult for heat to travel by conduction or convection. Giant helium dewars are filled with liquid helium when they are warm and then are cooled. The vacuum is almost completely free of air, with only a billionth of an atmosphere inside it. The dewars at the cryogenics facility contain many layers of aluminized mylar, which insulate the inner container reduces heat transfer through radiation. By reducing the ability to transfer heat through all three methods (conduction, convection and radiation), dewars make great insulators!
Three giant dewars can hold up to 4,000 liters of liquid helium each.
Smaller containers, similar to the ones we have at the AGO site, hold up to 2500 psi of compressed Helium gas.
Despite the great insulation in the dewars, about 15 liters of helium boil off of each dewar every day. It may seem strange that liquid helium boils. The helium is only in a liquid state because it is kept at a really low temperature. Therefore, the liquid helium is constantly boiling into gas much like how boiling water creates water vapor. Flint traps the vented helium gas and cools and condenses it back into liquid helium. This recycled liquid helium is used to launch the weather balloons at the facility next door.
Three cryogenic refrigerators, one for each helium dewar, allow escaped helium gas to be cooled and re-condensed back to liquid helium.
Constant monitoring
The three dewars in the cryogenic facility are constantly being monitored. Scales record the dewars' weights and instruments measure the depth of the liquid helium in each dewar.
A scale constantly measures the weight of the liquid helium from each dewar.
A probe sits inside the dewar tank to record the depth of the liquid helium in each dewar. The screen shows the depth.
There are also sensors that record the amount of oxygen in the facility. If the liquid nitrogen or liquid helium leaked out too much gas, it could displace the air and reduce the amount of oxygen in the room.
Helium: A precious resource
Flint works very hard to trap and reuse any vented helium. This is because helium is a precious resource. A helium atom has a full outer shell of electrons, making it a stable element. This means that it does not easily bond with other elements. Most elements that we use can be extracted from compounds. For example, aluminum readily bonds with oxygen and other elements, making it easier to extract from the earth. Helium, however, is a loner; it does not bond with other elements. Helium is difficult to capture because it is a lightweight gas that can easily diffuse into space. We get helium by that is trapped in natural gas wells. If there are radioactive rocks in the natural gas wells, they will decay and spit out alpha particles. Alpha particles are essentially helium nuclei, which can then form into helium atoms. This whole process takes millions of years and the helium in the wells only make 2 - 5% of the gasses that are trapped. Because of these factors, helium is a nonrenewable resource.
Questions
Why should the boiling helium gas escape from the dewar through the vent? Why shouldn't the gas be trapped in the dewar?
How is helium similar to fossil fuels? How is it different?
Math Connection
If 15 liters of helium leaks from each dewar every day, and there are 3 dewars, how much total helium leaks from the tanks per day? per week?
If a large dewar holds about 18,000 liters of liquid helium, and about 30 liters of helium boils and escapes a day, how long would it take for all of the liquid helium to boil and escape?