Environmental and health safety

Magnetic fields and medical implants

Persons with pacemakers or other electronic, electrical, or mechanical medical implants and devices must not enter room B-304 Stanley Hall due to the risk of damage to the device and possible injury or death.

Hazard analysis: The operation of electronic, electrical, or mechanical medical implants, such as cardiac pacemakers, bio stimulators, and neurostimulators may be affected or even stopped in the presence of either static or changing magnetic fields.

Besides electronic, electrical, and mechanical medical implants, other medical-surgical implants such as aneurysm clips, surgical clips, or prostheses, may contain ferromagnetic materials and therefore would be subject to strong attractive forces near the NMR magnet system. This could result in injury or death.

Safety zone and magnetic fields

The safety zone is outside of the 5 Gauss line, marked around each magnet in yellow and black tape. Do not bring ferromagnetic objects (examples: iron or steel wrenches, other tools, or ferromagnetic electronic equipment such as power supplies) into B-304 Stanley Hall unless absolutely necessary, and do not cross the 5 Gauss line with such objects.

Steel gas cylinders must be kept out of the 5 gauss line of all of the magnets. Use the non-magnetic aluminum cart to transport the cylinders.

Hazard analysis: Large attractive forces may be exerted on equipment in the proximity of the NMR magnet system. The force may become large enough to move the equipment uncontrollably toward the magnet. Small pieces of equipment may become projectiles.

Large equipment (e.g. gas bottles, power supplies) could cause bodies or limbs to become trapped between the equipment and the magnet, and may damage the magnet and cause a quench.

The closer a ferromagnetic object gets to the magnet, the larger the force. Also, the larger the equipment mass, the larger the force.

Safe handling of liquid nitrogen and liquid helium

The main hazards associated with handling liquid nitrogen and liquid helium are: A) burns when skin comes into contact with cold pipes or liquid, and b) asphyxiation if nitrogen or helium has expanded from its liquid form and the gas has displaced oxygen. To reduce the potential for injury, follow these guidelines:

• Avoid contact with cold unprotected pipes and vessels when working with liquid nitrogen or liquid helium.
• Wear proper protective equipment:
  • Dry leather or cryogenic gloves must be worn to avoid cold burns. The gloves must be loose fitting so that they can be removed easily.
  • Goggles or a face shield must be worn to protect the eyes and face.
  • Wear close-toed shoes and long pants while handling cryogens to protect feet and legs from accidental spills.
• Metallic objects (e.g. jewelry) should be removed from those parts of the body that may come into contact with the liquid.
• Liquid cryogens pose a potential asphyxiation hazard. Nitrogen expands by a factor of 680 and helium expands by a factor of 740 when changing from the liquid to gas phase. The expanding gas displaces breathable oxygen. To limit the asphyxiation hazard, liquid nitrogen and liquid helium should only be handled in well ventilated areas. Immediately evacuate the room if oxygen senor audible alarm sounds.
• The extreme cold of liquid nitrogen and helium can cause oxygen to condense from the air, resulting in fire danger. Keep cryogen use areas free of combustible materials (paper, cardboard, machine oil, etc) and eliminate any other sources of ignition.
• Never accompany cryogens in the elevator. If the elevator were to malfunction, the expanding gas could fill the elevator and pose a serious risk of asphyxiation. Load dewar on the elevator, post “No Passengers” sign on the dewar, and retrieve it after using a separate route (stairs or another elevator).
• When transferring cryogenic liquids, always direct the flow away from others.

Delivery of liquid helium dewars to Stanley Hall

Passengers should never accompany cryogens (liquid nitrogen or liquid helium dewars) in an elevator due to the risk of asphyxiation.

Procedure: Attach a sign to the dewar that states: Asphyxiation Hazard, PLEASE DO NOT ENTER ELEVATOR. Place the dewar in the elevator with the sign clearly visible, select the floor and close the elevator, walk down the stairs, and retrieve the dewar.

Hazard analysis: The insulation of the liquid helium provided by the dewar is not perfect. As heat enters the dewar, small amounts of liquid helium are converted to the gaseous state. Helium expands by a factor of 740 during this conversion and displaces oxygen.. This could result in an asphyxiation hazard, if the elevator were to stop for a prolonged period and passengers were not able to exit the elevator.

Magnet quenches and oxygen sensors

A magnet quenches when the electrical resistance in the coil of wire that forms the magnet suddenly increases. The electrical current is rapidly converted into heat, which then causes the liquid helium surrounding the magnet to boil and expand. The rapid boiloff of liquid helium displaces oxygen, which poses an asphyxiation hazard.

The greatest chance of a low oxygen condition occurs during the quench of a magnet. All of the liquid helium and liquid nitrogen are converted to gas in a short time. Helium expands by a factor of 740 and nitrogen expands by a factor of 680 when transforming from the liquid to gas phase. The expansion poses an asphyxiation risk due to the displacement of oxygen.

The volume of the room is approximately 100,000 cubic feet, with complete air exchange occurring four times an hour (average flow rate 6,666 cubic feet per minute). The quench of the largest magnet would release 6,500 ft3 of nitrogen gas and 7,000 ft3 of helium gas in approximately 1 minute. Based on the above calculations, even a quench of the largest magnet should not cause the oxygen content in the room to drop below the OSHA standard of 19.5% oxygen for more than 10 to 15 minutes.

Never-the-less, two oxygen monitors are located in B-304 Stanley Hall. The oxygen level should read approximately 20.8% and vary within a few tenths of a percent. The Cal/OSHA standard for minimum oxygen content is 19.5%. A value lower than this is hazardous and the alarm should alert you to leave the room immediately.

Compressed gas safety

• Most cylinders are made of steel, and because of their size, can be violently attracted to the magnets. Gas cylinders and gas carts should be kept out of the 5 Gauss lines of the magnets at all times. Also, use the non-magnetic wrenches provided to attach gas regulators. Do not bring magnetic wrenches into the room for this purpose.
• Cylinders must be stored in upright positions and immobilized by noncombustible restraints (chains) to prevent being knocked over.
• Cylinder valve caps must be in place when not in use.
• Cylinders must be moved only by a suitable hand truck.
• A gas regulator must be attached and in good working order before use.
• During use make sure that there are no leaks at the regulator. After use, make sure that the main valve is closed tightly and that there are no leaks.

Hazard analysis: Compressed gas cylinders containing low-grade helium gas are used to pressurize liquid helium dewars as part of the liquid helium fill procedure. Compressed gas cylinders containing high-grade liquid helium (research-grade – 6 nines) are used to supply the helium compressors associated with cryoprobes. The hazards associated with compressed gas use in this facility are as follows:

• Steel cylinders are magnetic and can be violently attracted to magnets.
• The cylinders contain high-pressure gas. In the event the valve is broken, the cylinder can become a lethal rocket.
• Leaks can lead to the displacement of oxygen and therefore pose an asphyxiation hazard.