The Cryofab CMSH Series liquid helium dewars are designed for minimum loss storage, transportation and dispensing of liquid helium. The CMSH container employs state of the art vapor cooled multishield technology in conjunction with superinsulation to achieve the highest efficiencies.
The CMSH Series is ruggedly constructed and simple to use, however, the extremely low temperature of liquid helium requires the use of techniques which are more involved than for other cryogenic fluids. The low latent heat of vaporization, characteristic of liquid helium, can cause unnecessary transfer losses to occur unless established procedures are closely followed.
General precautions regarding liquefied gases are covered in the pamphlet attached to the dewar, however, the unique properties of liquid helium require that even further precautions be taken. Liquid helium is a colorless, nontoxic liquid with a density of one-eighth that of water, inert under all temperature and pressure conditions. At atmospheric pressure it has a temperature of approximately -452°F (132 degrees colder than the freezing point of liquid nitrogen). It is the only substance known that remains liquid under ordinary pressure at temperatures close to absolute zero.
Air liquefies and solidifies readily when exposed to the extremely low temperature of liquid helium. For this reason, the fill and vent ports of storage containers should be kept closed at all times (except during filling) to prevent blockages from forming in the passages and a resultant pressure buildup.
CMSH Series cylinders should not be subjected to rough handling either full or empty. Bumping, jostling or excessive agitation of any filled liquid helium container will cause abnormal evaporation and may result in increased flow of gas through relief devices.
The CMSH container is designed for use in the vertical position and should not be laid on its side. If a container must be lifted, use a forklift or similar device beneath the base or hoist by means of the lifting slots in the halo ring struts. Do not attempt to lift by the handles or by means of slings around the shell.
Normal practice requires that weighing the container and then subtracting the stamped tare weight should determine the amount of helium in a container. Scale accuracy should be checked often and the scale serviced as often as necessary to maintain required accuracy. In order to obtain the greatest accuracy remember to correct for the vapor remaining over the liquid, which can be a significant portion of the observed net weight, especially when the dewar is near empty.
A Flutter Tube may be used only to give a rough estimate of the liquid contents. This method can be expedient but somewhat wasteful.
If the dewar is equipped with a Superconducting Level Indicating System, please refer to the manufacturer's instructions included in the documentation package. The height of the liquid observed may be converted to a volumetric equivalent by using the calibration chart attached to the dewar.
If the dewar is equipped with a differential pressure type liquid level gauge the contents can be read directly in liters although the accuracy is not quite as good as a superconducting level indicating system may deliver.
The pre-cooling operation is especially important to limit the post-fill flashoff to a reasonable amount. All warm vessels should be pre-cooled to -300°F or less using one of the methods below. If the container is already below -300°F the pre-cooling operation may be omitted.
a) Pre-cool by filling the container at least halfway with liquid nitrogen. Let the cylinder stand for approximately 1–2 days. Remove the liquid nitrogen and purge with gaseous helium according to the procedure below, prior to filling.
b) Pre-cool with -320°F or colder helium vapor. For best results, introduce approximately 50 CFH into the bottom of the cylinder while exhausting through the vent valve. Approximately 12 to 16 hours of pre-cooling is required prior to filling with helium.
c) Pre-cool by adding approximately 25 liters of liquid nitrogen to the cylinder and then introduce 25 CFH of gaseous nitrogen through a dip tube extending to the bottom of the cylinder. The warm gaseous nitrogen will bubble through the liquid nitrogen, producing -320°F gas. Exhaust the gas through the vent valve. About 16 hours are required for pre-cooling. Remove any remaining liquid nitrogen and purge the container with gaseous helium according to the procedure below, prior to filling.
To purge the container of residual nitrogen or other gases prior to filling proceed as follows. If the dewar is equipped with an optional differential pressure type liquid level gauge, the alternate procedure shown further below must be used.
An alternate purging procedure would consist of evacuating the inner vessel and backfilling with GHe. Proceed as follows:
In order to fill the container the following procedure should be used:
Transferring liquid helium from the container is accomplished by using the following procedure:
When a helium container is used for air transport an absolute relief valve set at 15.2 PSIA is installed in place of the primary relief valve. This will prevent atmospheric pressure variations experienced during flight from affecting the contents of the dewar.
When a helium container is to be used as a collector for a liquefier a flowmeter is provided in order to maintain high performance. In this type of application the flowmeter functions in lieu of the .5 PSI pressure control valve. The flowmeter diverts some of the exhaust gas from the dewar up the neck tube to keep the shielding refrigerated. The gas should then be directed to the helium recovery system or the low pressure side of the liquefier. The recommended flow rate should correspond to the normal boil-off of the container.
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