The transfer unit pumps carbon dioxide in its liquid phase only. This is true of all CO₂ transfer units. The amount of liquid carbon dioxide contained in a fully charged cylinder varies with t h e  p r e s s u r e  a n d  t e m p e r a t u r e ;  t h e r e f o r e ,  a standard 50-pound cylinder contains approx- imately 38 pounds of carbon dioxide in its liquid phase and approximately 12 pounds in its gaseous phase at an atmospheric temperature of 70°F. Therefore, the cooler the supply cylinder and the cylinder being recharged, the more efficient the operation of the transfer unit. Consequently, all cylinders should be kept in the coolest location possible. Conversely, the time required to charge an empty cylinder increases with increased temperature of the cylinder. When recharging a smaller cylinder, we found that if you invert the cylinder during the recharging period, it remains cooler and fills faster than it would if placed in an upright position. Larger cylinders should be placed horizontally on the scale when they are being recharged. After all the liquid carbon dioxide is trans- ferred from the supply cylinder, which is approx- imately 80 percent of the net contents, the transfer of CO₂ to the cylinder being recharged stops. After this, another fully charged supply cylinder must be used to finish recharging the cylinder to its full-rated capacity. The majority of gas remaining in the other supply cylinder can be used when you recharge another empty cylinder. The gas transfers itself under its own pressure until the pressure in both cylinders is equal. This method is called cascading. Through this method, the most economical use of the contents of the supply cylinder is made. To prevent expansion of carbon dioxide in the supply hose, and consequently blocking the hose with  CO₂ “ s n o w , ”  you should use a valve with an outlet opening of at least one-eighth inch in diameter-preferably three-sixteenths of an inch. Standard supply cylinders in 50-pound sizes are obtainable with or without a syphon tube. When you order cylinders, specify the ones with a syphon tube. Those without syphons must be inverted during the transfer process. MAINTENANCE M a i n t e n a n c e  m u s t  b e  p e r f o r m e d  o n  a ll carbon dioxide equipment on a periodic basis. These maintenance procedures are discussed in the following paragraphs. Once Every Month Check the level of the oil in the crankcase. See that it is within one-fourth inch of the top of the filling cup or to the upper groove in the measur- ing stick if the unit is so equipped. If you must add any oil, use only a good grade of SAE viscosity #30 automotive crankcase lubricating oil. Once Every 6 Months L u b r i c a t e  t h e  i d l e r  s h a f t .  T h i s  s h a f t  is equipped with a fitting of the variety that is commonly used in the automotive field. Two or three “ s h o t s” o f  l i g h t  c u p  g r e a s e  w i l l  be ample. Lubricate the gear teeth with a thin coating of light cup grease. With a small piece of wood, or preferably a small brush, apply a light coating of Vaseline to the piston rod. To do this, dip the brush in Vaseline, hold the brush against the piston rod, and manually rotate the gears until the piston rod i s  c o m p l e t e l y  a n d  t h o r o u g h l y  c o a t e d  w i th Vaseline. If necessary, tighten the packing at the piston stem. A special wrench is provided for this purpose. Do not tighten excessively. Because of the design of the packing, it must only fit snugly to hold tightly. Keep the motor commutator clean and main- tain a clean surface. Under normal operating conditions, the commutator will require only occasional cleaning with a dry piece of nonlinting cloth. Do not lubricate the commutator. Every 12 Months The oil should be drained from the crankcase and replaced with clean, fresh oil of the quality and viscosity specified. Once Every 2 Years The bearing housings of the electric motor should be removed and lubricated. To do this, disassemble the bearing housing. Then clean the inside of the housings, the plates or caps, and the bearings with carbon tetrachloride. Wipe off all grease and reassemble all parts except the outer caps or plates. Apply the new grease, either from a tube or by hand, over and between each ball. When you do this, do not apply more than one-half of an ounce of grease at each bearing. 3-4