When the steam-gage reaches the point desired, the safety valve is adjusted so that the steam will escape should the pressure continue to rise. Until the operator knows where to set the weight to the safety valve, leave it well out to the end of the rod until the pressure in the canner has reached the desired point. Then move the weight to the point on the arm of the valve which will just keep in the steam.
Fig. 122. Device for sealing tin can.
Be sure the cover is properly adjusted. Be sure to exhaust the air from the canner before closing the pet cock. Keep the fire so that the desired pressure will be maintained without the escape of steam from the safety valve. When steam escapes from the canner thru the pet cock at a rapid rate, it may cause liquid to flow out of the jars.
Be certain to let the canner cool until the indicator on the steam-gage has reached zero before opening the canner. When the indicator points to zero, open the pet cock. If a heavy stream of steam starts to escape from it, close it again and wait a few minutes longer. Test again by opening the pet cock; if a very little stream of steam escapes, leave the pet cock open and wait until steam has stopped escaping from it. Now loosen the screws holding the cover in place. Partially loosen each screw. When this is done, fully loosen all and lift off the cover. These precautions are taken to prevent the operator from being burned by steam or getting hurt by the cover being lifted by the steam. It also prevents the breaking of glass jars due to sudden pressure changes.
Never let the canner cool so long before the pet cock is opened that air will rush into it, due to the vacuum which is sure to form when the steam is cooled if the pet cock is not opened. Such a condition may break the jars.
Fig. 123. Dryer.
Tin cans are sealed with a device (Fig. 122) which folds the edge of the cover over the top of the can so tightly it will not leak.
220. Dryers. Dryers are devices to hold the food being dried in a thin layer so that the air can be circulated thru it freely. Sometimes they are devised to direct currents of air thru the drying material. If the air is heated, the drying is hastened (Fig. 123).