Closed Systems consist of bulb, capillary tube and Bourdon tube or capsular spring. They are either completely filled with mercury, another liquid or a gas ; or partially filled with a volatile liquid the vapour pressure of which is utilized.
Electric Systems possess three advantages : (I) applicability to high temperature work, (2) practicability of long distance regula tion, (3) replaceability of sensitive element. Radiation Pyro metric Systems are seldom used for automatic regulation. Resist ance Systems are applicable for temperatures up to 300° F with base metal (nickel) elements and up to l,500° F with platinum. Thermoelectric Systems may be of the galvanometric or poten tiometric type. The latter has several advantages for "medium" temperatures. Usual range limitations are 30o° to 1,800° F with base metal and ',coo° to 2,500° F with rare metal couples.
Other Conditions. Pressure.—The majority of pressure regu lators are of the self-operating type, but for process work the air operated type is being increasingly adopted. "Absolute pressure" and "differential pressure" regulators are in use in gas and by product plants.
Humidity.—In general, instruments regulating the humidity in dryers consist of a pair of automatic temperature regulators of the closed system type—the bulb of one being affected by "air temperature" while the bulb of the other is enclosed in a con stantly moistened porous sleeve exposed to the cooling effect of fairly rapid air circulation. The dry-bulb regulator unit controls the source of heat, while the wet-bulb unit controls the "spray."
Many ingenious humidity regulators have recently appeared, in cluding models which provide a "drying schedule." Timing.—The first automatic regulators for this purpose were "modified alarm clocks." Modern ones dependably perform the complex cycles of operations required in vulcanization, dyeing, pasteurization of milk, etc. (See ROBOTS.) Liquid Level.—An "automatic liquid level regulator" is to be found in every modem bathroom, but the industrial types neces sarily differ. They utilize auxiliary power to operate large valves. Extra heavy and corrosion-resisting models are used in petroleum refineries and chemical plants under severe temperature and pres sure conditions.
Flow.—While flow meters are being increasingly adopted in power plants and numerous industries, the demand for auto matic regulation of flow has not as yet brought about the full development of special instruments.
Specific Gravity.—Two types of devices which automatically regulate the density of the substance being processed are com mercially manufactured. One utilizes the hydrometric principle and embodies a float chamber ; the other utilizes the electrical resistance of the substance being processed.
Speed.—Standardized commercial devices consist essentially of tachometers provided with electric contacts or other means of operating levers, rheostats or other appliances.
See The Instrument World (London) and Instruments (Pittsburgh, Pa.). (M. F. BE.)