Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 5. Suction Gages

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION V - SUCTION GAGES

16. Purpose and use.-a. The suction gages described herein are used on aircraft to indicate the amount of suction that actuates the air-driven gyroscopic instruments. In instrument flying it is particularly important that all flight instruments on the instrument panel function properly. Any reduction in the suction which actuates the gyroscopic flight instruments below that specified as normal will impair their functioning. Therefore a means must be provided continually to inform the pilot of the exact amount being provided.

b. The specific uses of suction gages are to

(1) Indicate at all times the amount of suction in the vacuum systems of the aircraft.

(2) Give an indication of leaks that might develop in the vacuum system.

(3) Indicate the proper adjustment of the system relief valve.

17. Description.-The suction gage (figs. 7 and 8) consists of a case vented to the atmosphere, within which is a pressure sensitive capsule or diaphragm and a multiplying mechanism which amplifies the movement of the pressure sensitive element and transfers it to the pointer. The suction to be measured is admitted to the pressure sensitive element through a connection in the back of the case. The range of the gage is from 0 to 10 inches mercury (Hg.), the dial being graduated uniformly over the whole scale in increments of 1/5-inch Hg. Numerals are placed on the dial at 0, 2, 4, 6, 8, and 10 inches Hg. The scale utilizes 300° of the dial with the zero at the lower left side and the pointer moves clockwise with increasing suction. The hand, numerals, and "inch" graduations are painted with luminous material which under average conditions of night flying will give sufficient luminosity to be legible. All suction gages are provided with the standard 3-volt light. The light receptacle is built integral with the instrument case, which is of one-piece construction and has provision for holding the cover glass in position by use of a snap ring.

18. Operation.-a. The gage is connected directly into the vacuum system and indicates the amount of suction being provided to the gyroscopic instruments. The corrugated faces of the pressure capsule are reacted upon by suction through the connection on rear of the case, and the magnitude of deflection of the capsule walls depends upon the amount of suction. This deflecting, when transferred from the diaphragm through the rocker shaft, sector, and pinion to the pointer, gives a continuous indication, and any variation in the normal output of the pump or venturi tube will be immediately apparent.

b. On all standard installations the suction gage is connected by means of 3/16-inch tubing into the flight indicator because the air consumption of this instrument is greater than that of any of the other gyro instruments. With the suction relief valve adjusted so that the required amount of suction is available in this instrument, a reading on the gage indicates that there is suction in all of the gyro instruments. Variation in the amount of suction to these instruments is obtained by the use of restrictions placed in the line leading to them. The normal suction gage reading is l.75 to 4.25 inches Hg.

FIGURE  7. -Suction gage.

19. Installation.-a. The instrument is attached to the instrument panel in such a position that the pointer is horizontal when registering 2 inches Hg. When making the connection to the instrument. the threads on the nipple should be cleaned and a drop of oil added, care being taken to remove any deposits of solder from the union seat. The connecting nut is tightened until the nipple seals properly, producing a tight joint without excessive strain on the instrument. When the suction gage is mounted on a vibrationproof instrument panel and also connected to another instrument on the panel, no flexible connection is required; but if the connecting tubing is attached to a rigid portion of the airplane a suitable flexible connection must be used.

b. Suction gages equipped with built-in lighting are installed as above, except that they are mounted with three screws instead of four. The upper right-hand screw hole is notched out to accommodate the instrument lighting lug. Wiring is accomplished in accordance with the specific airplane installation drawing in which the equipment is installed.

20. Maintenance.-The general maintenance procedure outlined in section II is sufficient for this instrument.

FIGURE 8.--Schematic cross section of a suction gage mechanism