Chapter 2 — PRESSURE 



replaced by the marine barograph, but many 

 are still in use at shore stations. Instructions 

 for their operation and maintenance are con- 

 tained In the appropriate equipment manuals. 



MARINE BAROGRAPH 



The purpose of the marine barograph is to 

 register and record the atmospheric or baro- 

 metric pressure. Because of the magnified scale, 

 high sensitivity, and accurate temperature com- 

 pensation, it is often referred to as a micro- 

 barograph. The instrument is designed to maintain 

 its precision through the varied and exacting 

 conditions encountered in marine use. Its record 

 is neither interrupted nor rendered inaccurate 

 by pitch, roll, or vibration of the ship. An 

 adjustable, grease-filled damping cylinder pro- 

 vides a means of preventing rise and fall of 

 the ship from causing a corresponding wavy 

 trace on this chart. The unit, as seen in figure 

 2-2 is quite portable and can record pressure 

 either in its immediate vicinity or at some remote 

 external point while located within a pressurized 

 cabin system. This barograph has a total usable 

 range of 170 mb (915 to 1085 mb) over which 

 it has been calibrated and temperature-com- 

 pensated. 



The airtight case locks to the base by means 

 of two latches. When it is in place, a flexible 

 rubber tube running to the hose connector from 

 a remote source provides the means by which 

 "outside" readings are recorded independently 

 of the cabin pressure surrounding the instrument. 



The marine barograph has three principal 

 sections within the gasketed case: the chart 

 drive assembly, the element assembly, and the 

 pen shaft mechanism assembly. The last two 

 sections are of no interest to the regular user- 

 observer of the instrument. 



The chart drive assembly consists of a chart 

 drive mechanism, a chart cylinder, a chart, and 

 a chart clip. The chart drive mechanism is an 

 8-day, spring-wound clock with antibacklash gears 

 and a self-contained lever for winding. The 

 removable cylinder is driven at the rate of 

 one revolution in 108 hours (4 l/2 days) through 

 additional antibacklash gears. As a result, vibra- 

 tion and shock do not make the chart record 

 irregularly as a result of play in the cylinder 

 drive system. Inside the cylinder top is a knurled 

 nut which permits removal of the cylinder for 

 winding the clock. The chart is held in place 

 by a chart clip. 



The element assembly is the center section, 

 covered by the element cover. It consists of a 

 pair of vertically mounted bellows whose out- 

 board ends are secured to brackets. These 

 brackets are moved apart or together by the 

 adjustment knob on top for zeroing the pen 

 position. The inboard ends are flexibly connected 

 to a beam which moves as pressure variations 

 affect the bellows. Between the two flexible 

 connections is a temperature compensation de- 

 vice, which is adjusted to correct temperature 

 change errors occurring throughout the element 

 assembly. 



The pen shaft mechanism is the right-hand 

 section under the mechanism shield. The shield 

 and its spacers are to protect the mechanism 

 from damage as the case is removed or re- 

 placed. The mechanism includes range and lin- 

 earity adjustments, the damping device, and the 

 temperature compensating lever which compen- 

 sates for changes in temperature. The damping 

 device is a set of cylinders with a thin layer of 

 special fluid. The damping and spring loading 

 make it possible for the barograph to be subject 

 to tilting up to 22 l/2° in any direction and not 

 vary more than ±0.3 mb from the true reading. 

 Normal accuracy is within ±0.3 mb of true 

 pressure. Also, the instrument may be carried 

 about by its handle without any preparation. 

 If rougher treatment is anticipated, the pen arm 

 should be secured. 



Operation 



This barograph needs very little attention. It 

 records barometric pressure on a 108-hour chart 

 and can be read at any time by interpolation 

 to within one- to two-tenths of a millibar. 



WINDING AND CHART CHANGING.— The cor- 

 rect procedure for winding and changing the 

 chart is as follows: Lift the pen from the chart 

 by means of the pen lifter and remove the 

 cylinder. Every time a chart is changed, the 

 clock should be wound; approximately 8 pulls of 

 the lever is enough. Wrap a chart around the 

 cylinder so the tab end is covered and the 

 chart is held in place firmly against the bottom 

 flange of the cylinder and replace the chart 

 clip. Install the cylinder carefully in place 

 until the antibacklash gear teeth engage. By 

 rotating the cylinder, set the pen to indicate 

 the correct time. Check to see if the pen has 

 sufficient ink (about one-half full), then lower 

 the pen and replace the case. If the ink should 



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