Chapter 2— PRESSURE 



fail to feed, reopen the case and draw a piece of 

 glossy (bond) paper through the pen nibs to 

 start the flow. 



PEN AND INK.— Very little ink is needed; 

 a half-full pen should suffice. In fact, in damp 

 weather the nib may appear to become fuller. 

 This is because the instrument ink is hygro- 

 scopic and absorbs moisture. As this process 

 continues, the trace becomes paler because of 

 dilution. Wash and re-ink. A wide trace is 

 caused by dust accumulated on the point, or a 

 dull or bent point. 



USE BEYOND CHART RANGE.— The normal 

 barograph chart has a range of 965 to 1,060 mb. 

 When approaching any extreme pressure con- 

 dition which may carry the pen off the chart, 

 turn the adjustment knob to move the pen about 

 40 mb away from the close edge of the chart. 

 When the condition is passed, reset by the exact 

 same amount and mark the affected portion of the 

 chart accordingly. 



PLASTIC SHEET WINDOW.— Use a damp cloth 

 to clean the plastic sheet window in the case. 

 Do not use a solvent cleaner or a dry cloth, 

 as either can damage the plastic pane. 



Maintenance 



Very little maintenance is required for the 

 marine barograph. This section covers such 

 service, replacement, adjustment, and minor 

 repair as may be needed and can be performed 

 with no more than partial disassembly and 

 not require the use of special tools or test 

 equipment. 



Under normal operating conditions this in- 

 strument should be cleaned well once a year. 

 The element cover should be removed only to 

 clean out any bulky dirt, cobwebs, etc. Do not 

 wipe out this mechanism. Check the pen for 

 wear or roughness and replace as necessary. 



The chart drive mechanism should be cleaned 

 and oiled annually. This oiling should not be 

 attempted by Aerographer's Mates 3 or 2 un- 

 less they are properly instructed in the method 

 of doing it. 



For other troubles with the marine baro- 

 graph, consult the applicable instrument hand- 

 book. 



BAROMETERS 



MERCURIAL BAROMETER 

 (FORTIN) 



In 1643 Torricelli, an Italian physicist, made 

 the first crude barometer. The mercurial barom- 

 eters that are used in the Navy today operate 

 on the same principle. In the construction of the 

 barometer, a long glass tube, open at one end 

 and closed at the other, Is filled with mercury. 

 The open end is sealed temporarily and then 

 placed into a basin (cistern) of mercury, after 

 which the end is unsealed. This allows the mercury 

 in the tube to descend, leaving a nearly perfect 

 vacuum at the top of the closed end of the tube. 



When the atmospheric pressure is increased, 

 the mercury in the cistern is forced into the 

 glass tube. As the atmospheric pressure is 

 decreased, the mercury in the tube flows into 

 the cistern. The height of the mercury column 

 in the tube is therefore a measure of the air 

 pressure. (See fig. 2-3.) 



ATMOSPHERIC 

 PRESSURE 



CHANGE IN AIR PRESSURE 

 INDICATED HERE 



209.96 

 Figure 2-3. -Principle of the Mercurial 

 barometer. 



23 



