CHAPTERIVPRACTICALPROCEDURESFORMAGNETICCOMPASSADJUSTMENTNOTE: If the adjuster is not familiar with the theory of magnetic effects, the methods of analyzing deviation curves, and themethods of placing a ship on any desired heading, it is recommended to review Chapters II, V and VIl, respectively, beforebeginningadjustment.401. Dockside tests and adjustments. Chapter I, "Procedures for Magnetic Compass Adjustment" is, in general, self-explanatory,and brings to the attention of the adjuster many physical checks which are desirable before beginning anadjustment.The theoretical adjustment is based on thepremise that all the physical arangements are perfect, and much timeand trouble will be saved while at sea if these checks are made before attempting the actual magnet and soft iron correctoradjustments.Afewofthese checks are amplifiedbelow.402. Should the compass have a small bubble, compass fluid may beadded by means of the flling plug on the side of thecompassbowl.If anappreciableamountofcompass liquidhas leaked out,acareful check shouldbemadeontheconditionof the sealinggasketand fillingplug.U.S.Navy compass liquid maybea mixtureof 45%grain alcohol and 55%distilledwater,orakerosene-typefluid.These fluids areNOT interchangeable.403.Thecompassshouldberemovedfromtheshipandtakentosomeplacefreefromallmagneticinfluencesexcepttheearth'smagneticfieldfortestsofmomentandsensibility.Thesetestsinvolvemeasurementsofthetimeofvibrationandtheability of the compass card to return to a consistent reading after deflection,These tests will indicate the condition of thepivot,jewel,andmagnetic strengthof thecompassneedles.(SeeNAVSEA3120/3for suchtestdataonstandardNavycompassequipment.)404.A careful check should be made on the spheres and Flinders bar for residual magnetism.Move the spheres as close tothecompassaspossibleandslowlyrotateeachsphereseparately.Anyappreciabledeflection(2ormore)ofthecompassneedles resulting fromthis rotation indicates residual magnetism inthe spheres.This testmaybemade with the ship on anysteadyheading.TheFlinders barmagnetization checkis preferablymade with the ship on steady east or west compassheading.To make this check:(a)note the compass reading with the Flinders bar in the holder, (b) invert the Flinders bar intheholder and again note the compass reading.Anyappreciable difference(2°ormore)between these observed readingsindicates residual magnetismintheFlindersbar.SpheresorFlinders bars that showsignsof such residual magnetism shouldbe annealed, i.e. heated to a dull red and allowed to cool slowly.405. Correct alignment of the lubber's line of the compass, gyro repeater, and pelorus with the fore-and-aft line ofthe ship isofmajor importance.Suchamisalignmentwillproduce a constantA error inthecurveof deviations.All of these instrumentsmaybealigned correctlywith thefore-and-aft lineof theshipby using theazimuth circleand ametal tapemeasure.Shouldthe instrumentbe located on the centerline of the ship,a sight is taken on a mast or other object on the centerline.In the caseofaninstrumentoffthecenterline.ametaltapemeasureisusedtomeasurethedistancefromthecenterlineoftheshiptothecenterofthe instrument.A similar measurementfromthecenterline is madeforward or abaftthesubject instrumentandreferencemarks are placed on the deck.Sights are then taken on these marks.Standard compasses should always be aligned so that thelubber's line ofthe compass isparallel tothefore-and-aft lineofthe ship.Steering compasses may occasionally be deliberately misaligned in order to correct for any magnetic A errorpresent, as discussed in article 411.406. In addition to the physical checks listed in Chapter I, there are other precautions to be observed in order to assurecontinued satisfactory compass operation.These precautions are mentioned to bring to the attention of the adjuster certainconditions that mightdisturbcompassoperation.Theyare listed inChapterI andarediscussedfurther inChapterX.Expeditious compass adjustmentis dependentupontheapplicationof thevariouscorrectors inanoptimum sequence soasto achievethefinal adjustmentwitha minimum number of steps.Certainadjustments maybemade convenientlyatdocksidesoastosimplifytheadjustmentproceduresatsea.407.Inasmuch as theFlinders bar is subject to induction from several of the other correctors and, since its adjustment is notdependent on any single observation, this adjustment is logically made first.This adjustment is madeby one of the followingmethods:(1) Deviation data obtained at two different magnetic latitudes may be utilized to calculate the proper length of Flindersbarfor anyparticular compass location.Details of theacquisition of suchdata and thecalculations involvedarepresented inarticles605to609,inclusive.15
15 CHAPTER IV PRACTICAL PROCEDURES FOR MAGNETIC COMPASS ADJUSTMENT NOTE: If the adjuster is not familiar with the theory of magnetic effects, the methods of analyzing deviation curves, and the methods of placing a ship on any desired heading, it is recommended to review Chapters II, V and VII, respectively, before beginning adjustment. 401. Dockside tests and adjustments. Chapter I, "Procedures for Magnetic Compass Adjustment" is, in general, selfexplanatory, and brings to the attention of the adjuster many physical checks which are desirable before beginning an adjustment. The theoretical adjustment is based on the premise that all the physical arrangements are perfect, and much time and trouble will be saved while at sea if these checks are made before attempting the actual magnet and soft iron corrector adjustments. A few of these checks are amplified below. 402. Should the compass have a small bubble, compass fluid may be added by means of the filling plug on the side of the compass bowl. If an appreciable amount of compass liquid has leaked out, a careful check should be made on the condition of the sealing gasket and filling plug. U.S. Navy compass liquid may be a mixture of 45% grain alcohol and 55% distilled water, or a kerosene-type fluid. These fluids are NOT interchangeable. 403. The compass should be removed from the ship and taken to some place free from all magnetic influences except the earth's magnetic field for tests of moment and sensibility. These tests involve measurements of the time of vibration and the ability of the compass card to return to a consistent reading after deflection. These tests will indicate the condition of the pivot, jewel, and magnetic strength of the compass needles. (See NAVSEA 3120/3 for such test data on standard Navy compass equipment.) 404. A careful check should be made on the spheres and Flinders bar for residual magnetism. Move the spheres as close to the compass as possible and slowly rotate each sphere separately. Any appreciable deflection (2° or more) of the compass needles resulting from this rotation indicates residual magnetism in the spheres. This test may be made with the ship on any steady heading. The Flinders bar magnetization check is preferably made with the ship on steady east or west compass heading. To make this check: (a) note the compass reading with the Flinders bar in the holder; (b) invert the Flinders bar in the holder and again note the compass reading. Any appreciable difference (2° or more) between these observed readings indicates residual magnetism in the Flinders bar. Spheres or Flinders bars that show signs of such residual magnetism should be annealed, i.e. heated to a dull red and allowed to cool slowly. 405. Correct alignment of the lubber's line of the compass, gyro repeater, and pelorus with the fore-and-aft line of the ship is of major importance. Such a misalignment will produce a constant A error in the curve of deviations. All of these instruments may be aligned correctly with the fore-and-aft line of the ship by using the azimuth circle and a metal tape measure. Should the instrument be located on the centerline of the ship, a sight is taken on a mast or other object on the centerline. In the case of an instrument off the centerline, a metal tape measure is used to measure the distance from the centerline of the ship to the center of the instrument. A similar measurement from the centerline is made forward or abaft the subject instrument and reference marks are placed on the deck. Sights are then taken on these marks. Standard compasses should always be aligned so that the lubber's line of the compass is parallel to the fore-and-aft line of the ship. Steering compasses may occasionally be deliberately misaligned in order to correct for any magnetic A error present, as discussed in article 411. 406. In addition to the physical checks listed in Chapter I, there are other precautions to be observed in order to assure continued satisfactory compass operation. These precautions are mentioned to bring to the attention of the adjuster certain conditions that might disturb compass operation. They are listed in Chapter I and are discussed further in Chapter X. Expeditious compass adjustment is dependent upon the application of the various correctors in an optimum sequence so as to achieve the final adjustment with a minimum number of steps. Certain adjustments may be made conveniently at dockside so as to simplify the adjustment procedures at sea. 407. Inasmuch as the Flinders bar is subject to induction from several of the other correctors and, since its adjustment is not dependent on any single observation, this adjustment is logically made first. This adjustment is made by one of the following methods: (1) Deviation data obtained at two different magnetic latitudes may be utilized to calculate the proper length of Flinders bar for any particular compass location. Details of the acquisition of such data and the calculations involved are presented in articles 605 to 609, inclusive
(2)Iftheabovemethod is impractical.theFlindersbarlength will havetobesetapproximatelyby(a)Usinganempiricalamount ofFlindersbarthathasbeenfound correctforotherships ofsimilarstructure(b) Studying the arrangement ofmasts, stacks, and other vertical structures and estimating the Flinders bar lengthrequired.If these methods are not suitable, the Flinders bar should be omitted until data is acquired.The iron sections of Flinders bar should be continuous and at the top of the tube with the longest section at the top.Wooden spacers are used at the bottom of the tube to achieve such spacing408.Having adjusted the length of Flinders bar, place the spheres on the bracket arms at the best approximate position. If thecompasshasbeen adjusted previously,placethe spheres atthe best position as indicated by theprevious deviationtable.Inthe event the compass has never been adjusted, place the spheres at midposition on the bracket arms.409.The next adjustment is the positioning of the heeling magnet by means ofa properlybalanced dip needle, as discussedinChapterXI.41o. These three adjustments at dockside-Flinders bar, spheres, and heeling magnet -will properly establish the conditionsofmutual induction and shielding on the compass,suchthat a minimum ofproceduresat sea will completethe adjustment.411.Expected errors.Figure 318,"Summary of Compass Errors and Adjustment", lists six different coefficients or types ofdeviationerrorswiththeircausesand corresponding correctors.Adiscussion of thesecoefficientsfollowsTheA error is moregenerallycausedbythemiscalculationof azimuthsorbyphysical misalignments,rather than magneticeffectsofasymmetrical arrangementsofhorizontal softiron.Thus,ifthephysical alignmentsarechecked atdockside,andifcare is exercised in making all calculations, the A error will be insignificant Wherean azimuth or bearing circle is used on astandardcompasstodeterminedeviations,anyobservedAerrorwillbesolelymagneticAerror.Thisresultsfromthefactthatsuchreadingsaretaken onthefaceofthecompass card itselfratherthan atthelubber'slineof thecompass.Onasteering compass wheredeviations are obtained byacomparison of thecompass lubber's linereadingwiththe ship'smagnetic heading as determined by pelorus or gyro, any observed A error may be a combination of magnetic A andmechanical A (misalignment).These facts explain the procedure wherein only mechanical A is corrected on the standardcompass by realignment of the binnacle, and both mechanical A and magnetic A errors are corrected on the steering compassbyrealignmentofthebinnacle(seearticle405).Onthestandard compass,themechanical AerrormaybeisolatedfromthemagneticAerrorbymakingthefollowingobservationssimultaneously(1)Recordacurveofdeviationsbyusinganazimuth (orbearing)circle.AnAerrorfound will besolelymagneticA(2)Recorda curveofdeviations bycomparisonof thecompass lubber's linereading withtheship's magneticheading asdetermined by pelorus or by gyro. Any A error found wilil bea combination of mechanical A and magnetic A.The mechanical A on the standard compass is then found by subtracting theA found in the first instancefrom the total Afound in the second instance,and is corrected byrotatingthebinnacle in theproperdirection bythat amount.It is neitherconvenient nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyromayberemovedbyrotatingthebinnacleintheproperdirectionbythatamount.TheB error results from two different causes, namely:thefore-and-aft permanent magneticfield across thecompass, and aresultant asymmetrical vertical induced effect forward or aft of the compass.The former is corrected by the use offore-and-aft Bmagnets,and the latter is corrected by theuse of the Flinders barforward or aft of the compass.Inasmuch as theFlinders bar setting has been made at dockside, any B error remaining is corrected by the use of fore-and-aft B magnets.The C error has two causes, namely:the athwartship permanent magnetic field across the compass, and a resultantasymmetrical vertical induced effect athwartship of the compass. The former is corrected by the use of athwartship Cmagnets, and the latter by the use of the Flinders bar to port or starboard of the compass; but, inasmuch as this verticalinduced effect isveryrare, theC error is corrected byathwartshipCmagnets only.The D error is due only to induction in the symmetrical arrangements of horizontal soft iron,and requires correction byspheres,generallyathwartshipofthecompassThe existence ofE error of appreciable magnitude is rare, since it is caused by induction in the asymmetrical arrangementsof horizontal soft iron.When this error is appreciable it may be corrected by slewing the spheres, as described in Chapter VI.As has been stated previously,the heeling error is most practically adjusted at dockside witha balanced dip needle.(SeeChapterXI.)412.Asummaryoftheabovediscussion reveals thatcertainerrors are rare,andothershavebeen correctedbyadjustments atdockside.Therefore.formostships,thereremainonlythreeerrorstobecorrected atsea.namelytheB.C,andDerrors.These are corrected by the use of fore-and-aft B magnets, athwartship C magnets, and quadrantal spheres respectively16
16 (2) If the above method is impractical, the Flinders bar length will have to be set approximately by: (a) Using an empirical amount of Flinders bar that has been found correct for other ships of similar structure. (b) Studying the arrangement of masts, stacks, and other vertical structures and estimating the Flinders bar length required. If these methods are not suitable, the Flinders bar should be omitted until data is acquired. The iron sections of Flinders bar should be continuous and at the top of the tube with the longest section at the top. Wooden spacers are used at the bottom of the tube to achieve such spacing. 408. Having adjusted the length of Flinders bar, place the spheres on the bracket arms at the best approximate position. If the compass has been adjusted previously, place the spheres at the best position as indicated by the previous deviation table. In the event the compass has never been adjusted, place the spheres at midposition on the bracket arms. 409. The next adjustment is the positioning of the heeling magnet by means of a properly balanced dip needle, as discussed in Chapter XI. 410. These three adjustments at dockside - Flinders bar, spheres, and heeling magnet - will properly establish the conditions of mutual induction and shielding on the compass, such that a minimum of procedures at sea will complete the adjustment. 411. Expected errors. Figure 318, "Summary of Compass Errors and Adjustment", lists six different coefficients or types of deviation errors with their causes and corresponding correctors. A discussion of these coefficients follows: The A error is more generally caused by the miscalculation of azimuths or by physical misalignments, rather than magnetic effects of asymmetrical arrangements of horizontal soft iron. Thus, if the physical alignments are checked at dockside, and if care is exercised in making all calculations, the A error will be insignificant. Where an azimuth or bearing circle is used on a standard compass to determine deviations, any observed A error will be solely magnetic A error. This results from the fact that such readings are taken on the face of the compass card itself rather than at the lubber's line of the compass. On a steering compass where deviations are obtained by a comparison of the compass lubber's line reading with the ship's magnetic heading as determined by pelorus or gyro, any observed A error may be a combination of magnetic A and mechanical A (misalignment). These facts explain the procedure wherein only mechanical A is corrected on the standard compass by realignment of the binnacle, and both mechanical A and magnetic A errors are corrected on the steering compass by realignment of the binnacle (see article 405). On the standard compass, the mechanical A error may be isolated from the magnetic A error by making the following observations simultaneously: (1) Record a curve of deviations by using an azimuth (or bearing) circle. An A error found will be solely magnetic A. (2) Record a curve of deviations by comparison of the compass lubber's line reading with the ship's magnetic heading as determined by pelorus or by gyro. Any A error found will be a combination of mechanical A and magnetic A. The mechanical A on the standard compass is then found by subtracting the A found in the first instance from the total A found in the second instance, and is corrected by rotating the binnacle in the proper direction by that amount. It is neither convenient nor necessary to isolate the two types of A on the steering compass and all A found by using the pelorus or gyro may be removed by rotating the binnacle in the proper direction by that amount. The B error results from two different causes, namely: the fore-and-aft permanent magnetic field across the compass, and a resultant asymmetrical vertical induced effect forward or aft of the compass. The former is corrected by the use of fore-andaft B magnets, and the latter is corrected by the use of the Flinders bar forward or aft of the compass. Inasmuch as the Flinders bar setting has been made at dockside, any B error remaining is corrected by the use of fore-and-aft B magnets. The C error has two causes, namely: the athwartship permanent magnetic field across the compass, and a resultant asymmetrical vertical induced effect athwartship of the compass. The former is corrected by the use of athwartship C magnets, and the latter by the use of the Flinders bar to port or starboard of the compass; but, inasmuch as this vertical induced effect is very rare, the C error is corrected by athwartship C magnets only. The D error is due only to induction in the symmetrical arrangements of horizontal soft iron, and requires correction by spheres, generally athwartship of the compass. The existence of E error of appreciable magnitude is rare, since it is caused by induction in the asymmetrical arrangements of horizontal soft iron. When this error is appreciable it may be corrected by slewing the spheres, as described in Chapter VI. As has been stated previously, the heeling error is most practically adjusted at dockside with a balanced dip needle. (See Chapter XI.) 412. A summary of the above discussion reveals that certain errors are rare, and others have been corrected by adjustments at dockside. Therefore, for most ships, there remain only three errors to be corrected at sea, namely the B, C, and D errors. These are corrected by the use of fore-and-aft B magnets, athwartship C magnets, and quadrantal spheres respectively