Document RJxpGbw4GrjyGJxORpGjKq1bV

90 ROSS A. MC FARLAND Number of scale markings 38 50 70 100 150 200 300 TABLE 5 Minimum Dial Diameters50 (In inches) Minimum dial diameter for a viewing distance of 20 inches 36 inches 1 1 1 1.43 2.14 2.86 4.29 1 1.28 1.79 2.57 3.86 5.14 7.71 well as speed, the numbering and division markings should be iu the decimal system. If the numbers increase by 10's or 100's, there should be division marks to make the smallest division equal to 1 or 10.81'82 If accuracy in interpolating be tween scale markings is important, a large number of markings would be recom mended, but a great number of graduation marks produces a cluttered appearance and is likely to cause gross errors (see Figure 8). Scales with number intervals of Figure 8. Scale markings. A Beale should never have more markings or finer subdivisions than are needed to fulfill its function. A temperature scale such as the one at the left above could be replaced by the one in the center if only approximate quantitative readings were necessary. The scale at the right would be preferable if the safe operating range was all that was desired. As a general rule the fewest reading errors are made with dials haying the fewest markings. (From Baker amFGreffier?*) five with four divisions between numbered graduation marks are poorly designed, though they can be seen on many machines. Consistency of marking and number- ing..Hhould. appl-v.-.to. all, dials .that must.be used consecutively by the same, operator. On the basis of present knowledge, at least V2 inch linear distance between the numbered division marks is recommended for dials viewed at 30 inches. The height , ," W. R. Gamer and J. W. Gebhard, Machine dials and scales, Machine Design, 21 Nq. 8 98-104 (1949). . !. " M. D. Vernon, Scale and dial reading, Rept. No. If), Med. Research Council Unit in Applied Psychol., Cambridge University, England, June, 1946. HUMAN ENGINEERING AND INDUSTRIAL SAFETY 91 distances is shown in Table 6. numerals or letters, should be . Ak, Vi " ii TABLE 6 Height of Characters and Markings for Dial Design" '$$*'**> i'<t^V'1;ViewingdiBtanoe height (in.) Marking height (in.) Major Intermediate Minor in. or lea: .09 .22 .16 .09 1 ft. 8 in.-3 ft. .17 .40 .28 .17 S ?'* -3-6 ft. es .SS; 6-12 ft. .34 .78 .56 .68 1.57 1.12 .34 .68 u12-20 ft. 1.13 2.63 1.87 1.13 . juh.I-jSSiince most people expect the values of a scale to increase in magnitude when pointer turns clockwise, this relation should be incorporated into the original Ipspdesign if feasible.08 Otherwise, one can expect "reversal" errors to occur; ||ample, on a counterclockwise dial if the pointer is on the graduation 21 graduation 20 is numbered, the worker may read 19 instead of 21. nr W'inter ^->es^n- Experimental research on this topic, although limited, feMtoat--pointer-width--is--a-.relatively. -unimportant- consideration: in one reduction from */s* to Vaa inch caused no loss of legibility. Length, S$S|Ns important. Reading errors will increase if the tip of the pointer is Wm*1/* inh from the scale markings. Nor should the tip extend beyond the v 'W;>V - 0 8cak* general, the whole pointer, rather than only the pointer be of a" cooonnttrraassting color to the dial.84 In those jobs in which a worker lis|r^fil|ltp move a control in co-ordination with thb^pointer on a visual indi- i^^^Strl movement should always be in the same direction as the pointer the discussion of "Direction of Movement" on page 100). < 1 multirevolution Instruments. These displays consist of two or more TM^^^||^ingle dial face. A watch or clock would be a simple example of an 7'^W^higrtj^e--A-single-reading thus--requirel:-ah--&ndersilliidirig--of the ras|and interrelationships of all the pointers. One advantage of this type of ent seems to be that a large range of values can be' compressed into a `lal space. Nevertheless, the numerous reading errors found to be >3tb=.these .instruments , makes .their replacement desirable whenever Altimeters are a complex type of multirevolution instrument with a |e of reading errors. jj^gigtensen, The effect of the staircase scale on dial reading accuracy, Memoran- 'm)}f,MCREXD-69i-lP, Air Materiel Command, USAF, October 1, 1948. if}lts;-Psychological aspects of equipment design, Tech. Rept. No. 5819, Air MatedfijUSAF, August, 1949.