a7- Measuring Instruments

7/29/2019 a7- Measuring Instruments

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Brunel Technical College

CHAPTER 8

MEASURING INSTRUMENTS

Department of Aeronautical Studies . VOL 1- cprtv 1. I J U U L ICHAP 8

lntroduc tio n .k , ,F, J\ . *

1. ELec t r i c c i r c u i ts cover a very wide spectrum of ap pl ic at io ns andfrequencies. It i s high ly. des ira ble th at measurements in e l e c t r i ccircuits can take place no mafi-& to which use the c i r c u i t i s put.Clearly then a range of instruments i s required capable ofmeasuring the c i r c u i t s under, t e s t.

Moving Coil MeterFig. 8.1

C o i l of fine u;re on a nf o r m a r

Soft I r o n C y l i n d e rCons truc ti on Brass Supp o r t f o r Cy 1 i n d e rFig. 8.1 il lu s t ra t es the main constructional d et ai ls of a movingc o i l ins trument. The permanent magnet i s "agedf' to reduce the ratea t which the magnetic fl ux dens ity reduces. The magnet i s f i t t e dwith shaped pole pieces. The pole pie ces ar e shaped so th at a r a d i a lf i e l d i s produced in the a i r gap between the pole pieces and theso f t i ron cylinder. The s o ft ir on cylind er i s provided t o reduce thereluctance of the circuit thereby increasing the magnetic flux density.'Ihe soft iron cylinder i s fixed and held in posi t ion by brasssupports (Ekass being diamagnetic).


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VOL 1 --TSECT 1CHAP 8The c o i l i s wound from f i n e copper wire on an aluminium former.The former i s provided with a sp ind le a t each end and mountedwith jewelled bear ings i n non-magnetic supports a t each end. Theformer i s mounted so t h a t the s o f t iro n cylinder i s in the centreof the co il but the c o i l i s not attached to the cylinder and thec o i l i s fr ee to mve about the pivot i n the air gap between thecylinder and the pole pieces.The pointer i s attached to the spindle a t one end of the I:-ccil. -If the co il rotates, the spindle rot at es and the pointer moveswith the spin dle. The end of th e po int er moves acr oss t he l in e a rscale. Ihe spindles are insulated from the aluminium former andthe ends of the c o i l a r e ternrinated on the spindles, one a t eachend.

*

The coiled springs ar e als o connected to the spindle, one a t eachend. They a re used to prov ide back pressur e, ag ai nst movement ofthe co il and provide el ec tr ic al connection t o th e coi l. The coilsar e contra-wound, so t h at a s one co i l opens out, the ot her tightens;this compensates for temperature variations.Ac t ionThe moving co i l works on the motor prin cip le . When cu rrent passesthrough the coil i t se t s up a magnetic f i e l d tha t d i s t or ts thepermanent magnetic field. The dis tor t io n of the f i e l ds re su lts i nthe motion of the c o i l and the po int er.TorqueThe torque or turming moment produced by th e current in the co i lcan be found:-The force experienced by a current carrying conductor i n a magneticf i e l d (F)

(where B = flux density in t e s l a 1I = Current in ampere 1F = BlI newtons 1 = E f f e c t i v ~ l e n g t h o f c o n d u c t o r )

( . in metres 1Thus there i s a torque about the axis of rotation.Torque = l inear force x radius of action.Hence the torque due to each si d e of the co i l i s F x radius andtotal torque = Force x Diameter'Iherefore: T = BLIND Nm where N i s the number of turnsHowever a s d = the diameter of the co il and length x diameter = area,the formula m y be fi na li se d

T = BINA newton metre ( ~ m ) here A = Area of the coil .?he quantities B, N and A ar e fixed, therefore torque T va r ie s l inea r lywith current I. a


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" VOL 1SECT 1. . CHA P 8-- n l n n i nr A u r v r yle ~f %vexat of the C o U

8. No curren t in the Coi l Curren t i n the c o i ls o f t i r o n c y l i n d e r

/ \

Fig. 8.2 i l lu s tr a te s th e pr inc iple of ac t ion of the moving co i l .In the zero cur ren t condition, there i s a uniform permanent f i e l d i nth e d r gap. When current i s caused to flow through the c o i l i nthe d ir ec ti on ind ic ate d, the f i e l d s e t up by the c ur re nt @istortsthe p ~ z m ~ n ~i e l d . The f i d d below A i s s treng thened whi ls t thef i e l d above A i s weakened. The magnetic l i n e s of fo rc e cause theconductor A to move in an upward direction. S imi la r ly the f ie ld aboveB i s s treng thened w h i l s t the f i e l d below B i s weakened, B i s causedto move downward by th e magnetic l i n e s of fo rc e. Ihe conductors Aand B are pa r t o f the c o i l so the co i l w i l l ro t a t e abou t i t s cent repivots. The magnetic f i e ld round t he c o i l continues t o d is to r t thepermanent f i e l d as i t s moves round i n th e air gap.

9. If the re was no co nt ro lli ng for ce to oppose th e mvement of theco il , even a small cur ren t i n the c o i l would cause the c o i l toro ta te r ig h t round as fa r a s i t could go i.e . give f u l l sca l e de f l ect i on.-. of the po inte r. Control f o r the mvement i s provided by the contra-

wound springs. As the c o i l moves, one spri ng i s compressed and theot he r i s extended. Hooke 's Iaw st a t es t ha t the extension of a bodyi s d i r ec t ly p ropor ti onal t o t he fo rce app li ed , w i th in t he e l a s t i c .limit. This means t h a t the rel at io ns hi p between the ext ensio n andappl ied force i s l i n e a r . The appl ied force i s the torque on the c o i land th i s has a l in ea r re la t ion sh ip wi th the cur ren t . Therefore theextension of the sprin g and the current in t he co i l have a l i n ea rrel atio nsh ip. This means th at th e meter sc al e i s l i n e a r ,

10. The c o i l and the poi nt er w i l l come to r e s t when the torque providedb y t he c u rr e n t i n t he c o i l i s equa lled by th e back pr es su re of thesprings. The la rg er the curre nt, the larger th e torque, and thefurther the poi nte r moves across the scale . The cur re nt required tocause th e movement to take th e po in te r to fu r th es t end of the sca lei s ca l l ed t he FULL SCALE DEFLECTION CURRENT (I f s d).


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VOL 1 -EC T 1Overshoot

ll. I f t he c u rr e nt i n t he c o i l i s l e s s t ha n I f s d., the po in te r w i l ltake up i t s posit io n between 0 and f u l l scale. However, the movementi s designed to move quickly so tends to overshoot th e co rr ec treading, ret urn ing when the back press ure of the sp rin gs gai nscon trol. This could le ad to os ci l la t io n of the po inte r and a timedelay in th e meter ta king up i t s f i na l posi ti on. To avoid th i sD W I N G -1s provided.I--D a m p i n g i s achieved by th e presence of the alurrrinium former forthe co i l , !he aluminium i s a conductor that i s moving in a magneticf i e l d and Ewaday's Law s ta t e s t ha t whenever there i s r e l a t i v emotion between a conductor and magnetic flux, an .e.m.f. w i l l beinduced i n the conductor. This induced e.m.f. w i l l g i v e r i s e t oent s and the direc tion of the eddy cur ren ts w i l l be sucheE f d d s e t up by them w i l l oppose We m tion causing them( ~ e n z ' sLaw). Therefore the eddy cu rr en ts i n th e aluminium formerw i l l slow the movement of the pointer and prevent i t from osci l lat ing.

Fig. 8.3

13. A s can be seen from Fig . 8.3 th e movement can be over damped andthe meter could then take longer to come to r e s t a t the corre ctreading than with an undamped movement. The , o m mount ofd a m p i n g i s shown by the curve lab elle d 'k ri t i c al ly dampedrr; i nthis case the meter comes quickly t o r e s t a t the f i n a l reading.When th e poi nt er comes to r e s t without o sc il la ti ng the lnovement i s-said to be DEAD BEAT.

\----

Parallax Errora. Many instruments a r e provided w i t h a s t r ip of mirror c lose t o thescale.. This i s provided to reduce PAR+= error . Paral lax err or

i s the error produced by reading the meter from an angle so tha t theobserver thinks the meter pointer i s a t a dif fer ent posi t io n on thesca l e t o the actua l posi t ion. Paral lax erro r i s avoided i f themeter i s read from dir ec t ly i n f r on t or above the scale. The positionmay be checked by the observer posi tio ning himself so that he cannotsee the image of the pointer in the mirror. This means that theimage of the poi nt er i s hidden by the po inte r i t s e l f and the observerms t be seeing the sc ale correct ly.

I


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VOL 1SECT 1CHAP 8

Meter PositionAnother cause of e r ro r in reading i s the po sit i on of the meter.I f the meter i s ca l i b r a t ed in the hor izontal posi t ion, i t mst beplaced i n t hat po si t i on before readings ar e taken. Simil-arly i fthe meter i s c a l i b ra t e d i n t he v e r t i c a l p os it io n, i t should beread with the meter i n the ve r t i ca l pos i t ion , Errors can be causeddue to the ef fe ct of gra vi ty act ing on the pointer ,Summary of Cons tr uc t io n and Ac t io n of the moving C oi l Meter -a ) fig ne t: Aged to reduce va ri at io n in B. =aped po le pie ceswhich together with the so ft ir on cylinder provide a high density,uniform, r a d i a l magnetic fi el d . ,b) Springs: Provide the con tro llin g for ce th a t opposes the torqueproduced by the cu rre nt, Restores th e movement t o zero po sit io nwhen c ur re nt i s removed. Provides connection from the terminal oft he meter t o t he c o i l i t s e l f , Linear relationship between thecurrent and torque and dis to rt i on of the springs provide theinst rument wi th a l inear scale ,c ) Aluminium Former : Provides damping fo r the movement (by meansof eddy currents) and renders the movement dead-beat,Features of the moving Coil &tera) A v e ry v e r s a t i l e ins trmment, i t can be adapted to indicate awide range of variables,b) It i s robust and accurate f or most genera l purposes.c ) It has a l inear sca le . It i s not a f fec ted by externa l f i e ld sbecause the lkorkingfrf i e l d i s concentrated in the airgap betweenpole pieces and sof t i ron cyl inder ,d) It maintains accuracy with passage of t im e , due t o using anaged magnet,Shunts and M i l i ~ l i e r sThe ba sic moving c o i l movement i s , in general, f a r too s ens i t i ve fo ruse in p r a c t i c a l c i r c u i t s , Typical val ues of F.S,D, a r e cu rr en t 100 pAand voltage 0.005 volts. The range of use may be extended by theuse of shunts, fo r high er curre nt F-S-D, or rrmltiplie rs for h ighervoltage F.S.D.lbving Coil lkt e r a s an AmmeterThe instrument must be connected i n se r i es wi th the ci r cu i t , Themaximum cu rr en t which may pass through the instru men t i s t ha t cu r ren twhich w i l l give F.S.D. If t h e c i r c u i t i s carrying more currentthan F.S.D. the n th e exces s curr en t must bypass the meter movementby a pa ra l le l pa th or $T& ?he meter however, w i l l normally beca l i b r a t ed t o i n d i c a te t h e t o t a l c i r c u i t c u rr en t,


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VOL 1 '7SET 1To Determine the Shunt Res istance- - -F'rom the F.S.D. current and the instrumentresistance, obtain the F.S .D. voltage.In par a l le l c i rcu i t s , the vol tage i scommon to both arms. ' \Shunt Current Is = 5 - Im

n- S\

e ?C I n . I T - I Rm = Mete r ~es i s t ance?he shunt, when connected in the cir cu it, shoudir ec t l y to the main circuit conductorsand the should then b e c ~ n n e c t g d ,,

I t-o the shunt.i e t , n 6he event of any accidentalopen circuit. 4Id always be connected

Meter as a Voltmeter21. The instrum ent must be connected acro ss the vo ltage to be measured.I n such a s it ua ti on , the c ur rent which would flow, would be

determined by the c ir c u i t voltage and the meter res ista nce . Thiscur ren t might we ll be f a r great er than the F.S.D. cu rr en t of themeter. Thus the current m st be l imi ted by a se r ie s res i s to r or

To Determine the M ilt ip lie r Resistance22. ?he t ot al resistance RT = mult ip l ie r res is tan ce % + meter resistance R

to ta l current = $eSeDe - -R~Hence RT = % + m =

%.s.D.

Note th a t shunts and multi p1 ier s~ :sho uld e made from low d-(temperature coefficient of resistance) material. (see Chapter 2Para. 2.18). 1


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VOL 1S C T 1CHAP 8

Numerical &ample23. Amoving c o i l meter has a res is tance of 18 ohms and a f u l l s c d edeflect ion current of 2 mA. H b i may i t be converted to read:-

(a) 0-20 mA?(b) 0-2V?-1: 1 ~ - - -- . -(a) - -\

Rshunt res is tance required Rs = IFSD mIT - I

= 2 ohms-vk l t i p l i e r r e s is ta n ce r e qu ir ed RM = - - Rm%SD

= 982 ohms-i rcuit Loadinq24. Whenever a moving c o i l instrument, tog ether wit h i t s shunt orraiLtipl ier , i s connected to a c ir cu it , the total c i r c u i t r e s is t an c ei s chagged. This means that the current and voltage measured willbe s l i g h tly d i f fe re n t f ~ mhe tr ue values, due t o this loading--ffect .


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VOL 1 "\SECT 1CHAP 8

25. As an ammeter i s ins erted i n se ri es with the te s t c i r c u i t; i t sres ist an ce (mainly the shunt res ist an ce ) should be a s low a spossible.

26. A s a voltmeter i s inser ted in paral le l wi th the te s t c i r c u i t ,i t s resis tanc e (mainly the mul tiplie r resista nce ) should be ashigh as possible.Q. When using a mv ing c o i l instrument gr ea t car e should be taken toensure tha t loading ef fe ct s are negligible, otherwise serious e rrors

in readings can result.Ohms per Volt

28. When used a s a voltmetey, the degree of c i r c u i t loading i s ind icatedby the fbhms per v o l t f f+due . Consider two mvi ng c o i l meters:. --\(a) Meter resistance (including multiplier) = 100 kohmsF.S.D. voltage = 100 vOhms/volt = 100 lo3 = 1000100(b ) h t e r r e s is t ance ( in clu din g m l t i p l i e r ) = 2 MohmsF.S.D. voltage = 10)v0hms/vol t = 2 x 1 0 = X),ooo .loo&ter (b) w i l l have a smaller loading effect than meter (a)'.Ohms/volt i s a ls o an indic atio n of meter sen sit ivit y. The ohms/voltvalue i s sometimes called the

29. ( An al te rn at iv e method of c alc ula ting the ohms/volt va lue i s to take\ the re ci pr oc al of th e F.S.D. cu rren t e.g.I 13 (a) If. F.S.D. current = 1mA, ohms/volt = - = lo3 O ~ / Vii 103

I1 (b) If F.S.D. current = 50 u~,ohms/volt = - -6 = 20k ohms/VL 50 x 10

30. It i s us efu l to be a ble 0 o use a moving c o il instrument to measureres is tance d i rect ly . This i s done by including a b at t ery i n ser ieswith the movement, together with a protect ive resistor R, (whichl l in r i t s the maximum poss ible current to a saf e value) and a varia bler e s i s t o r Rv, to s e t a zero reading. Notice th at sinc e i t i sli

necessary t o connect the ba tte ry +ve t o the meter + ve, and th a t themeter -ve w i l l alr ea dy be connected to an ex te rn al te rmin al nominated-ve, the a c ~ ~ li r cu i t w i l l fin d +ve voltage on the -ve terminaland -ve voltage',on the +ve terminal.


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VOL 1SECT 1) 4. CHAP 8-

L Method of Use.31. .When the unknown res is tance i s zero, i.e . the ohmmeter terminalsare short-circuited, % ' i s adjusted so that the met.er showsscal e defl ect ion. When the unhown re si st an ce i s i n f i n i t e , -

e the terminals ar e bpencircuited, the meter cu rre nt i s zero and thepoicter shows zero deflection. Any f i n i te value o f r es i s tance w i l lcause the pointer t o take up a po si ti on between the se two extremes,and the scale may be ca lib rat ed i n ohms.3 2 , Note th at f u l l sca le d efl ec tio n corresponds t o zero ohms and zerodefl ect ion corresponds to i n fi n i t e ohms. 'Ihe ommeter scale is non-li n e a r because of the hyperbolic re la ionship be tween cu rre nt andresistance- - see Figs. 8.8 and 8.9. The scale i s cramped a t thehigh res2stance end. t\3\C 3 < C < - c f : t -- - 1 . ;

Fig 8.81

Measurement of ac with a moving Coil Meter33 . If a.c. was appl ie d to a moving co i l meter, the movement would attemptto move across the sc al e on the po si ti ve half-c ycl e and rev ers edirection on the negative half cycle. I the wvement would- take up an qer_ilg.e po si tion . Bu t the average position for anysymmetrical a.c. waveform (e.g. a sine wave) i s zero, therefore, themeter would read zero. So th e a.c. must be converted to d.c. byrec 5f ic a ion. t . .


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VOL 1 ,,SECT 1CHAP 8

34. A s i q l e form of r ec t i f i c a t i o n i s obtained by connecting a diodei n seri es with tge meter as in ' Fig. 8.10.

Fig. 8.10. -- -- - -A diode i s a "one-way cu rr en t f devide i. . current w i l l only flow

\ through i t in one dir ec tio n. Taking conventional cu rr en t fl&,current w i l l flow i n the dir ec tio n of the arrow head. The r e s u l twould be as shown in Kg. 8 . n i .e . pos i t ive '@ulsesff f currentthrough the meter. The meter would take up an average posit;ion.'Ihe average value of one half cy cle of a s in e wave, taken over awhole cycle, iso peak .value. Therefore this method i s notvery ef fic ie nt , the meter will only take up a position that i s0.319 of the peak value of c urr en t passing though i t . A m r eef f i c i en t mthod i s to employ f'tiLl wave rectification.

35. Bbst mving c o i l meters, connected to measure a.c., a r e ca lib ra te din- values. Note t ha t some meters ar e ca li br a te d f o r s ine waveinputs only.

Fig. 8 . n

Iherm-Junc tio n MeterJ:ss;m ,\aC

36. Consider a c i r . formed by y o wires x and y, of d% fT~en%materials, as in Fig. 8.12.

Fig. 8.12.


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VOL 1S Z T 1CHAP 8

A small e.m.f. may be measured a t each ofthe value of the e.m.fs on the jIf the junctions ar e a t the same temperatequal and opposite and no current w i l l flow. If the temperatureof J1, say, i s higher than the temperabe unequal and a current w i l l flow. ?he value of the current i san in di ca tio n of the temperature di ffe ren ce between the junctions andprovides a method of temperature measurement.?he thermocouple p ri nci p le may be used to measure cur re nt . .Thecurrent to be measured heats one junction, w h i l s t the other junctionremains a t room tempera ture (see Fig. 8.13). The heating elementi s oft en a sho rt length of wire, bonded to th e junction, bu tinsu lated from i t . 'Ihe hot junction i s usually munted in a smallplug-in interchangeab le sub assembly, with i t s calibrat ion rangemarked. 'he se p lug- in elements a re e a s i ly damaged byc m t . The cu rren t flowing in the thermocouple, produces a read ingin th e meter shown i n Fig. 8.13. -meter reading may b e ca li br at edto show the value of the unknown curr en t. -- - - L _- -_ --

i1

$:

ct - - / 'Fig 8.13.

Features of ihermo-Junction & te r38. (a) Operates on a.c. or doc.

(b) Can be used a t rad io frequ encie s(c) Slow response due to thermal capacity of junction(d ) Can be very accurate.kasurement of Very Hi gh and Very Low R es is ta nc es

39. 'Ihe ohmmeter i s not sui table for measuring res is tances less thanabout 0.5 ohms or larger than about 2 megohms. Smaller resistancesare usually measured by a bonding test er , higher re si st an ce s by amegger. Both instrum ents employ the pr in ci p le of t he ratiome ter.The Ra iome te r

40. A ratiometer i s ba si ca ll y a moving c o i l instrument b ut using twoco ils , one to measure curre nt, the othe r voltage. Both c o i l s a r emunted on the same axis but th ei r torques are i n opposition.The final posit ion of the co ils i s a measure of the r a t i o of voltageto current i .e. res is tances .


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VOL 1SECT 1CHAP 8

42. If the unknown resistance i s low, the cu rr en t through i t i sr e l a t i v e l y l a r g e . !h e c u r r e nt t h m u g h ' th e p a r a l l e l v o lt a ge c o i l( c i r c u i t 1) i s negl ig ib le but the voltage across the s er ie s current- -7c o i l ( c i r c u i t 2 ) i s not negl ig ib le . Thus in t h i s case C i rcu i t 1i s preferred. 4

. 'Ihe coils, marked V and A, mar be connected ei th er a s i nCircui t 1 or Circui t 2 i n Fig. 8.a. In Circui t 1, the currentc o i l measures th e cu rre nt through the unknown re si st an ce p lu s thecurrent through the voltage coil. In c i r c u i t 2, t he vo l t age co i lmeasures the voltage across the current c o i l i n addi t io n to thevolta ge acro ss th e unknown re sis tan ce s.

c,Rc~tr Fig. 8 . a .

43. If the unknown resistance i s high, the cu rr en t through i t i sre la t ive ly low. ?he current through the pa ra l l e l vol tage c o i l i sno t neg l igib le ( c i r cu i t 1 ) bu t the vol tage ac ross the se r i e s cu r ren tco i l (C i rcu i t 2 ) i s negl ig ib le . ?bus in this case Circui t 2i s preferred.

,

BONDING TESTER

1 '

Fig. 8.15

&


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X - Gor~trolCurrent) Col l- VOL 1SECT 1CHAP 8

Fig. 8.17 SCALEREADS0 to 0.1 ohms in 0.002The -Bonding Tester Divisions.

44. ?he prin cip le i s shown i n Figs 8.16 and 8.17. As only small r e s i s t -ances a re to be measured a NIFE c e l l b a t t er y i s used as the powersupply and the co il s ar e connected as i n Circu it 1of Fig. 8.16.Rro t e s t leads a re used. A 60 f t . long r rs ta t i c" ead and a 6 f t .long lead. 2he 60 f t . s ta t i c lead i s connected to a fixed terminalpo int , ensuring a good contact. The 6 f t . te s t lead i s onlypressed on to the metal casing or racking and care must be taken toensure a good contact.

45. Fig. 8.18 represents the ci rc ui t when the t es t leads only areshort c ircui ted.

Fig. 8.18The current through Coils A and B i s th e same, bu t t he torque producedby coil B i s arranged to be gr ea ter than the torque produced by c o i lA. Consequently a f u l l scale reading is .ob tain ed - high resistanceend.


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VOL 1 -ECT 1CHAP 8

When a l l l e ads a r e sho r t c i r cu i t ed i . e . both s t a t i c and t e s tleads, Coil B i s s k r t e d . Consequently the po int er moves togive a zero reading.

Fig. 8.20In the case of a f a u l t y bond as shown i n Fig. 8.20, Coil Bw i l l no longe r be sho rted. Some current w i l l flow through Coil Band the poin ter w i l l take up an intermediate position showing thevalue of the bond resistance.

48. The megger uses the ratiometer principle to measure high values ofres i s tance . The two c o i l s ar e connected a s i n C ir c ui t 2 of Fig. 8.I-4,a s shown in Fig. 8.21. A s high resistances are to be measured ahigh supply voltage i s required.. This i s obtained by a handwound Ygenerator which also avoids the need for an on/off switch. Contactres is ta nce s ar e neg l ig ible compared with the resis tan ce under tes t .


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VOL 1SECT 1--

f i e

bc;'CLa c c jccrl. Fig. 8.21Ac t on

49. I f the te s t terminals are open-circui ted, the cu rren t throughthe de f l ec t i on co i l i s zero. "he generator current f lows ent i re lythrough the v o l t ~ g e o r con t ro l ) c o i l and the po in t e r i s arrangedto i nd i ca t e i n f i n i t y on the s ca l e . I f t he t e s t t ermina ls a r e sho r t -c ir cui ted , curre nt flows through both coi ls and the po inte r i s -arranged to in dic ate sca le zero. A f i n i t e value of r e s i s t a nce.between th e te s t te rmina ls wi l lc ause the po in te r to take up anintermediate posi t ion.Preca uti ons t o be observed when us ing th e Megger

50. 1. h s u r e tha t th e voltage output from the megger genera tor doesno t exceed the normal working va lue of th e component under t e s t .2. Disconnect th e component under t e s t be fo re us ing th e megger.3. Ensure that good contact i s made between the ends of the.component and th e le ad s of the mg ger .4. &sure th a t the lea ds ark i n good condition.5. Ensure th at th er e ar e no moisture beads between th e ter min alson the case.6 . When making measurements on outside cables and aerials, allowancemust be made f o r weather co nd itio ns .


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VOL 1 .nSECT 1CHAP 8The Digital Multimeter

51. Measurements ar e in cr ea si ng ly being ca rr ie d ou t by means ofd i g i t a l c i r c u i t r y . I n analogue type meters th e accuracy i s l imi tedby the accuracy of the ind ica tin g instrument used, ty pi ca ll y 1%.The accuracy of d i g i t a l equipment depends on i t s ci rc ui ty . Evena simple d ig it a l meter giv es accuracies gre ate r than 0.1%

2 . Dig ita l meters ar e simple, more accurate, ea si er to read thanconventional analogue meters. Furthermore t he measurements canbe retained. They are capable of being used by all grades of-personnel.53. Several types of d i g i ta l multimeter exist, bu t even th e most ba si chave input impedances ,of 2 Megohms or greater f o r doc-. , e n d y

1Pbhm or gr ea te r f o r a. c. The frequency range of o pe ra ti on fo r -.c. va ri es from approximately 0 - 20 KHz upwards depending on thele ve l of sophis tic at io n of the instrument. Values of inputimpedances and frequency ranges sh ou ld be checked befor e us ing themeter.

'he Hot Wire Meter54. ?his type of instrum ent i s used to measureRF current. A s the cur e n t flows through5the wire i t ge t s ho t (I R) and expands.The inc rea sed len gth i s taken up, v i a thetension wire and f ib re, by the c o i l spring.

A s the fibre moves i t turns the spindlearound which the fibre i s wrapped and thepointer moves. e';&s.Fig. 8.22. -

5 . ?he basic response i s squa re law bu t ca re fu l arrangement of thepos i t ions of the jo in ts a t A and B can c on ve rt t h e s c d e t oapproximately linear. The grub screw pr es se s on the l e a f spri ngwhich takes up the sla ck i n the ho t wire. The instrument workson a.c. or d.c. It has negligible reactance which makes i t . 'par t i cu la r ly su i t ab le fo r R.F.

56. These types of meter f a l l in to two groups, the a t tr a c ti o n andrepulsion types.They are cheap and sturdy.Ihe ope rate on e it he r a.c. or d.c.


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57. Attr acti on Type Pbving Iron k t e r .Fig. 8.23

VOL 1SECT 1-I',HAATTRACTION TYPE?he current flows throughCoil C. This c rea tes a- magnet ic f ie ld , a t t rac t ingVthethe---e--- .

eccentr ic Disc.needle movesa t r a c t io n t o. .-

I., hence3I and tne

58. Repulsion Type Moving Iron h t e rFig. 8.27

Repulsion Type:When th e c u rr en t flows i n theco i l , both p ieces of i ron .become s im il a r ly magne t i s e dand hence repel each other.


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VOL 1SECT 1PTT n n o

N..dtir an g e Ins trument60. This i s a combination of ammeter (shunts), voltmeter (multipliers)and ohmmeter,

'Ihe instsument role i s changed by variation of switches and notemust be paid to t he importance of low switch con tact res is ta nc e f o rnnc i n 1 - v m n -nn- fin m i 7 +VIIU -L 0-0 a W cI.A VcI. WJ. A(-Uwiper action, - - -

Avo 8 Mk.3 claims accuracy of 2% FSD d.c. voltage1%SD dac e cu r ren t

Notes on Reverse :Use meter fa ce upwards, Lead re si st ance . 0 2 ~ p e rpa i r .'Ihermis to r temperature compensation inc luded.DOC. CURRENT & VOLTAGE

A l l DOC. volt age ranges a re 20,OOO ohms per vo lt .( 5 ~or f u l l sca le def lec t ion)On DOC* curr ent ranges a po te nt ia l drop of approximately O.SVoccurs a t the terIl linals a t f u l l load except for the 50 p A rangewhich absorbs 125 mV,

AoCo ClJRRENT & VOLTAGEVoltage ranges f o r ~ S V / ~ O Vpwards a re 2,000 ohms pe r vo l t , (0.51nAf.s.d.) the. ~ . S V / ~ . O Vange i s 100 ohms p er v o l t (1 0 m ~ .s.d.)and the 10V range i s 1,000 ohms per volt (1mA f.s.d.)Variation due to frequency from 15 Hz to 15 I(Hz on A. C. ur rentranges and A.C. voltage ranges from 10V t o 250V does not exceed2% of in di ca tio n,On the 2.5V/3.0V range, i t does no t exceed 4$,vo lt drop on A.C.current ranges does not exceed 3 9 mV.

I


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VOL 1SECT 1CHAP 8

Ranges are: D.C. 10A 1A lOOmA lOmA imA 250 uA 50 uAlO0OV soov 250v l0OV 25v 10v 2.5 v

BRUNEL TECHNICAL COLLEGE,BRISTOLJrrrJE 1979 - 19 -


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a7- Measuring Instruments

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