Nhiet Dien Tro

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Nhiet Dien Tro

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<ul><li>1.III. NHIT IN TR 1. Nguyn l chung Nhit in tr l dng c o nhit da trn hiu ng bin thin in tr sut ca cht bn dn, bn kim loi hoc kim loi khi nhit thay i. Nhit tng th in tr tng hoc nhit gim th in tr gim. u im ca nhit k in tr l chnh xc cao, s ch n nh, c th t ghi v truyn kt qu i xa. Trong trng hp tng qut, s thay i in tr theo nhit c dng: R(T) = R0.F(T T0) R0 : l in tr nhit T0 F: l hm c trng cho vt liu v F=1 khi T=T0 Hin nay thng s dng ba loi in tr o nhit l: in tr kim loi, in tr silic v in tr ch to bng hn hp cc oxyt bn dn. Trng hp in tr kim loi, hm trn c dng: R(T) = R0.(1+AT+BT2+CT3) Trong nhit T o bng 0C, T0 =00C v A, B, C l cc h s thc nghim </li></ul> <p>2. Cc h s c xc nh chnh xc bng thc nghim khi o nhng nhit bit trc. Khi bit gi tr cc h s, t gi tr ca R ngi ta xc nh c nhit cn o. 3. 2. Nhit k in tr kim loi a. Vt liu Yu cu chung i vi vt liu lm in tr: - C in tr sut ln in tr ban u R0 ln m kch thc nhit kvn nh. - H s nhit in tr ca n tt nht l lun lun khng i du, khng trit tiu. - C bn c, ho nhit lm vic. - D gia cng v c kh nng thay ln. Cc cm bin nhit thng c chto bng Pt v Ni. Ngoi ra cn dng Cu, W. - Platin : + C th ch to vi tinh khit rt cao (99,999%) do tng chnh xc ca cc tnh cht in. + C tnh tr v mt ho hc v tnh n nh cu trc tinh th cao do m bo tnh n nh cao v cc c tnh dn in trong qu trnh s dng. + H s nhit in tr 00C bng 3,9.10-3/0C. + in tr 1000C ln gp 1,385 ln so vi 00C. + Di nhit lm vic kh rng t-2000C 10000C. 4. - Nikel: + C nhy nhit cao, bng 4,7.10-3/0C. + in tr 1000C ln gp 1,617 ln so vi 00C. + D b oxy ho khi nhit cao lm gim tnh n nh. + Di nhit lm vic thp hn 2500C. - ng c sdng trong mt s trng hp nh tuyn tnh cao ca in tr theo nhit .Tuy nhin, hot tnh ho hc ca ng cao nn nhit lm vic thng khng vt qu 1800C. in tr sut ca ng nh, do chto in trc in tr ln phi tng chiu di dy lm tng kch thc in tr. - Wonfram c nhy nhit v tuyn tnh cao hn platin, c thlm vic nhit cao hn. Wonfram c thchto dng si rt mnh nn c thchto c cc in trcao vi kch thc nh. Tuy nhin, ng sut d sau khi ko si kh btrit tiu hon ton bng cch do gim tnh n nh ca in tr. 5. b. Cu to nhit in tr - t rnh s lm nng u o dng in chy qua in tr thng gii hn gi tr mt vi mA v in tr c nhy nhit cao th in tr phi c gi tr ln. - Mun vy phi gim tit din dy hoc tng chiu di dy. Tuy nhin khi gim tit din dy bn li thp, dy in tr d b t, vic tng chiu di dy li lm tng kch thc in tr. hp l ngi ta thng chn in tr R 00C c gi tr vo khong 100, khi vi in tr platin s c ng knh dy c vi m v chiu di khong 10cm, sau khi qun li s nhn c nhit k c chiu di c 1cm. Cc sn phm thng mi thng c in tr 00C l 50, 500v 1000, cc in tr ln thng c dng o di nhit thp. - Nhit k cng nghip: s dng cho mc ch cng nghip, cc nhit k phi c v bc tt chng c va chm mnh v rung ng, in tr kim loi c cun v bao bc trong thu tinh hoc gm v t trong v bo v bng thp. Hnh minh ha nhit k cng nghip dng in tr Pt. 6. - Nhit k b mt: Nhit k b mt dng o nhit trn b mt ca vt rn. Chng thng c ch to bng phng php quang ho v sdng vt liu lm in trl Ni, Fe-Ni hoc Pt. Chiu dy lp kim loi cvi m v kch thc nhit k c 1cm2 . Hnh minh ha nhit k b mt c trng chnh ca nhit k b mt: - nhy nhit : ~5.10-3/0C i vi trng hp Ni v Fe-Ni ~4.10-3/0C i vi trng hp Pt. - Di nhit s dng: -1950C 2600C i vi Ni v Fe-Ni. -2600C 14000C i vi Pt. Khi s dng nhit k b mt cn c bit lu n nh hng bin dng ca b mt o. 1 2 1. in tr 2. Tm vt liu cch in 7. 3. Nhit k in tr Silic Silic tinh khit hoc n tinh th Silic c h s nhit in m Tuy nhin khi c kch tp loi n th trong khong nhit thp chng li c h s nhit in tr dng H s nhit in tr ~0,7%/ C 25 C. Phn t cm nhn nhit c ch to c kch thc 500*500*240 m c m kim loi mt pha, pha kia l b mt tip xc. 8. Silic tinh khit hoc n tinh th Silic c h s nhit in m Tuy nhin khi c kch tp loi n th trong khong nhit thp chng li c h s nhit in tr dng H s nhit in tr ~0,7%/ C 25 C. Phn t cm nhn nhit c ch to c kch thc 500*500*240 m c m kim loi mt pha, pha kia l b mt tip xc. Trong di nhit lm vic t -55 C n 200 C, gi tr gn ng in tr ca cm bin nhit theo cng thc: Trong R0 v T0 l in tr v nhit im chun. S thay i nhit ca in tr l tng i nh nn c th tuyn tnh ha bng cch mc thm mt in tr ph. 2 000 1 TTBTTARRT 9. 4. Nhit k in tr oxyt bn dn Hn hp bt oxit c trn theo t l thch hp c nn nh dng v thu kt nhit ~1000 C. Cc dy ni kim loi c hn ti hai im trn b mt v c ph bng mt lp kim loi. 1 2 3 Cu to 1. V bc 2. in tr 3. Dy ni 10. S ph thuc ca in tr ca nhit in tr vo nhit theo biu thc: H s nhit in tr: Gn ng: v Vi B = 3.000 - 5.000K. Nhit in tr c nhy nhit rt cao nn dng pht hin nhng bin thin nhit rt nh c 10-4 n 10-3K. Kch thc cm bin nh o nhit ti tng im. Nhit dung cm bin nh thi gian hi p nh. Di nhit ca cm bin nhit in tr t 0C n 300 C 11. 5. Mt s hnh nh ca nhit k in tr </p>