CẢM BIẾN NHIỆT ĐỘ

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<p>CM BIN NHIT (TEMPRATURE SENSOR)</p> <p>1. Khi nim chung 1.1. Khi nim cm bin nhit</p> <p>Cm bin nhit l cc vt liu kim loi hoc l bn dn c gi tr in tr thay i theo nhit hoc dng in phn cc ngc cc lp tip gip thay i khi b tc dng bi nhit . -Nhit c vai tr quyt nh nhiu tnh cht cu vt cht.Mt trong nhng c im tc ng ca nhit l lm thay i 1 cch lin tc cc i lng chu nh hng ca n. Do trong khoa hc, cng nghip v i snghng ngy vic o lng v khng ch nhit l mt iu rt cn thit. o lng nhit v t kt qu o lng c th khng ch nhit ta dng ti cm bin nhit. cm bin nhit c dng ph bin trong dn dng v cng nghip vi mc ch khng ch v diu chnh nhit . 1.2. Thang o nhit Ta c 3 thang o: --Thang Kenvin (0K). Trong thang ny nhit ca im cn bng 3 trng thi nc-nc -hi nc c gn tr s 273,150K. --Thang Cenlsius-thang bch phn (0C). T(0C)=T(0K) - 273,15 --Thang Farenheit (0F). T(0F).=1,8T(0C) + 32 2. Nhit in tr kim loi</p> <p>Nhit in tr kim loi l 1 loi cm bin s dng cc kim loi: Platin, Niken v 1 s oxit kim loi c in tr thay i theo nhit m n b tc ng. 2.1. Nhit in tr Platin (Pt). a. c im. Nhit in tr platin c ch to t kim loi platin kt hp vi 1 s hp cht kim loi khc tng in tr thanh o nhit hoc tng bn. y l loi cm bin o nhit rt thng dng trong cng nghip, c di nhit o c t -2000C n 8500C. Quan h gia in tr ca in tr platin vi nhit mi trng o xc nh theo a thc: +Di o t -2000C n 00C R(t)=R0.(1 + A.t + B.t2 + C.[t-1000C].t3) +Di o t 00C n 8500C R(t)=R0.(1 + A.t + B.t2) Trong A,B,C l cc h s thc nghim A = 3.9082.10-3 B = -5,082. 10-7 C = -4,2735. 10-12 R0 : nhit in tr ca in tr Pt 00C. Trong khong nhit trn 00C nhit c tnh theo s thay i ca in tr Pt nh sau: t = (-R0.A + [(R0.A)2 4.R0.B.(R0 R)]1/2)/(2. R0.B) *u &amp; nhc im: -u im:</p> <p>+Di o rng. + chnh xc cao. +ng dng ph bin trong cc l nhit. -Nhc im: Do phi s dng hp cht platin trong mi trng nhit cao nn tnh cht vt l ca n s b thay i theo thi gian v vy u o phi c thay mi nh k. b. Cc nhit in tr Pt thc t (hnh nh minh ha) Trong thc t ngi ta ch to cc nhit in tr: Pt-100, Pt-500, Pt-1000.. Nhit in tr Pt-100 Pt-500 Pt-1000 2.2. Nhit in tr Niken -c ch to t kim loi chnh l Niken theo tiu chun DIN 43760. Thng trong thc t ch ta Ni-100 c di o t -600C n 2500C. -Tr s in tr c trng 00C l R0 = 100. Quan h gia nhitd din tr v mi trng o: R(t) = R0.(1 + A.t + B.t2 + C.t4 + D.t6) Trong : A =0,5845.10-2 B = 0,665.10-2 R0 100 500 1000 S thay i nhit 0,4/0K 2/0K 04/0K</p> <p>C = 2,805. 10-4 D = 2,111. 10-7 -c im: Ni-100 c dung sai khng ln nn lc no ta cng c th thay th cci khc c cng cp dung sai m ko cn chun nh li. Trong thc t Ni-100 c ng dng trong khng ch nhit ca iu ha. 2.3. K thut ni dy. in tr ca nhit in tr thay i theo nhit ,vi mt dng in khng i qua nhit in tr ta c th o c V = R.I nh vy khi R thay i dn ti V thy i. cm bin khng b nng ln khi o th dng in cn phi nh khong 1mA. Vi Pt-100 1000C ta c U = 0,1V. in th ny cn a ti my o vi sai s thp nhp. thc chin iu ny l phi c cc k thut ni dy o. Ta c 4 k thut. a. K thut ni 2 dy</p> <p>Phng php ny t s dng do sai d vn ln. b. K thut 3 dy</p> <p>K thut ny c dng rt ph bin. K thut ny yu cu 3 dy o c cng tr s k thut v cng nhit . c. K thut 4 dy</p> <p>K thut ny cho kt qu tt nht. d. K thut 2 dy vi b bin i tn hiu o Vn dng 2 dy o nhng khng b sai s bng ch thay i tn hiu o. B bin i bin i tn hiu ca cm bin thnh 1 dng ienj chun tuyn tnh vi nhit v cng t 4mA n 20mA. Dng in nui cho b bin i c ti qua hai dy o vi cng khong 4mA. 2.4. Nhit in tr bn dn:(Thermistor): Lm t hp cht bt oxit nn nh hnh,nung nhit 10000C v c trng 2 lp kim loi 2 mt kt ni vi h thng mch bn ngoi. MgO ; MgAl2O4 ; Mn2O3 ; Fe3O4. u im: c nhy nhit cao,di o t 50C n 150C. Nhc im ca in tr bn dn l c tnh nhit c phi tuyn cao, kh khc . Khng c s dng lm cm bin o nhit m dng cnh bo v khng ch nhit . 2.5. Cu trc ca nhit in tr kim loi a. Nhit in tr v gm bc kim loi</p> <p>Si Pt hoc Ni c qun bn trong ng gm s bn ngoi bc bng kim loi chu nhit cao tng bn c hc. Cu trc ny cho di o t -200 0C n 8000C. b. Nhit in tr v thy tinh Si Pt c qun bn trong 1 ng thy tinh. Loi cm bin ny c nhy v bn c hc cao cho di o t -2000C n 4000C dng trong mi trng ha cht v n mn cao. c. Nhit in tr v nha Dy Pt c ng knh khong 30m c dn kn gia 2 lp nha poliamit. ng dng o nhit b mt cc ng hoc cun dy bin th cao p. Di o t -800C n 2300C. d. Nhit in tr k thut mng mng. Platin c phun ph bng phng php phun ion hoc bc hi chn khng ln mt nn nhm oxit vi dy khong 1m. Sau lp Pt s c to thnh 1 dng gp khc bng phng php quang khc hay bng tia laze. Cui cng c ph ln bng 1 lp thy tinh. Di o ca loi ny t -500C n 4000C. u im: cc nhit in tr k thut mng mng c thi gian hi p rt b v quan tnh nhit b. Vi k thut mng mng nhit in tr c s n nh lu di,d thay th v di o rng. 3. IC cm bin nhit Hin nay c rt nhiu loi IC bn dn dng o nhit cho di o t - 55 C n 1500C. Sau y l 1 s loi IC cm bin nhit :0</p> <p>3.1.</p> <p>LM35 IC cm bin nhit ca hng National Semiconductor</p> <p>LM35 c in p ng ra t l trc tip vi nhit thng o 0C, in p ng ra l 10mV/0C v sai s khng tuyn tnh l 1,8mV cho ton thang o. in p ngun nui c th thay i t 4V n 30V. LM35 c ch to cho 3 thang o:</p> <p>-550C n 1500C loi LM35 v LM35D. -400C n 1100C loi LM35C v LM35CA. 00C n 1000C loi LM35DA.</p> <p>3.2 AD22100 IC cm bin nhit ca hnh Analog Devices IC ny c h s nhit 22,5mV/0C. in p ng ra c cng thc:</p> <p>Vout = (V+/5V).(1,375V + 22,5mV/0C.T)Trong : V+ l tr s in p cp. T l nhit cn o. Cc IC trong h AD 22100: AD22100 KT/KR cho di o t 00C n 1000C. AD22100 AT/AR cho di o t -400C n 850C. AD22100 ST/SR cho di o t -500C n 1500C.</p> <p>4. Cm bin nhit in tr. Cu to: c lm t bt oxit kim loi. 4.1. Nhit in tr NTC (Negative Temperature Cofficient).</p> <p>L in tr c h s nhit m,c bn cht l cc in tr bn dn c in tr gim khi nhit tng. in tr ca NTC gim mnh khi nhit gia tng. T 00C n 1500C in tr ca NTC gim i 100 ln. NTC l hn hp a tinh th nhiu oxit gm c chy nhit cao nh Fe2O3, Zn2TiO4, MgCr2O4, TiO2 hay NiO, CO, Li2O. - ng dng: +Khi lm vic vi dng in b, NTC c dng lm thit b o nhit . +Khi dng in lm vic ln,dng NTC o mc cht lng. 4.2. Nhit in tr PTC (Positive Temperature Cofficient).</p> <p>in tr c h s nhit dng,c bn cht l mt in tr bn dn c in tr tng khi nhit tng. nhit nh hn 1100C in tr ca n nh c trm m v bin i khng ng k. Khi nhit vt qu 110 th in tr ca n tng ti hng ngn mga m. Vt liu ch to PTC gm c hn hp: Ba 2CO3, StO, TiO c p v nung nhit cao. -ng dng: dng bo v ng c in khi xy ra s c ngn mch hay qu ti hoc l iu khin mc nhit.. 5. Cp nhit (Thermocouple). Cp nhit l loi cm bin nhit nguyn l hot ng da trn hiu ng Peltier, Thomson v Sheebek. 5.1. Hiu ng Peltier Hai dy dn A v B khc nhau, tip xc vi nhau v c cng mt nhit s to nn mt hiu in th tip xc. Hiu in th ph thuc vo bn cht vt dn v nhiu .</p> <p>5.2. Hiu ng Thomson Trong mt vt dn ng nht A. Nu hai im M v N c nhit khc nhau s sinh ra mt sc in ng. Sc in ng ny ph thuc vo bn cht vt dn v nhit ti hai im:</p> <p>5.3. Hiu ng Sheebek Nu c mt mch kn to thnh t hai vt dn A,B v hai u chuyn tip c nhit khc nhau T1 v T2. Chng to thnh mt mt cp nhit in v c sc i ng do kt qu ca hai hiu ng Peltier v Thomson v gi l sc in ng Sheebek</p> <p>Sc in ng ny ch ph thuc vo T1 , T2 v c th biu din di dng:</p> <p>Khi mt u tip xc gi nhit n nh (v d T2 = C) v u kia (T1) t mi trng c nhit thay i. Sc in ng s l hm s ca nhit T1</p> <p>C - Hng s T1 - Nhit u lm vic T2 - Nhit u t do (mi trng) 5.4. Vt liu ch to cp nhit Vt liu ch to cp nhit cn c sc in ng nhit in ln, gi c bn khi t nng nhit cao, in dn ln, h s nhit nh, c tnh cht nhit n nh. C kh nng chng c xy ho nhit lm vic.Vt liu ch to cp nhit c tnh ng nht v c thnh phn n nh. Khi ch to cp nhit cn trnh gy nn sai s do sc in ng nhit k sinh do dy gp khc, mi hn c khch thc ln..v.v Dy cp nhit t trong ng s cch in, bn ngoi l mt lp v bc kn.V thng c lm bng thp. 5.5. Cc vn khi dng cp nhit. - Tnh cht ca cp nhit in theo thi gian. -C sai s trong php o vi cp nhit.</p> <p>- Ni di dy ni cp nhit ti thit b o. -Thi gian hi p lu. 6. MT S NG DNG THC T CA CM BIN NHIT a.</p> <p>Cm bin nhit c np trn cnh tn nhit ca mt s b ngun v mt s thit b trong mch in t o v iu khin cnh bo nhit:</p> <p>b. Cm bin nhit c np trong cc my p trng o v duy tr nhit n nh</p> <p>c.</p> <p>Ngoi ra cm bin nhit c s dng trong my sy cng nghip a nng (sy dc phm,sy nng sn,thc phm..) dng iu khin mc nhit khc nhau tu theo yu cu ca tng loi i tng. Cm bin nhit cn c s dng trong cc nghnh cng nghip ln nh sn xut xi mng,sn xut thp,bng si, p nha...v.v..</p>