06_chuong 5_cam bien luc va ap suat

Chng 5: Cm Bin Lc V p Sut

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Chng 5

CM BIN LC V P SUT (Sensors for

force and pressure)

5.1 Khi nim

Khi dy dn chu bin dng c kh th in tr ca n cng thay i. Hin

tng gi l hiu ng tenzo. Chuyn i in tr lm vic da trn hiu ng ny

c gi l chuyn i in tr tenzo hay chuyn i in tr lc cng.

Chuyn i in tr kiu lc cng ny c chia lm 3 loi:

o Chuyn i in tr lc cng dy mnh

o Chuyn i in tr lc cng l mng

o Chuyn i in tr lc cng mng mng

Loi cm bin ny c 3 thng s chnh:

o Kch thc ca

o Gi tr in tr ca cm bin

o Dng in ti a cho php

Ph bin nht l loi chuyn i in tr lc cng dy mnh: Trn mt tm giy

mng bn, ngi ta dn 1 dy in tr theo kiu hnh rng lc c ng knh khong

0,02 0,03mm. Dy c ch to t vt liu constantan, nicrom, hoc hp kim

platin-iridi. Hai u dy c hn vi l ng dng ni vi mch o. Pha trn

c dn mt lp giy mng c nh dy. Chiu di l0 ca rng lc l chiu di

tc dng ca chuyn i.

cc chuyn i in tr lc cng lm vic tt trong thc t, yu cu vt liu

ch to chuyn i c nhy ln nhy t c cao. Mt khc, h s nhit

ca chuyn i in tr lc cng () cn b v trong kim loi bin dng tng i

1 trong gii hn n hi khng ln hn 2,5.10-3, do R vo khong (1,25 10)


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tc l s thay i in tr tng i khng vt qu 1% khi i tng o chu p

sut ln nht.

5.2 Cm bin lc (Force Sensor)

5.2.1 Cm bin lc cng (Strain Gauges sensor)

a. Khi nim

Cm bin lc cng da trn s thay i in tr ca dy dn hoc cht bn

dn khi n b bin dng v c s dng o s co - gin khi vt th b nn li

hoc ko cng ra. C nhiu ng dng nh l o lc, p sut, gia tc

Hnh 5.1 Khi Strain Gauges cha c lc tc ng

Hnh 5.2 Khi Strain Gauges c lc tc ng.

b. in tr ca dy dn

o Strain gauge n gin l mt dy dn nh hnh sau:

Hnh 5.3 in tr dy dn.


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o in tr ca dy dn :

( )=A

lR (5.1)

: in tr sut

Khi c lc tc ng vo th strain gauge s b thay i v hnh dng, dn ti

in tr thay i.

c. gin n

gin n hay cn gi l n di c nh ngha bng bin dng

chiu di trn tng chiu di ban u

=

(5.2)

Hnh 5.4 Cc dng cm bin lc cng

d. Hot ng ca cm bin lc

in tr thay i ca strain gauge l rt nh, c vi phn trm, nh hn mt

Ohm. pht hin ra s thay i nh nh th, ta phi p dng cc mch cu

Hnh 5.5 Hai in tr chnh v ph.

e. Phng php o bng mch cu


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Hnh 5.6 Phng php o bng mch cu.

o u tin ta s chnh cc gi tr in tr sao cho V1=V2

G

GS

RR

RVV

+=

11 (5.3)

D

DS

RR

RVV

+=

22 (5.4)

o Sai lch in p trn mch cu (V1 V2)

+

+==

D

D

G

GS RR

R

RR

RVVVV

2121 (5.5)

( )( )( )DG

DGS RRRR

RRRRVV

++

=

21

12 (5.6)

o n gin ta cho cc gi tr in tr l bng nhau = R. Khi c s thay i

in tr strain gauge R th:

RR

RVV S +

=

24 (5.7)

4R s ln hn rt nhiu so vi 2R v th:

RRR 424 + (5.8)

R = s thay i in tr ca strain-gauge

R = gi tr in tr ca cc in tr trong mch cu

V = sai lch in p


Trang 152

Vs= ngun in p cp trong mch cu nn ta suy ra c

SV

VRR

4

(5.9)

f. Mi lin h gia n di v in tr

Khi strain gauge b tc ng, in tr n tng ln. Mi quan h gia n

di v in tr c biu din nh sau:

GF

RR /= (5.10)

Vi:

= n di(L/L)

R = in tr strain-gauge

GF =H s gauge, h s ny c cung cp bi nh sn xut vi

RL

LR

LL

RRGF

.

.

/

/

=

= (5.11)

Youngs modulus (E): l hng s m t mi quan h gia lc tc ng v

n di ca vt:

=E (5.12)

Vi:

E = Hng s (hng s ph thuc vo tng loi vt liu)

= Lc tc ng trn mt vng

= n di

Youngs modulus (E)

Cht liu lb/in2 N/cm2

St 30.106 2,07.107

ng 15.106 1,07.107

Nhm 10.106 6,9.106

7,3.106 5,0.106


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G cng 1,5 . 106 1,0.106

V d: Cho mch o strain gauge nh hnh. B mt tr (cu to bng thp) l 2

in2, R= 120 v GF bng 2. Ngun cung cp 10 V. Khi thanh cha b tc ng

ng ra cu l 0V. Khi thanh chu tc ng, and ng ra cu l 0.0005 V. Tm lc

tc ng ln thanh.

Gii

o thay i in tr khi thanh chu tc ng

=

=

024.010

0005.0*120*44

V

V

V

VRR

S

o n di :

./.0001.02

120/024.0/inin

GF

RR==

=

o p dng cng thc:

=E

o tnh ton lc tc ng ln thanh. Tra bng Youngs modulus. T bng,

ta tm c 30,000,000 lb/in2 cho bi:

22 /3000/.0001.0*/000,000,30. inlbinininlbE ===

o l lc tc ng ln mt in2 mt ct, theo l 2.in2 nn lc tc ng phi

l 6000 lb/in2


Trang 154

5.2.2 Load cell

a. Cu to v phn loi

Load cell l thit b dng chuyn i lc thnh tn hiu in. Loadcell c

rt nhiu loi khc nhau nh:

o Loi theo lc tc ng: chu ko (shear loadcell), chu nn (compression

loadcell), chu un (bending), chu xon (tension loadcell) Hnh 5.7

Hnh 5.7 Loi Loadcell theo lc tc ng.

o Loi theo hnh dng: dng a, dng thanh, dng tr, dng cu, dng ch

SHnh 5.8 l loadcell dng tr

Hnh 5.8 Loi Loadcell hnh tr.

(a) Gauge chnh v gauge ph c lp li n khi no chng c cng nhit .

(b) Loadcell dng cu strain gauge

Phn in tr b c t gn strain gauge chu tc ng, m bo cng

nhit . N khng b thay i di tc ng ca lc.

b. Nguyn l hot ng

Ty theo loi loadcell m mi loi c nguyn tc hot ng khc nhau nh:


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o Hydraulic load cells: l loi loadcell thy lc, n o trng lng nh vo s

thay i p sut cht lng. Khi n chu ti lc tc ng s chuyn ti mt pit-

tng nn mt lng cht lng trong bung cha. Khi lc tc ng tng ln, p

lc cc cht lng thy lc cng tng theo. p lc ny c th c xc nh cho

ch nh cc b hoc iu khin t xa. u ra l tuyn tnh v tng i khng

b nh hng bi hm lng ca cht lng hoc nhit ca n. Nu cc

loadcell c ci t ng v hiu chun, chnh xc c th l 0,25% trong

ton thang o hoc tt hn trong hu ht cc ng dng o trng lng. Bi v

cm bin ny khng c thnh phn in, nn n rt thch hp s dng trong

cc khu vc nguy him. Cc ng dng in hnh ca loadcell thy lc bao gm

cn cc bn cha, gu v phu. t c chnh xc ti a nn dng nhiu

loadcell kt ni vi nhau ri tng hp kt qu u ra.

o Pneumatic load cells: Loadcell kh nn hot ng trn nguyn tc cn bng lc.

Cc thit b ny s dng nhiu bung gim chn nn c chnh xc cao hn

loadcell loi thy lc. Trong mt s thit k, bung gim chn u tin c s

dng nh mt bung trng lng b. Loadcell kh nn thng c s dng

o trng lng tng i nh trong cc ngnh cng nghip m vn sch s

v an ton l mi quan tm chnh. Nhng u im ca loi loadcell ny l

khng nhy cm vi s thay i nhit . Ngoi ra, chng khng cha cc cht

lng c th gy nhim qu trnh nu v kt cu. Nhc im ca loi loadcell

ny l tc p ng tng i chm v kh cn thit phi sch s, khng kh

kh quy nh, hoc nit.

o Strain-gauge load cells: Hot ng da trn nguyn l cu in tr cn bng

Wheatstone. Gi tr lc tc dng t l vi s thay i in tr cm ng trong cu

in tr, v do tr v tn hiu in p t l.

c. Mch o cm bin lc

o Dng bn cu:


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Hnh 5.9 Loadcell dng cu i xng.

y c hai strain gauge chnh, 2 strain gauge ph


Ng ra bn cu:

( ) ( )

++

+++

=RRR

R

RRR

RRvv So

( ) ( )

++

=RR

RRRvv So 2

(5.13)

Nu RR


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Hnh 5.11 Loadcell dng cu ton phn.


Ng ra mch cu y :

( ) ( )

+

++

+=

RRR

RR

RRR

RRvv So (5.15)

( ) ( )( ) ( ) ( )( )( )( ) ( )( )

+++++++

=RRRRRR

RRRRRRRRRRvv So (5.16)

Nu RR


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Bng so snh cc loi Loadcell Loi Tm o

chnh xc

ng dng u im Nhc im

Mechanical Load Cells Load Cell thy lc

Ln n 10,000,000 lb

0.25% Bn cha , x lan ch bn ;khu vc nguy him.

khng b nh hng bi nhit .

Chi ph cao; phc tp

Load Cell kh nn

Tng i rng

Cao Cng nghip thc phm: khu vc nguy him.

An ton: thnh phn khng cha cc cht nguy hi

p ng chm: i hi kh phi sch v kh.

Strain Gauge Load Cells Load Cell chu un

10-5k lbs. 0.03% Bn cha, bn cn.

Chi ph thp, cu to n gin.

Strain gauge l ra bn ngoi nn cn c bin php bo v.

Load Cell chu ko

10-5k lbs. 0.03% Bn cha, bn cn, ti trng lch tm.

D dng lp rp

Load Cell dng hp

Ln n 500k lbs.

0.05% Cn xe ti, bn cha, cn hnh phu

Ti c th chuyn ng.

Khng c bo v theo phng ngang.

Load Cell dng vng v dng a

5- 500k lbs.

Bn cha, cn. Cc b phn lm bng thp khng g .

Ti khng c chuyn ng trong khi cn.

Load Cell dng nt

0-50k lbs

1% Cc loi cn nh

Nh gn, chi ph thp.

Ti phi t ngay v tr trung tm v khng c chuyn ng.

5.3 Cm bin lu lng v p sut (Pressure and Flowmeters sensor)

Cc cm bin o lu lng c s dng o c cht lng v cht kh

trong nhiu ng dng gim st v iu khin. Vi cht lng, khi lng ring c

th coi l hng s nn vic o lu lng nhn chung d thc hin hn. Mt s k

thut hot ng vi c cht lng v cht kh, mt s ch hot ng vi dng lu

cht xc nh. Vic o lu lng thng bt u bng vic o tc dng chy.


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5.3.1 Cm bin o lu lng: (Flowmeters Sensor)

Mt trong s cc tham s quan trng ca qu trnh cng ngh l lu lng

cc cht chy qua ng dn. Mun nng cao cht lng sn phm v hiu qu ca

h thng iu khin t ng cc qu trnh cng ngh cn phi o chnh xc th

tch v lu lng cc cht.

Mi trng o khc nhau c c trng bng tnh cht l ha v cc yu

cu cng ngh, do ta c nhiu phng php o da trn nhng nguyn l khc

nhau. S lng vt cht c xc nh bng khi lng v th tch ca n tng

ng vi cc n v o (kg, tn) hay n v th tch (m3, lt). Lu lng vt cht l

s lng cht y chy qua tit din ngang ca ng dn trong mt n v thi

gian.

o Lu lng th tch: t

VQV = (5.18)

n v o m3/s; m3/gi

o Lu lng khi: Q m t

m= (5.19)

n v o kg/s; kg/gi; tn/gi

Cn phn bit s khc nhau gi lu lng tc thi v lu lng trung bnh.

Chng hn lu lng th tch: Lu lng trung bnh Qtb=V( 21 ), lu lng

tc thi: QV = dV/d (V l th tch vt cht o c trong thi gian ( 21 ))

i vi cht kh, kt qu o khng ph thuc vo iu kin p sut v

nhit , ta qui i v iu kin chun (nhit 200C, p sut 760 mm thy

ngn).

thch ng vi cc nhu cu khc nhau trong cng nghip, ngi ta

pht trin rt nhiu phng php khc nhau o lu lng cht lng, hi nc,

kh...

c trng ca lu cht:

Mi lu cht c c trng bi nhng yu t sau:


Trang 160

o Khi lng ring

o H s nht ng lc

o H s nht ng hc

Khi lng ring:

Khi lng ring l khi lng ca 1 n v th tch lu cht

)/( 3mkgV

m= (5.20)

m: khi lng ca khi lu cht

V: th tch ca khi lu cht

Hnh 5.13 Khi lng ring ca nc v hi nc trng thi bo ha vi cc iu

kin nhit khc nhau.

Tnh nht:

Tnh nht l tnh cht chng li s dch chuyn, n biu hin sc dnh phn

t hay kh nng lu ng ca lu cht. y l 1 tnh cht quan trng ca lu cht

v n l nguyn nhn c bn gy ra s tn tht nng lng khi lu cht chuyn

ng. Khi lu cht chuyn ng, gia chng c s chuyn ng tng i, ny

sinh ma st to nn s bin i 1 phn c nng thnh nhit nng v mt i. Tnh

nht c c trng bi h s nht ng lc, h s ny ph thuc vo loi lu

cht. C nhiu cch o nht, cch thc n gin thng c cc phng

th nghim cc trng i hc s dng chng minh s tn ti nht v xc

nh gi tr l: Cho 1 qu cu ri trong cht lng di tc dng ca trng lc. o

khong cch (d) v thi gian (t) qu cu ri, tnh vn tc u.


Trang 161

H s nht ng lc s c tnh theo phng trnh sau:

u

rg

.9

...2 2

= (5.21)

: H s nht ng lc (Pa s)

: S khc nhau gia khi lng ring qu cu v cht lng (kg/m3).

g: Gia tc trng trng 9,81 m/s2.

r: Bn knh qu cu (m).

u: Vn tc ri ca qu cu u= d/t (m/s)

n v ca h s nht ng lc:

Pa s= Ns/m2= 103 cP (centiPoise) = 10 P (Poise)

Hnh 5.14 H s nht ng lc ca nc v hi nc trng thi bo ha vi cc

iu kin nhit khc nhau.

nhn mnh mi quan h gia tnh nht v khi lng ring ca lu cht

ngi ta a ra h s nht ng hc.

v =

310. (5.22)

v: h s nht ng hc, n v centistokes (cSt)

: H s nht ng lc.

: Khi lng ring ca lu cht (kg/m3).

n v h s nht ng hc l cSt (centistokes), St (stokes), m2/s

1St = 100 cSt = 1 cm2/s = 10-4 m2/s


Trang 162

nht ph thuc vo nhit v p sut. nht ca cht lng tng khi

nhit gim v khi p sut tng, i vi cht kh th ngc li.

Tr s Reynold: (Re)

Tt c cc yu t k trn u c nh hng ti dng chy ca lu cht

trong ng dn, ngi ta kt hp chng vi nhau to ra 1 i lng duy nht th

hin c trng ca lu cht

Tr s Reynold:

Du

Re..

= (5.23)

: Khi lng ring ca lu cht (kg/m3)

D: ng knh trong ca ng dn lu cht (m)

u: Vn tc ca lu cht (m/s)

: H s nht ng lc (Pa s)

Hiu chun khi lng ring:

Khi lng ring ca cht lng, cht kh trong mi trng o nh hng

n php o lu lng. Thc cht khi lng ring thng khng l mt hng

s.

Khi lng ring ca cht lng ty thuc vo nhit . Trng hp ny

hiu chun khi lng ring ta ch cn o nhit .

Kh thng l mt hn hp gm nhiu thnh phn. Khi lng ring ca n

l thuc vo p sut v nhit . hiu chnh sai s cn o c hai i lng ny

(ly chun l khi lng ring iu kin 00C, p sut kh quyn).

Trng thi dng chy:

Nu b i nh hng ca nht v s ma st vi thnh ng dn th vn

tc dng chy s nh nhau mi v tr trn mt ct ngang ca ng dn (hnh

5.15).


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Hnh 5.15 Vn tc dng chy. (trng hp l tng)

Tuy nhin ch l trng hp l tng, trong thc t nht nh hng

n tc ca dng chy, cng vi s ma st ca ng dn lm gim vn tc ca

lu cht v tr gn thnh ng (hnh 5.16).

Hnh 5.16 Vn tc dng chy vi nh hng ca tnh nht v lc ma st.

Cc kh ( trng thi bo ha) v hu ht cc cht lng thng c vn

chuyn bng ng dn trng thi dng chy ri. Hnh 5.17

Hnh 5.17 Tr s Reynold.

5.3.2 Cm bin o p sut: (Pressure sensor)

a. Phng php o lu lng da trn nguyn tc s chnh lch p sut


Trang 164

dng cm bin p sut o lu lng, ngi ta o s chnh lch p sut

(hiu p) gia 2 v tr ng c tit din dng chy khc nhau. Cc lu lng k o

da trn hiu p (differential pressure flowmeter) c s dng rt ph bin, c

bit l dng vi cc cht lng. Cc thit b ny cng nh hu ht cc lu lng k

khc gm hai thnh phn c bn.

Thnh phn 1: l nguyn nhn gy nn s thay i trong nng lng ng

hc, to nn s thay i p sut trong ng. Thnh phn ny phi ph hp vi kch

thc ca ng ng, iu kin dng chy, tnh cht ca lu cht.

Thnh phn th 2: o s chnh lch p v tn hiu u ra c chuyn i

thnh gi tr lu lng.

b. nh ngha p sut

p sut l lc tc dng trn mt n v din tch

dS

dFp = (5.24)

p: p sut

A: din tch

F: lc

o p sut ngi ta s dng mt nguyn tc ging nhau, p sut c

cho tc dng ln mt b mt xc nh, nh th p sut c bin thnh lc. Vic

o p sut c a v o lc. Tt c cc lc tc dng ln mt mt phng xc

nh l thc o p xut.

S

Fp = (5.25)

c. n v p sut

y ban quc t cho vic o c vi lut nh chn Pascal (Pa) = N/m2 l

n v p sut (ISO 1000, DIN 1301).

o Vic phn chia thang o ca my o p sut c dng vi bi s ca n v

Pa.


Trang 165

1 mbar = 102 Pa

1 bar = 105 Pa

Nhng n v c dng ph bin trc y:

1 mmHg = 1,0000 Torr

1 atm = 760 Torr (atm n v p sut kh quyn vt l)

1Torr =1,333224 102 Pa

1kp/cm2 = 0,980665. 105 Pa

1at =1kp/cm2 = 0,980665. 105 Pa (at n v p sut kh quyn k thut)

1mm nc =9,80665 Pa

o Ngoi ra cc nc Anh, M ngi ta cn dng cc n v p sut sau:

1 pound-force/ square yard (Lb/yd2)

1 pound-force/ square foot (Lb/ft2)

1 pound-force/ square inch (Lb/in2 = psi)

1 ounce/ square foot (oz/ft2)

1 ounce/ square inch (oz/in2)

1 Ton/ square foot (Ton/ft2)

1 inch of water (trong nc)

1 inch of mecuri (trong thy ngn)

= 5,425. 10-5 at

= 4,883. 10-4 at

= 7,031. 10-2 at

= 3,052. 10-5 at

= 4,394. 10-3 at

= 2,540. 10-3 at

= 2,40. 10-3 at

= 3,455. 10-2 at

d. B phn to nn s chnh lch p sut

D hin nay c nhiu phng php o lu lng c pht trin,

phng php o lu lng bng ng co vn c ng dng rt rng ri trong

cng nghip v cc lnh vc khc. ng co dng to s chnh lch p sut

(gia v tr ng cha co v ng co) nn ng co phi l nhng linh kin c hc rt

bn b, cu trc n gin v khng c cc phn t di ng chu c nhng

iu kin v cng khc nghit trong cng nghip. Phng php o s dng Pitot

tube cng da trn s chnh lch p sut nhng khng to s co trc tip trn

dng chy.

e. B bin i p sut


Trang 166

Hnh 5.18 B bin i p sut.

Li dng cu to ca cm bin lc ta c th s dng o p sut.

p sut chnh l lc tc ng trn mt n v din tch.

Cm bin p sut thng dng l loi cm bin p sut cht lu, nguyn l

hot ng thng l bng cht lng cn bng thy tnh hay o bng phn t nhy

cm bin dng (c s bin dng n hi ca cc phn t cm bin). Cm bin p

sut th c nhiu loi : loi p tr, lai in dung ...

f. ng co Venturi

Phng php o lu lng bng ng co da trn nh lut lin tc v

phng trnh nng lng ca Bernoulli.

o Phng trnh lin tc:

A1u1 =A2u2 (5.26)

o Phng trnh Bernoulli:

p1 + 1.. hg + 2 2

1u = p2 + 2.. hg + 2 2

2u (5.27)

o p dng cho trng ng co venturi:

p1 + 2

21u = p2 + 2

22u (2.28)


Trang 167

Hnh 2.19 Phng php o lu lng ng co venturi.

o Trong :

A1: Din tch trc co.

A2: Din tch v tr co.

u1: Vn tc trc v tr co

u2: Vn tc v tr co

p1: p sut tnh trc v tr co

p2: p sut tnh v tr co

: Khi lng ring.

h 1: cao v tr v tr trc co

h2: cao v tr v tr sau co

ni din tch ng b thu nh, vn tc dng chy gia tng. Vi phng

trnh nng lng ca Bernoulli, nng lng ca dng chy l tng nng lng p

sut tnh v ng nng (vn tc) l mt hng s.

)(2

21

2221 uuppp ==

(5.29)

o Gii phng trnh trn theo v2:

22

2

1

221

2121

22 .)(

2)(

2u

A

Appuppu

+=+=

(5.30)


Trang 168

o t 2

1

21

1

=

A

A l hng s dng chy

Ta c:

212 .2

ppu =

(5.31)

o T ta c lu lng tnh theo th tch v khi lng nh sau:

pkppAuAQV === ...2

... 21222 (5.32)

pkppAuAQm === ...2....'

21222 (5.33)

Nh th lu lng t l vi cn s bc hai ca hiu p khi khi lng ring

l hng s.

Hnh nh thc t loi ng co Venturi H800 do hng Tetratec Instruments

sn xut.

Hnh 5.20 Hnh nh thc t loi ng co Venturi H800.

g. Orifice plate

Orifice plate l mt trong nhng cch thc n gin nht (v cng kinh t

nht) tc ng n dng chy, t c th tnh c lu lng.

Hnh 5.21 Cc dng ng co Orifice plate.


Trang 169

Orifice plate dy khong 1/16 n n inch. C 3 loi Orifice plate:

concentric (ng tm), eccentric (lch tm), segmental (hnh cung) nh hnh

5.21.

Trong 3 loi, loi concentric c s dng nhiu nht. Khi lu cht i qua

Orifice plate, dng chy hi t, tc lu cht tng ln mc ti a. Ti im

ny, p sut l nh nht. Khi dng chy phn k, tc lu cht gim tr li mc

ban u.

Hai loi Eccentric v Segmental cng c chc nng hon ton ton ging

nh Consentric. Thit b c lp t ng tm vi ng dn lu cht (ng dn t

nm ngang), vi loi Segmental, v tr phn cung trn (phn c ct) ph

thuc vo dng cht lng c th trn hoc di nhm mc ch ngn chn cc

vt l t trong dng chy. Kt qu t c l php o tng mc chnh xc.

Loi eccentric cng c thit k vi cng mc ch trn.

Nguyn tc hot ng:

Hnh 5.22 Phng php o lu lng dng ng Orifice plate.

Nguyn tc o lu lng khi s dng Orifice plate cng da trn phng

trnh Bernoulli.

ghv 2= (5.34)

ghAvAQV 2.. == (5.35)


Trang 170

Trong thc t gi tr vn tc thc s s nh hn gi tr vn tc trn l thuyt

(do ma st). S khc bit ny c iu chnh vi h s VC .

VC =vn tc thc t/ vn tc l thuyt.

Bn cnh din tch dng chy hi t s nh hn din tch ca Orifice

plate, iu ny tip tc c iu chnh vi h s CC.

CC = din tch vng hi t / din tch Orifice plate

Hai h s VC , CC kt hp vi nhau c c h s iu chnh C.

Q v= C. ghA 2. (5.36)

Qv: Lu lng m3/s

A: H s iu chnh.

A: Din tch Orifice plate.

h: S chnh lch v p sut (m)

g: Gia tc trong trng (9,8 m2/s)

H s C (c nh ngha theo tiu chun ISO 5167 2003) c th tra cc

gi tr da vo da vo tiu chun ISO 5167.

Hnh 5.23 Yu cu lp t (theo tiu chun ISO 5167)


Trang 171

Cn c cc on ng dn lu cht khng cong, thng v tr trc v sau v

tr lp t Orifice Plate. di ti ti thiu ca cc on ng ny phi t mc:

o v tr sau Orifice Plate (cui ngun): 5 ln ng knh ng dn lu cht.

o v tr trc Orifice Plate (u ngun): Ph thuc vo t s v cch lp

t.

D

d= (5.37)

d: ng knh ca Orifice Plate (ng knh ming ca vi).

D: ng knh ng dn lu cht.

Cn c vo gi tr v cch lp t (xem hnh 5.24) m s dng 1 trong

cc h s A, B, C. Tnh c di ti thiu cn thit = h s * D.

H s


Trang 172

p1 + 2

21v = p2 + 2

22v (5.38)

p1: p sut tnh trong dng chy ca lu cht ng dn.

p2: p sut tnh trong Pitot tube.

v1: Vn tc dng chy.

v2: Vn tc dng chy v tr ng (bng 0).

: Khi lng ring ca lu cht.

o Do v2=0

p1 + 2

21v = p2 (5.39)

2112 2vppp

== (5.40)

o Gii phng trnh trn theo v1

p

v

=.22

1 p

v

=.2

1 (5.41)

o T ta c lu lng tnh theo th tch v khi lng nh sau:

p

AvAQv

==.2

.. 1 (5.42)

Trong thc t, Pitot tube c th c tch hp trong cng 1 khi, b phn o

tng p sut tnh v ng nng vi b b phn o p sut tnh c t trong

cng mt thit b.

Do dng n gin ca pitot tube ch o ti 1 im, m gi tr ca vn tc

dng chy ti nhng v tr khc nhau l khng ging nhau (trn mt ct ngang

ca dng chy). Do v tr o mang tnh cht quyt nh.


Trang 173

Hnh 5.25 Dng n gin ca Pitot tube.

khc phc c im vn tc dng chy khc nhau, dng pitot tube

trung bnh c s dng. ng nng ca dng chy c o cc v tr khc

nhau, kt qu ly gi tr trung bnh.

Hnh 5.26 Pitot tube trung bnh.

S chnh xc ca php o

o Khi din tch ct ngang ca dng chy v khi lng ring ca lu cht coi

nh c nh, trong phng trnh

Qv = h s. p (5.43)

o Hiu p t l vi bnh phng vi lu lng QV, do khi QV = 30%

thang o th = 9% thang o.

o Hiu p cng thp, sai s tng i cng ln. Khi p = 9% thang o, sai s

tng i lc ny ln gp 11 ln sai s cui thang o.

o Do ch nn thc hin vic o lu lng bng ng co t 30% n 100%

thang o.

B phn o s chnh lch p sut

p

p


Trang 174

B phn o s chnh lch p sut ny c thit k o p sut da trn cc

nguyn tc:

o Chuyn i p sut kiu in dung

o Chuyn i p sut kiu bin p vi sai

o Chuyn i p sut kiu in tr p in

o Chuyn i p sut kiu p in

o Chuyn i p sut kiu mng sc co gin...

Cm bin p sut loi in tr p in

Cm bin p sut loi in tr p in thay i in tr tng ng vi bin

dng trn bn thn n.

o Cm bin bin dng p in tr kim loi

o Cm bin bin dng p in tr bn dn

Cc thng s c bn:

o di bin dng: T s ca s thay i kch thc vi chiu di ban u

l

l= (5.44)

n v l Strain v thng s dng microstrain

o H s bin dng: t s thay i in tr vi thay i chiu di.

R

R

llR

RGF

=

= (5.45)

H s GF ca kim loi: 2, bn dn: 80 120

Nguyn l o s dng cm bin dng p in tr:

Cc php o bin dng t khi c gi tr ln hn vi millistrain (. 10-3)

o V d: C cu chu lc bin dng 500. H s bin dng GF = 2.

Gi tr thay i in tr: R/R = GF. 500. 10-3 = 0. 001


Trang 175

o s thay i nh gi tr in tr, cc cm bin bin dng p in tr s

dng mch cu.

So VRR

R

RR

RV .

21

1

43

3

+

+= (5.46)

Khi R1/R2 = R3/R4 th in p ra bng 0. Nu thay th R4 bng cm bin th

khi c lc tc dng ln, in p ra s thay i

Hnh 5.27 Dng mch cu .

o thay i in tr: R = RG. GF.

o Chn R1 = R2 v R3 = RG ta c:

+=

2.1

1

4

..

GF

GFVV SO (5.47)

o Tuy nhin GF*/2

V0 = Vs. GF.

5.3.3 Cm bin p sut loi in

Cm bn p sut in

i hay xut hin phn cc in khi mt s cht in mi b bin dng d

dng ca lc.

B cm bin gm ba lp m

mng gia l cht nn (cao su silicon) v

bin mng PVDF ch

in tch thay i to n

lc tc ng vo b cm bin.

Cm bin p i

1000 ms-1(100N/mm

u im ca cm bin loi n

cy cao, c kh nng o cc i l

5.3.4 Cm bin p sut loi m

Mng sc co gi

c pht trin u ti

ca loi cm bin ny l tr

vi cu Wheatstone. c chnh xc, mch in cn nhiu in tr b

sa sai.

a. Nguyn tc hot ng

Chng 5: Cm B

Trang 176

. GF.

m bin p sut loi in p

m bn p sut in p c nguyn tc hot ng da tr

i hay xut hin phn cc in khi mt s cht in mi b bin dng d

cm bin gm ba lp mng polyvinylidene flourid (PVDF)

t nn (cao su silicon) v lp p Khi tc d

ng PVDF chu ng sut v hnh thnh cc in tch trn b

Hnh 5.29 Cm bn p sut in p.

n tch thay i to nn bin thin in p ra. Bin

cm bin.

p in c dng o lc bin thin (n 10 kN), o p sut

(100N/mm2) v gia tc (ti 1kg) trong di tn t 0,5

m ca cm bin loi ny l cu trc n gin, kch th

y cao, c kh nng o cc i lng bin thin nhanh.

m bin p sut loi mng sc co gin kim loi

c co gin l loi cm bin rt quan trng dng

c pht trin u tin M trong nhng nm cui thp ni

y l tr s o chnh xc, kch thc b. Mch o th

i cu Wheatstone. c chnh xc, mch in cn nhiu in tr b

t ng

Bin Lc V p Sut

(5.50)

c hot ng da trn hin tng thay

i hay xut hin phn cc in khi mt s cht in mi b bin dng di tc

ng polyvinylidene flourid (PVDF) c dt

p Khi tc dng lc ln b cm

n b mt.

in p ra t l vi

n 10 kN), o p sut

c (ti 1kg) trong di tn t 0,5 100kHz.

n, kch thc nh, tin

ng o p sut, lc...

M trong nhng nm cui thp nin 40. u im

c b. Mch o thng dng

i cu Wheatstone. c chnh xc, mch in cn nhiu in tr b tr v


Trang 177

Khi 1 si dy dn in b ko cng ra, n tr nn di v m hn nn in

tr ca dy tng ln. Khi si dy b nn, co li, n tr nn ngn hn v mp hn

nn in tr gim i. Nu gi vic nn v ko dn ny trong gii hn n hi th

sau khi b bin dng n vn gi nguyn kch thc v tr s nh ban u. Nu

gn cht dy dn ny vo phn t cn o c, chiu di dy dn thay i theo s

bin dng ca phn t ny. Sao cho s thay i in tr ca dy dn tng ng

vi lc, p sut lm bin dng phn t cn kho st.

S

lR = (5.51)

R: in tr dy dn.

L: Chiu di.

: in tr sut

S; Din tch mt ct ngang ca dy dn

b. ng dng o lu lng bng ng co vi cm bin p sut loi in tr p in

o s chnh lch ca p sut 2 v tr c th s dng cm bin p sut

loi in tr p in.

Loi cm bin ny c 2 mt:

o Mt trc cn gi l mt tch cc (mt cng tc), v l do cch in nn ch

chu ng c cc kh sch nh khng kh v kh Freon.

o Mt sau cn gi l mt th ng ch gm cc mt silic chu ng c nhiu

mi trng khc nhau.

Cho nn o hiu s p sut ca mt dng chy ta dng 2 cm bin v

cho mi trng tc dng mt sau cm bin.

o C th dng loi cm bin 240PC Series ca Honeywell.

o Cc loi cm bin ny c cc khong o: 0..0,345 bar, 01 bar, 0 2 bar,

04 bar, 07 bar, 010 bar.

o Hot ng vi in p 8 V.


Trang 178

o iu kin nhit mi trng: -400C+850C.

Hnh 5.30 Cm bin 240 PC Series.

o Vt liu ng dn l cao su buna - N nn c th chu ng c du la, du

nht, du thy lc, cn, kh Freon

i lng ng ra ca cm bin l gi tr in p. Tn hiu ra ca 2 cm bin

c a n mch x l cui cng c c kt qu ch th.

06_chuong 5_cam bien luc va ap suat

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