Transcript
Sensors of thermal quantities (thermometers) -Temperature = thermodynamic state quantity -temperature scales: Kelvin (triple point of water 273,16 K), Celsius, Fahrenheit ITS-90 (4 ranges) electrical
contact
sensors of temperature:
• resistive-metal(RTD) • resistive bulk semiconductor: -thermistors NTC - thermistors PTC -monocrystalic Si • semicon. with PN junction • quartz • thermocouples therm. expansion (liquid,gas,...) special (noise, acoustic,…) thermal
noncontact
quantum
comparison of sensors: thermocouple
ra nge s e ns itivity line a rity inte rcha nge a ge ing pa s s ive s pe e d e ndura nce
-200..2300
-+ -+ ++ ++
RTD
NTC
-200..850 -80..150 + + -++ e xpe ns ive s e lfhe a ting + ++ -+
PN junction
-40..100 ++ + +
-
1. noncontact sensors • thermal: - gas:
pV = RmT
V = konst ⇒
thermodynamic equation
po p =T To
2. contact sensors 2.1 metal resistive thermometers - principle: dependence of resistance of metal on temperature
R = R0 (1 + α ⋅ ∆T )
wire wound resistive thermometer Worth remembering
α[%/K] range [oC] Pt 0.39 -200/850 Ni 0.69 -80/320 Cu 0.43 -200/260
a)
thin layer resistive thermometer kovová vrstva resistive layer passivation layer pasivační vrstva izolační podložka insulating pad kontaktní vrstva contact layer b)
© Omega
Platinum resistance thermometer (RTD-Resistance Temperature Detector) - criterion of Pt purity -normalized resistance (1,3910 in GB, USA, Japan, Russia) tolerances of Pt standard according to IEC
-standard value of Pt resistance: Pt100: 0oC R= 100 Ω
B
-2 tolerance classes: A range– 200/650 B range –200/850 oC
4 tří da
oC
3
Pt
R 100 ≥ 1,385 Ro
tolerance [°C]
W 100 =
2 P
aA d í ř tt
1
or 200, 500, 1000, 2000 Ω -200
0
200 400 600
ϑ[°C]
resistance - temperature equation:
R ϑ = R 0 [1 + Aϑ + Bϑ 2 + Cϑ 3 ( ϑ − 100 )] according to international recommendation IEC
1,385
1,45.10
0
50
α.100°C
-3
100
ϑ[°C]
W100 = 1,385 R0 = 100 Ω A = 3,90802.10-3 K-1 B = -5,802. 10-7 K-2 for ϑ < 0 oC C = -4,27350.10-12 K-4 ϑ > 0 oC C = 0
Nickel resistance thermometer ☺ high sensitivity,quick response, small dimensions limited temperature range
∆ϑ[°C]
for –60..180 oC : R ϑ = R 0 [1 + Aϑ + Bϑ 2 + Cϑ 2 ( ϑ − 100 )]
+1 ϑ[°C]
A = 5,49.10-3 K-1 B = 6,80. 10-6 K-2 for ϑ > 0 oC, C = 9,24.10-9 K-3 ϑ < 0 oC, C = 0 Ni 100, 1000, 10000
Copper resistance thermometer -copper is used in range from –200 to +200 oC
small resistance, oxidation of Cu -for range –50 to +200 oC is valid: R = R0 (1 + αT )
α = 4 ,26.10 −3 K −1
- application: e.g. direct measurement of temperature of motor windings
2.2 Semiconductor resistive sensors of temperature NTC, negastors α < 0
thermistors
PTC, posistors α > 0
classification
monocrystalline resistive sensors
2.2.1 Thermistors:
thermal dependance
NTC (−80°C až +200°C)
150*
R=AeB/T
PTC
3
R=RreAT 180*
Ni (−60°C až +200°C)
2
Pt(-200°C až +1000°C)
850*
1 Rmin
-100
-50
0
50 ϑj
100
ϑ [°C]
negastor posistor * Omega
NTC, or negastors α<0 -produced by sintering technology from the powder of metal oxides -usable range - from 4,2K to 1000 oC
R = AeB/T
R1 = Rr e
1 1 B − T1 Tr
R1 - resistance at T1 Rr - resistance at Tr= 289,15K, i.e. 25oC B[K] - thermal „constant“ (in fact it depends on TR1, R2) A[Ω] – const. dependent on geom. and material
R[Ω] SiC Pt
10
7
10
6
10
5
10
4
103 -100 a)
100
200
300
b)
Al2O3
1 dR B α= =− 2 R dT T
0
α ≈ −2%
for high ϑ
α ≈ −8%
for low ϑ
Stein + Hart
400
ϑ[°C]
error 0,1K (0-100)oC
1 = a + b ln R + c(ln R ) 3 T
PTC, posistors α>0 - made from polycristalline ferroelectric ceramics e.g. (BaTiO3) - resistance decrease with increasing temperature – then after Curie point rapid increase of resistance -relation for increase of resistance:
R=RreAT
A = 0,16 K-1
-application: two state sensors (thermal switches – indication of max.temp.excess)
2.2.2 semiconductor monocrystalline sensors of temperature region of operation
resistivity rezistivita ρ [Ω cm]
30
20 10
10
ρ
16
3,0 5
n
10
3.10
15
1.10
15
14
20 0
16
1.10
5
1,0
3.10
ni 50
3.10
100 150 200 250 300 350 400 temperature teplota ϑ [°C]
resistance of sensor
ρ R= βd
β - geometrical factor D – diameter of contact ρ - resistivity
-3 Charge carriers concentration nosičů náboje n[cm ] koncentrace
ρi
internal structure of sensor Si3N4 kontakt Al
∅d
N+
N+
Siemens KTY 81-87 H
SiO2
α = 1%/K
Back conductor zpětný kontakt
-55…+150 oC +300 oC
∅D
transfer curve of Si sensor
2 elements in series - R independent on polarity
R[kΩ] 4 3,5 3
R = R r + k( ϑ − ϑ r ) 2
2,5 2 1,5 1
-50
0
25 50
100
ϑ[°C]
Signal conditioning circuits - self - heating by measuring current I error
RI 2 ∆ϑ = D
D – thermal resistance, „loading constant“
∅12
bead negastor
∅ 0,1
Pt 100: for ∆ϑ = 0.1oC → Imax=1mA thermistors [I] = µA Volt- ampére characteristics: 4
I[mA] IK
U[V] 10
10
posistor
0
IR
10
-1
10
-2
-1
10
10
0
I[mA]
UK
UB
Umax
U[V]
• influence of lead resistance two wires connection of resistor to bridge
Ust R1
RCu Rϑ
RCu
R2 A
Rj R3
RV = 2 RCu + R j
RCu = RCuo (1 + α Cu ϑ p ) δ = 2 ∆RCu ∆R ϑ
Uv
three wire connection of resistor to e.g. automatically balanced bridge with servo recorder °C R2
R1
M
Rvp
R4
R3
Rj
Rj
Rj
Rϑ ϑ
- reduction of resistance leads influence
measuring circuit with four wire connection of sensor with current source IST and auxiliary source of voltage U (RV=Rcu) Rv + Ist
Rv
+ Rϑ −
Rv Rv
−
−
U
iv
+ uv Rvst→∞
0 - 20mA (4mA-20mA)
converter voltage-current, output current ranges 0-20 mA or 4-20 mA advantages of current output (current loop)
three wire connection of sensor to active bridge: suppression of wire resistance
RV1 Rϑ
+ UZ
RV2 R1
UA
UR2 RV3 I=
U ST R3
−
Ust
R3 R2 A Ri→∞
U A = U ST + IRV U Z = U ST + 2 IRV + IRϑ ⇒
UV
IR U UZ − U A = ϑ − ST 2 2 2
• linearisation of resistance sensors of temperature converter for linearization using Pt RTD RS + Ust
− i
U
1
+
Rϑ
2
Uv Ust
RS
a)
Rϑ b)
UR v
linearized real
transfer characteristic of sensor
0
ϑ
ϑ
Uv
XTR 103 by Burr–Brown (now Texas Instruments) bridge with current sources: better linearity, elimination of leads resistance RLin
Offset
+UB
Iref + A2 −
Iref
-
UIn
− +A1
+ In
U Rϑ
Rz
Io= 4..20mA U
Ust
I RL
Rs I0 R CM
+ -
• linearization of thermistors and Si sensors - linearization by parallel R connection point of inflexion inflexní bod Ist
Si-senzor
1,0 negastor
RP
0,5
Rϑ
0
a)
Ji 0
50
b)
100 J[°C]
Linearity error DJ [°C] linearity chyba
1,5
5 4 3 2 +1 0 -1 -2 -3 -4 -5 0
negastor
Si-senzor
c)
50
100 J[°C]
- linearization by means of voltage divider: Rϑ
RS
I
negastor
RS
a)
Si sensor
Rϑ b)
Uv
series-parallel connection of negastors for linearization :
Rϑ
Rϑ
R2
Rϑ Rϑ
RP R2
RS
a)
R1
b)
R2
R3
Rϑ
Rϑ
Rϑ
c)
Remark to linearization: digital nonlinearity correction better than analogue (e.g. smart sensors) - more important is reproducibility of transfer characteristic than its linearization
