Transducers Temperature sensors (i) thermistors (ii) RTD’s (iii) thermocouples (iv) digital November 5, 2015 MECH4600.9.pptx 1 Resistance Temperature Detector (RTD) • The thermistor (thermal resistor) is a semi-conductor device, that relies on
the change in resistance of a semi-conductor material as the temperature
changes.
• It’s precision is limited and it is not useful at high temperatures.
• The RTD is made of a metal that changes in resistance as the temperature
changes.
• Examples of metals used for RTD’s are platinum, copper, nickel and silver.
• RTD’s can be used between -260°C to 1000°C. The higher limits are
associated with the melting points of the metals.
• RTD’s have a highly linear resistance vs temperature curve.
• However, the change in resistance of an RTD is very small relative to that
of a thermistor.
• Most RTD elements consist of a length of very fine wire coiled around a
ceramic or glass core.
• The element is very fragile, and is therefore placed inside a sheathed probe
to protect it. November 5, 2015 MECH4600.9.pptx 2 Copper RTD ⎛ L⎞
• The resistance of a electrical conductor is
R = ρ⎜ ⎟
⎝ A⎠
• ρ is the resistivity, L is the length of the conductor, and A is the cross-sectional
area of the conductor.
• The resistivity is a function of the temperature, with resistivity increasing with
temperature.
(
)
ρ = ρTref ⎡⎣1+ α T − Tref ⎤⎦
• The reference temperature is usually taken to be 20°C.
• For copper:
−8
ρTref = 1.68 × 10
ohm·m
α = 0.00386 °C-1
• The standard copper RTD has a resistance of 10 ohms at 20°C.
November 5, 2015 MECH4600.9.pptx 3 Resistance$of$Conduc9ng$Metal$
1.00$
Not an RTD! A stator winding with 18 m of 21 AWG copper wire Resistance$of$conduc9ng$metal$(ohms)$
0.95$
0.90$
0.85$
0.90 ohms at 80°C 0.73 ohms at 20°C 0.80$
0.75$
R = 0.73 ohms, i = 2.5 amps
0.70$
Ploss = i 2 R = 2.5 2 × 0.73 = 4.6 W @ 20°C
0.65$
Ploss = i 2 R = 2.5 2 × 0.90 = 5.6 W @ 80°C
0.60$
0$
10$
20$
30$
40$
50$
60$
70$
Temperature$of$conduc9ng$metal$(°C)$
Resistivity.xlsx
November 5, 2015 80$
90$
100$
2015/10/19
MECH4600.9.pptx 4 Resistance$of$Conduc9ng$Metal$
14.0$
Cu10 RTD: 1.207 m of 44 AWG copper wire (D = 51 µm) Resistance$of$conduc9ng$metal$(ohms)$
13.5$
13.0$
Diameter of human hair is about 100 µm 12.5$
12.0$
11.5$
12.32 ohms at 80°C 10.00 ohms at 20°C 11.0$
10.5$
10.0$
9.5$
9.0$
0$
10$
20$
30$
40$
50$
60$
70$
Temperature$of$conduc9ng$metal$(°C)$
Resistivity.xlsx
November 5, 2015 80$
90$
100$
2015/10/19
MECH4600.9.pptx 5 PlaPnum RTD • Platinum is the most widely used metal for RTD’s.
• Platinum can be employed for temperatures ranging from -200°C to 600°C.
• For platinum:
ρTref = 1.04 × 10 −7 ohm·m α = 0.00385 °C-1
• The typical platinum RTD has a resistance of 100 ohms at 20°C (Pt100 RTD).
• They are generally fabricated from 50 AWG wire (D = 25 µm).
• Platinum is very strong and it is not a problem to handle 25-µm thick
platinum wire.
Pt wire wound around a ceramic core Pt thin film on a ceramic substrate Pt wire coils inside a ceramic core November 5, 2015 MECH4600.9.pptx 6 Resistance"of"Conduc6ng"Metal"
Resistance"of"conduc6ng"metal"(ohms)"
350"
300"
Pt100 RTD: 0.49 m of 50 AWG Pt wire (D = 25 µm) 250"
200"
150"
284.7 ohms at 500°C 100.0 ohms at 20°C 100"
Slope =
284.7 − 100.0 ohms
= 0.38 ohms/°C
500 − 20°C
50"
0"
'100"
Need a very sensiPve ohmmeter 0"
100"
200"
300"
400"
Temperature"of"conduc6ng"metal"(°C)"
Resistivity.xlsx
November 5, 2015 500"
600"
2015/10/19
MECH4600.9.pptx 7 Using a Pt100 RTD (for 0°C < T < 300°C) ⎛ 92.3 ⎞
V 2 0°C = 11.5 ⎜
= 2.706
⎝ 92.3 + 300 ⎟⎠
IN
OUT
PE
PRTD = ( 29 mA ) × 92.3 ohms=78 mW
2
November 5, 2015 300
R14
R17
V2 2.4k
R15
243
IC2
• Over a ∆T of 300°C we have a 2000 mV
voltage change, that is 6.7 mV/°C, or
0.15°C/mV.
• We should be able to detect
temperatures changes of 0.15°C.
• Consider the “self-heating” of the RTD.
2
PRTD = iRTD
RRTD
iRTD ADJ
⎛ 207.8 ⎞
V 2 300°C = 11.5 ⎜
= 4.706
⎝ 207.8 + 300 ⎟⎠
11.5 VDC
iRTD =
= 29 mA
92.3 + 300 ohms
11.5 VDC TRTD = 0°C RTD = 92.3 ohms This will “heat” the RTD RTD
TRTD = 300°C RTD = 207.8 ohms MECH4600.9.pptx 8 2015-10-19
12:49 PM f=2.69 /Users/peterallen/Documents/Docs1/Dal.Docs/MECH4600/Thermistor/RTD.lo
Using a RTD (for 0°C < T < 300°C) 11.5 VDC IN
OUT
ADJ
1.46 k R14
PRTD
0.005
=
= 7.4 mA
RRTD
92.3
11.5 V
11.5 V
Rtotal =
==
= 1.55 kohms
iRTD
7.4 mA
iRTD =
PE
92.3 ⎞
⎛
V 2 0°C = 11.5 ⎜
= 0.684
⎝ 92.3 + 1460 ⎟⎠
207.8
⎛
⎞
V 2 300°C = 11.5 ⎜
= 1.433
⎝ 207.8 + 1460 ⎟⎠
R15
R17
V2 2.4k
R17 = Rtotal − 92.3 = 1.46 kohms
November 5, 2015 243
IC2
iRTD 300
• The self-heating coefficient for an RTD
is 30 – 60 mV/°C (in water, nat. conv.).
• The 78 mW of power dissipation could
lead to a 2 – 3°C error.
• Reduce the current through the RTD to
5 mW of power, which will lead to an
error of about 0.1 – 0.2°C.
TRTD = 0°C RTD = 92.3 ohms RTD
• Over a ∆T of 300°C we have a 749
mV voltage change, that is 2.5 mV/
°C, or 0.4°C/mV – still OK.
2015-10-19 12:49 PM f=2.69 /Users/peterallen/Documents/Docs1/Dal.Docs/MECH4600/Thermistor
MECH4600.9.pptx 9 Low Temperature Controller 207.8
⎛
⎞
V 6 = 11.5 ⎜
⎝ 207.8 + 1460 ⎟⎠
V 6 = 1.433
R19
R16
300k
If TRTD is “low” then turn on a heater with the op amp. R29
2k
R8 = 0.65 k (adjustable) 1.46k
10k
11.5 VDC 5
YELLOW
LED1
1.5k
IC1B
7
R8
0.65 + 0.75 ⎞
⎛
V 5 = 11.5 ⎜
⎝ 0.65 + 0.75 + 10.0 ⎟⎠
6
LM324N
R21
PE
V5 < V6, therefore, V7 = 0 (the “low T” switch is OFF) November 5, 2015 R23
750
V 5 = 1.412
TRTD = 300°C RTD = 207.8 ohms RTD
If TRTD drops, RTD will drop, V6 may drop below V5 (fixed), and the op amp will be acPvated. MECH4600.9.pptx THERMISTOR-5
10 Self-­‐heaPng of an RTD • Consider an RTD cylindrical element that is 15 mm long and 1.5 mm in diameter.
• The RTD is used to measure the temperature of air flowing at 1 m/s.
• The heat transfer coefficient in this case is about 85 W/m2·K, assuming cross flow
over the cylinder.
q = hA∆ T
q
= hA
∆T
A = π DL = π × 1.5 mm × 15 mm = 71 mm 2
Neglects any heat removed by the leads of the RTD q
= hA = 85 W/m 2·K × 71 mm 2 = 0.0060 W/°C
∆T
q
= 0.0060 W/°C = 6.0 mW/°C
∆T
∆T
= 0.17°C/mW
q
November 5, 2015 5 mW means an error of 1°C. We may have to reduce RTD current by a factor of 3, which will reduce heaPng by a factor of 10. MECH4600.9.pptx With water h is orders of magnitude higher, which means ∆T/q is orders of magnitude lower. 11 Ceramic Wire-Wound
Platinum RTD Elements
-200 to 60
0
(-330 to 11 °C
10°F)
Class A (IEC751), Alpha = 0.00385
We calculated 0.17°C/mW, The “KN” Series RTDs are suitable for applications requiring extremely high temperature stability and high temperature shock resistance.
neglecPng the RTD leads. Deviation from the IEC751 characteristic curve is minimal over the entire temperature range. The small diameter tolerances of the sensor body
allow easy installation in protective tubes. Applications are found in chemical and power generation plants and with analytical equipment.
To Order 15 mm x 1.5 mm Dimensions†
in millimeters
(1 mm =
0.03937")
15
25
30
30
30
Nominal
Resistance Temperature
(Ohms)
Range, °C (°F)
Model Number
C°/mW
Flowing Air
V = 1 m/sec
Response Time in Seconds
Flowing Water
Moving Air
V = 0.4 m/sec
V = 1 m/sec
50%
90%
50%
90%
Response Response Response Response
1.5
1 x 100
-200 to 600
(-330 to 1110)
1PT100KN1515CLA
0.08
0.2
0.6
5
18
1.5
1 x 100
-200 to 600
(-330 to 1110)
1PT100KN2515CLA
0.08
0.2
0.6
5
18
1 x 100
-200 to 600
(-330 to 1110)
1PT100KN3045CLA
0.21
0.2
0.6
2.5
9
1 x 100
-200 to 600
(-330 to 1110)
1PT100KN3026CLA
0.06
0.3
0.7
15
50
2 x 100
-200 to 600
(-330 to 1110)
2PT100KN3045CLA*
0.08
0.2
0.6
5
18
2 x 100
-200 to 600
(-330 to 1110)
2PT100KN3026CLA*
0.06
0.3
0.7
15
50
4.5
2.6
4.5
2.6
30
0.08°C/mW = Heating
12.5 mW/°C SelfError
in
† Leads are 10 mm long.
* Dual element.
Ordering Examples: 1PT100KN1515CLA, 1 x 100 1 ceramic wire-wound element.
2PT100KN3026CLA, 2 x 100 1 ceramic wire-wound.
Enlarged to show
Discount Schedule
construction.
November 5, A2015 MECH4600.9.pptx See above
(for Class
and Class B elements)
for dimensions.
1 to 4 units . . . . . . . . . . . . . Net
e For
12 Resistance#of#Conduc8ng#Metal#
120#
Suppose we need to measure temperatures precisely in the range 50 to 70°C. Resistance#of#conduc8ng#metal#(ohms)#
119#
118#
117#
116#
The range of resistance is from 111.5 to 119.2 ohms, which is a very small change in resistance to detect.. 115#
114#
113#
112#
111#
110#
50# 51# 52# 53# 54# 55# 56# 57# 58# 59# 60# 61# 62# 63# 64# 65# 66# 67# 68# 69# 70#
Temperature#of#conduc8ng#metal#(°C)#
Resistivity.xlsx
November 5, 2015 2015/10/29
MECH4600.9.pptx 13 The Wheatstone Bridge • The Wheatstone bridge is used
1. to measure an unknown resistor by
comparison to standard resistors
2. to detect small changes in a
resistance transducer (e.g. a
thermistor).
• For the first application, we need a calibrated variable resistance (R4), two
fixed known resistors (R1 and R2), and the unknown resistance (R3).
• R4 consists of the series combination of a fixed resistance and a variable
resistance (to make fine adjustments possible). R4 is adjusted until the
detector voltage is zero, at which point R3/R4 = R1/R2. Usually, R1 is
made equal to R2, so R3 = R4 at balance.
• For the second application, R3 is the resistive transducer. The bridge is
initially balanced to "zero the baseline", then any changes in the
transducer's resistance (R3) are detected by recording the detector voltage
as it varies from zero.
November 5, 2015 MECH4600.9.pptx 14 The Wheatstone Bridge • The Wheatstone bridge is two voltage dividers.
• One voltage divider is formed by R1 and R3.
• The other is formed by R2 and R4.
• The outputs (the junctions of the two resistors) are
connected to a sensitive voltmeter, assumed to have very high input
resistance (typically 10 Mohms).
⎛ R3 ⎞
• The output voltage of the first voltage divider is
Vb ⎜
⎟
⎝ R3 + R1 ⎠
⎛ R4 ⎞
• The output voltage of the second voltage divider is
Vb ⎜
⎝ R4 + R2 ⎟⎠
• When two voltages are equal the bridge is balanced.
• This happens when
⎛ R3 ⎞
⎛ R4 ⎞
Vb ⎜
=
V
b⎜
⎝ R3 + R1 ⎟⎠
⎝ R4 + R2 ⎟⎠
R3
R4
=
• Cancelling out Vb
R3 + R1 R4 + R2
November 5, 2015 MECH4600.9.pptx 15 The Wheatstone Bridge and the RTD • We wish to measure a resistance that is close to 115 ohms (R3 = RRTD= 115 ohms).
• Set R4 = 115 ohms.
• We wish to keep the current to a low value to
prevent self-heating, say 6 mA,
which will yield 4 mW of heating (very small).
• Set R1 = 1.5 kohms, which will produce a current in R3 of 6.2 mA (4.4 mW
of heating; OK).
115
⎛ R4 ⎞
⎛
⎞
V
=
10
=
10
⎜⎝
⎟⎠
⎜⎝
⎟⎠ = 712 mV
2−4
R4 + R2
115 + 1500
• If the RTD senses a temperature of 50°C, then R3 = 111.5 ohms.
111.5
⎛ R3 ⎞
⎛
⎞
V1−3 = 10 ⎜
=
10
⎜⎝
⎟⎠ = 692 mV
⎝ R3 + R1 ⎟⎠
111.5 + 1500
• The voltmeter will detect a voltage difference of -20 mV.
• Similarly, if the RTD senses a temperature of 70°C, then the voltmeter will
detect a voltage difference of 732 – 712 = +20 mV.
• Over a temperature range of 20°C, the voltage changes from -20 to 20 mV.
November 5, 2015 MECH4600.9.pptx 16 Temperature vs resistance for 10 kohm thermistor!
65!
64!
63!
Temperature (°C)!
62!
61!
60!
59!
Thermistor response (red) Linear response (green) 58!
57!
56!
55!
2.0!
2.1!
2.2!
2.3!
2.4!
2.5!
2.6!
Resistance (kΩ)!
Thermistor 10 k
November 5, 2015 2.7!
2.8!
2.9!
3.0!
2015-10-29
MECH4600.9.pptx 17 Temperature vs resistance for 10 kohm thermistor!
25!
24!
Thermistor response 23!
Temperature (°C)!
22!
Linear response 21!
20!
19!
18!
17!
16!
15!
10.0! 10.5! 11.0! 11.5! 12.0! 12.5! 13.0! 13.5! 14.0! 14.5! 15.0! 15.5! 16.0!
Resistance (kΩ)!
Thermistor 10 k
November 5, 2015 2015-11-02
MECH4600.9.pptx 18 Temperature vs resistance for 10 kohm thermistor!
25!
Thermistor response (red) 24!
Temperature (°C)!
23!
22!
21!
Linear response (green) 11.3 kohm resistor in parallel with the 10-­‐kohm thermistor 20!
19!
18!
17!
16!
Why an 11.3 kohm resistor? At ≈ 22°C, Rth ≈ 11.3 kohms. This axis is the combined resistance of the thermistor and its parallel resistor. 15!
5.2!
5.4!
5.6!
5.8!
6.0!
Resistance (kΩ)!
6.4!
6.6!
2015411402
Therm_Wheatstone.xlsx
November 5, 2015 6.2!
MECH4600.9.pptx 19 20k
R2
20k
+
RTH
11.3k
R4P
6.27k
R3
THERMISTOR-5
3V60R
mV
-
5VOLTS
R1
Wheatstone Bridge -­‐ Thermistor Wheatstone Bridge
2015-11-03 10:32 AM f=3.08 /Users/peterallen/Documents/Docs1/Dal.Docs/MECH4600/Thermistor/Whetastone_thermistor.sch (Sheet: 1/1)
November 5, 2015 MECH4600.9.pptx 20 Wheatstone Bridge -­‐ Thermistor R1-3
R2-4
Req
i
26.27
25.93
13.05
0.38
kohms
kohms
kohms
mA
Therm_Wheatstone.B.xlsx
November 5, 2015 mV
49.5
-43.3 ∆mV
-34.2 9.1 0.11
-25.1 9.1 0.08
-15.9 9.2 0.06
-6.7
9.2 0.04
2.6
9.3 0.02
11.9
9.3 0.01
21.2
9.3 0.01
30.6
9.4 0.00
40.0
9.4 0.00
49.5
9.4 0.00
58.9
9.5 0.00
68.4
9.5 0.01
78.0
9.5 0.01
87.5
9.5 0.02
97.1
9.6 0.02
106.7 9.6 0.03
116.3 9.6 0.03
125.9 9.6 0.04
135.5 9.6 0.04
145.1 9.6 0.04
9.4 0.17
18.8 mV/°C
Thermistor"in"a"Wheatstone"Bridge"
25"
24"
y"="0.053x"+"17.351"
R²"="0.99993"
23"
22"
Temperasture,"°C"
T
NTC
0.50
a
2,923.9
20 °C 15.00
b
4.8657
Tt
15.50
c
-0.0054 293.2 K 16.00
Y
3889
16.50
R
12.492 kohms
17.00
17.50
18.00
R1
20.00 kohms
18.50
R2
20.00 kohms
19.00
R3
6.27 kohms
19.50
RP4
11.30 kohms
20.00
RTH
12.49 kohms
20.50
R4
5.93 kohms
21.00
21.50
Vin
5.00 volts
22.00
22.50
V1-3
1.193 volts
23.00
V2-4
1.144 volts
23.50
∆V
49.5 mV
24.00
24.50
iTH
0.10 mA
25.00
i2RTH
0.12 mW
21"
20"
19"
18"
17"
""T"vs"mV"
""T"vs"mV"
16"
Linear"(""T"vs"mV)"
15"
350"
MECH4600.9.pptx 0"
50"
mV"3"voltage"difference"
100"
150"
2015511503
21 Thermocouples • In 1821 Thomas Seebeck, a German physicist, discovered that a voltage is
created when two dissimilar metals are joined in a loop, and the two
junctions are held at different temperatures.
Copper wire T1 T2 (≠ T1) V, i Thermocouple Constantan wire • One of the temperatures is the temperature that we wish to measure (T2).
• The other temperature (T1) is a known reference temperature.
• T1 has been measured by an RTD, a thermistor, or by some other nonthermocouple means.
• The thermocouple must be connected to
the DVM with copper-constantan leads.
T2 • The above is known as the thermoelectric DVM effect, or Seebeck effect.
November 5, 2015 MECH4600.9.pptx 22 Type#T#Thermocouple#(Tref#=#0°C)#
18#
16#
14#
Thermocouple#volatge#(mV)#
12#
Every temperature measuring device has its own reference temperature. For thermocouples it is 0°C. 10#
8#
6#
4#
2#
0#
4.3 mV at 100°C !2#
0.0 volts at 0°C !4#
!6#
!8#
!300#
!200#
!100#
0#
100#
Temperature#(°C)#
Type_T_TC.xlsx
November 5, 2015 200#
300#
400#
2015/10/19
MECH4600.9.pptx 23 Type#T#Thermocouple#(Tref#=#0°C)#
6#
A Type T thermocouple (at Tx) is measured with a voltmeter. The measured voltage is 2.76 mV. The voltmeter is at 20°C. Thermocouple#volatge#(mV)#
5#
4#
2.76 V + 0.79 V = 3.55 mV 3#
3.55 mV at 84°C (T2) 0.79 volts at 20°C (T1) 2#
1#
A high resoluPon digital voltmeter is required. Most DVM only read to the nearest 0.1 mV 0#
!1#
!20#
0#
20#
40#
60#
Temperature#(°C)#
Type_T_TC.xlsx
November 5, 2015 80#
100#
120#
2015/10/19
MECH4600.9.pptx 24 Electronic Digital Temperature Sensor Dime Electronic digital temperature sensor DQ NC GND November 5, 2015 MECH4600.2.pptx 25 temperature register that stores the digital output from the temperature sensor. In addition, the scratchpad
provides access to the 1-byte upper and lower alarm trigger registers (TH and TL). The TH and TL
registers are nonvolatile (EEPROM), so they will retain their data when the device is powered down.
Electronic Digital Temperature Sensor DS18B20-PAR BLOCK DIAGRAM Figure 1
VPU
PARASITE POWER
CIRCUIT
4.7K
MEMORY CONTROL
LOGIC
DS18B20-PAR
DQ
TEMPERATURE SENSOR
INTERNAL VDD
64-BIT ROM
AND
1-wire PORT
SCRATCHPAD
CPP
ALARM HIGH TRIGGER (TH)
REGISTER (EEPROM)
ALARM LOW TRIGGER (TL)
REGISTER (EEPROM)
CONFIGURATION REGISTER
(EEPROM)
GND
8-BIT CRC GENERATOR
Only 2 leads: ground (GND) and data (DQ) November 5, 2015 2 of 19
Electronics inside the “sensor” MECH4600.2.pptx 26 Electronic Digital Temperature Sensor • The DS18B20 digital thermometer provides 9 to 12–bit celcius temperature
measurements and has an alarm function with nonvolatile user-programmable
upper and lower trigger points.
• The DS18B20 does not need an external power supply. It derives power
directly from the DQ (data line; “parasite power”).
• The DS18B20 communicates over a 1-Wire bus, which requires only one data
line (and ground) for communication with a microprocessor.
• It has an operating range of –55°C to +100°C and is accurate to ±0.5°C over a
range of –10°C to +85°C.
• Each DS18B20 has a unique 64-bit identification code, which allows multiple
DS18B20’s to function on the same 1–wire bus.
• It is possible to use one microprocessor, e.g., an Arduino, to control many
DS18B20’s distributed over a large area.
• Applications include HVAC environmental controls, temperature monitoring
systems inside buildings, equipment or machinery, and process monitoring and
control systems.
• http://www.hacktronics.com/Tutorials/arduino-1-wire-tutorial.html
November 5, 2015 MECH4600.9.pptx 27 Sign: plus or minus November 5, 2015 MECH4600.2.pptx 28 How to read a digital temperature sensor? DATA GND USB cable to computer November 5, 2015 5 VDC MECH4600.2.pptx 29