Transcript Slide 1

Measurement of Temperature
•Practical Temperature Measurement
•Temperature Measurement Presentation
•Defining and measuring
temperature
•Thermal Time Constant
•Measurement Errors
•RTD’s
•Thermistors
•I.C. Sensors
•Thermocouples
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Defining Temperature
• A scalar quantity that determines the
direction of heat flow between two bodies
• A statistical measurement
• A difficult measurement
• A mostly empirical measurement
http://www.m-w.com/dictionary.htm Temperature: degree of
hotness or coldness measured on a definite scale
http://www.m-w.com/dictionary.htm Empirical:
originating in or based on observation or experience
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The Reaumur temperature scale is named after the French scientist (1683-1757).
He proposed his temperature scale, in 1731. Reaumur divided the fundamental
interval between the ice and steam points of water into 80 degrees, fixing the ice
point at 0 Degrees and the steam point at 80 degrees. The reaumur scale,
although of historical significance, is no longer in use.
Temperature Systems
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Measuring Temperature
• Don't let the measuring device change the
temperature of what you're measuring.
• Response time is a function of
– Thermal mass (mass of the device e.g large
Thermistor vs small Thermistor)
– Measuring device (type of device e.g. RTD or
Thermocouple)
• The Thermal Time Constant for a thermistor is the
time required for a thermistor to change its body
temperature by 63.2% of a specific temperature span
when the measurements are made under zero-power
conditions in thermally stable environments.
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The dominant factors that affect the T.C. of a thermistor are:
– The mass and the thermal mass of the thermistor itself.
– Custom assemblies and thermal coupling agents that
couple the thermistor to the medium being monitored.
– Mounting configurations such as a probe assembly or
surface mounting.
– Thermal conductivity of the materials used to assemble
the thermistor in probe housings.
– The environment that the thermistor will be exposed to
and the heat transfer characteristics of that
environment.
• Typically, gases are less dense than liquids so
thermistors have greater time constants when
monitoring temperature in a gaseous medium than in
a liquid one.
http://www.betatherm.com/t_c.html
Thermal Time
Constant
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750C
56.60C
250C
3τ
1τ
2τ
4τ
5τ
• Example: A thermistor is placed in an oil bath at 25°C and allowed to
reach equilibrium temperature. The thermistor is then rapidly moved to
an oil bath at 75°C. The T.C. is the time required for the thermistor to
reach 56.6°C (63.2% of the temperature span [difference]).
Thermal Time Constant
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Temperature Errors
• What is YOUR normal
temperature?
• Thermometer
accuracy, resolution
• Contact time
• Thermal mass of
thermometer, tongue
• Human error in
reading
95%
Confidence
interval
http://www.amstat.org/publications/jse/v4n2/datasets.shoemaker.html
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The Resistance Temperature
Detector (RTD)
• RTD: Most accurate, Most stable, Fairly linear
–
–
–
–
Expensive (platinum)
Slow (relative)
Needs I source (changing resistance)
Self-heating (don’t change the measurement due
to the internal current!)
– 4-wire measurement (must take the resistance of
the leads into account)
http://www.temperatures.com/sensors.html
http://www.minco.com/sensorsg.php
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• RTDs are among the most precise
temperature sensors commercially used.
They are based on the positive temperature
coefficient of electrical resistance.
RTD’s
http://www.efunda.com/designstandards/sensors/rtd/rtd_intro.cfm
http://www.omega.com/
http://www.sensorsmag.com/articles/article_index/index.htm
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RTD Linearity
R=RRef[1+α(T-TRef)]
R=100[1+.00385(70-60)]
=103.85 ohms
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RTD
Measurement
DDC RTD Measurement
To balance the bridge:
R1R3=R2R4
Dissipation Constant
The power in milliwatts required to raise a thermistor 1°C above the
surrounding temperature is the dissipation constant.
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http://www.tiptemp.com/sense/Sense_RTD_TechData.pdf
To estimate leadwire error for a 2-wire configuration, multiply the total
length of the extension leads by the resistance per foot in the table shown
below. Then divide by the sensitivity of the RTD, given in the table below
to obtain an error in C°.
Example: You are using a 100 platinum RTD with a TCR of 0.00385
and 50 ft. of 22 AWG leadwire.
R = 50 ft. x 0.0165/ft. = 0.825
Approximate error = 0.825 / 0.385 = 2.14°C
4-wire circuit
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Thermistors
• Advantages:
– High output
– Fast
– 2-wire
measurement
NTC Thermistor Shown
RT
R25
• Disadvantages
–
–
–
–
–
Very nonlinear
Limited range
Needs I source
Self-heating
Fragile
http://www.embedded.com/story/OEG20020125S0100
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• Commonly used for sensing air and liquid temperatures in
pipes and ducts, and as room temperature sensors. Unlike
RTD's, the temperature-resistance characteristic of a
thermistor is non-linear, and cannot be characterized by a
single coefficient.
• The following is a mathematical expression for thermistor
resistance1: R(T) = R0 exp[b (1/T - 1/T0)]
• Where: R(T) = the resistance at temperature T, in K,
R0 = the resistance at reference temperature T0, in K,
b = a constant that varies with thermistor composition
T = a temperature, in K,
T0 = a reference temperature (usually 298.15 K)
• Because the lead resistance of most thermistors is very
small in comparison to sensor resistance, three and four
wire configurations have not evolved. Otherwise, sensing
circuits are very similar to RTD's, using the Wheatstone
bridge
1Beckwith,
Thomas G., Roy D. Marangoni, and John H. Lienhard V. Mechanical
Measurements. New York: Addison and Wesley, 1993. Pp. 673
DDC Thermistors
Thermistors
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http://www.omega.com/Temperature/pdf/44000_THERMIS_ELEMENTS.pdf
Thermistor
Equation
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ACI Thermistor Data
8,000
7,000
Ohms
6,000
5,000
4,000
3,000
2,000
1,000
0
0
20
40
60
80
100
Degrees Centigrade
Thermistor Curve
http://www.workaci.com/pdf/t-19.pdf
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Eq. Temp
273.15
-0.01241
9.990243
14.9908
19.99065
24.99121
29.99101
34.99036
39.98917
44.98954
323.15
49.98872
54.98818
59.988
64.98751
69.98734
74.98829
79.98832
84.9899
89.99019
94.99223
373.15
99.99313
Thermistor Circuit
The Omega Thermistor equation is:
1/T =A+B*Ln(R)+C*(Ln(R))3
.003661=A+8.903B+705.65C
.0030945=A+6.698B+300.52C
.0026799=A+5.0293B+127.21C
To use this equation you write 3 simultaneous
eqs. In 3 unknowns and solve. The eqs. Used
the values at 0, 50 and 100 Celsius, with the
Kelvin values shown on the left.
The final equation is:
1/T =A+B*Ln(R)+C*(Ln(R))3 with
A = 1.472E-3, B=237.5E-6, and C=105.9E-9
The resulting temperatures from the equation
are shown here and are almost identical to the
given values.
The resulting graph from the Eq. is
indistinguishable From the original graph from
the table.
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• Advantages
– High output
– Most linear
– Inexpensive
I.C. Sensors
•Disadvantages
–Limited variety
–Limited range
–Needs V source
–Self-heating
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I.C. Sensors
LM34: $2.33 from DigiKey
AD590: $5.24 from Analog Devices
• AD590 (Analog Devices)
– Current Output – Two Terminal IC Temperature Transducer
– Produces an output current proportional to absolute
temperature. For supply voltages between +4 V and +30 V
the device acts as a high impedance, constant current
regulator passing 1 µA/K.
• LM34 (National Semiconductor)
– The LM34 is a precision integrated-circuit temperature
sensor, whose output voltage is linearly proportional to the
Fahrenheit temperature.
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AD590 & LM34
Circuits
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Conversion from Kelvin to
Fahrenheit
We know that 273.150K = 00C = 320F AND
373.150K=1000C=2120F so
we can write two linear equations in two unknowns.
32 = 273.15m + b
212=373.15m + b
Solving these for m and b yields:
m = 1.8
b = -459.67
the linear conversion equation is
0F
= 1.8*(0K) – 459.67
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AD590 Conversion to
Fahrenheit in mV
+10
-.45967volts
AD590
1KΩ
180KΩ
100KΩ
1mV/0K
180KΩ
-
-1mV/0F
+
0F
= -1.8*(0K) + 459.67
in mvolts
Use an inverting amplifier to get positive output
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Thermocouples
• Advantages:
–
–
–
–
Wide variety
Cheap
Wide T. range
No self-heating
• Disadvantages
–
–
–
–
Hard to measure
Relative T. only
Nonlinear
Special connectors
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Seebeck and Peltier
Effects
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Seebeck coefficient in a
circuit exhibiting the
Seebeck effect, the ratio of
the open-circuit voltage to
the temperature difference
between the hot and cold
junctions.
Thermocouples
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Thermocouples
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Thermocouples
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Thermocouples
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Thermocouples
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Summary
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Defining and measuring temperature
Thermal Time Constant
Temperature Errors
RTD’s
Thermistors
I.C. Sensors
Thermocouples
Next
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