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
Dec 24, 2015
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
2
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 Empirical: originating in or based on observation or experience
http://www.m-w.com/dictionary.htm Temperature: degree of hotness or coldness measured on a definite scale
3
Temperature Systems
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.
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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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Thermal Time Constant
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
6Thermal Time Constant
• 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]).
250C
750C
1τ 5τ4τ3τ2τ
56.60C
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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
http://www.amstat.org/publications/jse/v4n2/datasets.shoemaker.html
95%Confidenceinterval
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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.minco.com/sensorsg.php
http://www.temperatures.com/sensors.html
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RTD’s• RTDs are among the most precise
temperature sensors commercially used. They are based on the positive temperature coefficient of electrical resistance.
http://www.sensorsmag.com/articles/article_index/index.htm http://www.omega.com/
http://www.efunda.com/designstandards/sensors/rtd/rtd_intro.cfm
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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 ConstantThe power in milliwatts required to raise a thermistor 1°C above the surrounding temperature is the dissipation constant.
124-wire circuit
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. = 0.825Approximate error = 0.825 / 0.385 = 2.14°C
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Thermistors
• Advantages:– High output– Fast– 2-wire
measurement
• Disadvantages– Very nonlinear– Limited range– Needs I source– Self-heating– Fragile
http://www.embedded.com/story/OEG20020125S0100
NTC Thermistor Shown
RT
R25
14Thermistors
• 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
DDC Thermistors
1Beckwith, Thomas G., Roy D. Marangoni, and John H. Lienhard V. Mechanical Measurements. New York: Addison and Wesley, 1993. Pp. 673
15ThermistorEquation
http://www.omega.com/Temperature/pdf/44000_THERMIS_ELEMENTS.pdf
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Thermistor Curvehttp://www.workaci.com/pdf/t-19.pdf
ACI Thermistor Data
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
0 20 40 60 80 100
Degrees Centigrade
Oh
ms
17Thermistor 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
273.15
323.15
373.15
To use this equation you write 3 simultaneouseqs. In 3 unknowns and solve. The eqs. Used the values at 0, 50 and 100 Celsius, with the Kelvin values shown on the left.
-0.01241
9.990243
14.9908
19.99065
24.99121
29.99101
34.99036
39.98917
44.98954
49.98872
54.98818
59.988
64.98751
69.98734
74.98829
79.98832
84.9899
89.99019
94.99223
99.99313
Eq. Temp
The resulting temperatures from the equationare shown here and are almost identical to thegiven values.
The resulting graph from the Eq. is indistinguishable From the original graph from the table.
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
18I.C. Sensors
• Advantages– High output– Most linear– Inexpensive
•Disadvantages–Limited variety–Limited range–Needs V source–Self-heating
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I.C. Sensors
• 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.
LM34: $2.33 from DigiKeyAD590: $5.24 from Analog Devices
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AD590 & LM34Circuits
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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 + b212=373.15m + b
Solving these for m and b yields:
0F = 1.8*(0K) – 459.67
the linear conversion equation is
m = 1.8b = -459.67
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AD590 Conversion to Fahrenheit in mV
0F = -1.8*(0K) + 459.67 in mvolts
AD590
+10
1KΩ +
-100KΩ
180KΩ180KΩ-.45967volts
Use an inverting amplifier to get positive output
1mV/0K -1mV/0F
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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
25Thermocouples
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.
26Thermocouples
27Thermocouples
28Thermocouples
29Thermocouples
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Summary
• Defining and measuring temperature• Thermal Time Constant• Temperature Errors• RTD’s• Thermistors• I.C. Sensors• Thermocouples• Next