Transcript met163_lecture_4

Met 163: Lecture 4
Chapter 4
Thermometry
This week
TUE: thermometry lecture
THU: how to obtain data from RAWS, RAWS data analysis using MATLAB
software, RAWS break-down.
Outline
• Introduction
• Thermal expansion
- bimetallic strip, Liquid-in-glass thermometer
• Thermoelectric Sensors
- thermocouple
• Electrical Resistance Sensors
- thermistors, Resistance Temperature Detectors (RTD)
• Exposure of Temperature Sensors
Air Temperature
The measurement of air temperature dates back to the
time of Galileo(1564-1642).
It is perhaps the premier measurement of meteorological
parameters.
Measurements of temperature are not free of errors or
problems due to response time, accuracy of instrument,
and operational need.
Errors in the measurement of air temperature in excess of
2-3 °C are not uncommon.
Galileo thermometer
- made of a sealed glass cylinder
containing a clear liquid and several
glass vessels of varying densities.
- As the temperature of most liquids
increases, their density decreases.
Air Temperature
Errors in the measurement of air temperature in excess of
2-3 °C are not uncommon.
These errors are acceptable for the general public.
However, numerical models of all scales of motion are
greatly affected by errors as large as 1 °C.
Errors of just 1°C in mesoscale models have been shown
to be the deciding factor between no storms initiated and
intense storms.
Additionally, errors as small as 0.2 °C can change the
prediction of a global climate model.
Errors in temperature measurements…
http://usatoday30.usatoday.com/weather/story/2012/09/14/death-valley-now-thehottest-spot-in-the-world/57775492/1
September 2012: The Libyan temperature record of 136 degree F from 1922 was
determined to be invalid because of the combination of a poor weather instrument, a
location in a bad spot for accurate readings and an inexperienced record-keeper.
http://www.wunderground.com/deadheat
Air Temperature
Temperature sensors can be categorized according to the
physical principle that they use:
- Thermal expansion
- Thermoelectric (thermocouples)
- Electrical resistance
- Electrical capacitance
and some other effects…
Direct indicating instruments can use displacement
directly: (liquid-in-glass thermometers, bimetallic strips)
Sensors designed to work with data loggers usually
convert the output to a voltage signal.
Air Temperature
Raw output of a temperature sensor may be displacement,
voltage, resistance, capacitance, etc.
T
Y1 = X, V, R, C
Temperature Sensor
Air Temperature: Thermal Expansion
One way thermal expansion is exploited to sense
temperature is through the use of bimetallic strips and
liquid-in-glass thermometers.
In both cases the difference between expansion
coefficients of two materials is used.
Linear expansion is given by:
ΔL=αL0Δ
ΔV=βV0ΔT
Where α is the coefficient of linear expansion, L0 is the length of the
material when ΔT = 0, ΔL is the change in length and ΔT is the temperature
change from some arbitrary temperature where L0 was measured.
Air Temperature: Thermal Expansion
Air Temperature: Thermal Expansion
Bimetallic Strip: A bimetallic strip is a pair of metals with
different thermal expansion coefficients that have been
bonded together.
At the reference temperature, the temperature at which
bonding took place, the strip maintains its original shape.
When the temperature changes, the strip bends in a
circular arc, for small deflections, due to the differential
expansions of the two components of the strip, where one
end is held in a fixed position.
http://video.mit.edu/watch/mit-physics-demo-bimetallicstrip-12046/
Air Temperature: Thermal Expansion
KTL
y
t
2
Bimetallic strip applications
Fire alarm: the heat of the fire causes the brass and iron strips to
expand, resulting in the bending of the bimetallic strip, which on
bending touches the screw adjacent to it, thus completing the
circuit. Once the circuit is complete, the bell begins to ring.
Thermostats: these sense the air temperature using the differential
expansion of two metals to actuate an on/off switch; the system
will be switched on when the temperature drops below the set
point on the thermostat, and switched off when it rises above,
- By coiling a very long strip it becomes much more
sensitive to small temperature changes.
Air Temperature: Liquid-in-glass Thermometer
A Liquid-in-glass thermometer is a glass tube with a bulb at
one end filled with the liquid and a scale fastened to or
etched on the glass tube.
The liquid is usually mercury or alcohol. The freezing and
boiling points of mercury are -39 °C and 357 °C,
respectively. Alcohol can be used down to -62°C.
Q: Why isn't water used as the liquid in the common
thermometer?
A: If water was used in the common liquid thermometer we
wouldn't be able to measure anything over 100 °C or under
0°C
Air Temperature: Liquid-in-glass Thermometer
Air Temperature: Liquid-in-glass Thermometer
One way to classifying these thermometers is by the
immersion required. Immersion types are partial, total and
complete.
A partial immersion thermometer should be placed in the
bath liquid until the bulb and a small portion of stem
(indicated by an immersion line) are immersed in the liquid
to be measured.
For a total immersion thermometer, the bulb and the portion
of the stem containing the thermometric fluid are immersed.
Partial and total types are used for calibrating other
sensors.
Air Temperature: Liquid-in-glass Thermometer
A complete immersion thermometer: the bulb and the entire
stem are immersed. This type is used for air temperature
measurement.
Air Temperature: Liquid-in-glass Thermometer
Two special liquid-in-glass thermometers are used to
measure the minimum and maximum temperature: a
minimum and maximum thermometer.
Air Temperature: Liquid-in-glass Thermometer
The minimum thermometer uses alcohol with a dumbbell in
the stem.
The minimum thermometer must be mounted horizontally
to work properly.
The alcohol flows around the dumbbell as the temperature
increases and leaves the dumbbell in a fixed position.
When the temperature decreases, the meniscus of the
alcohol does not let the dumbbell pass but drags it down to
indicate the minimum temperature.
To reset it, the thermometer is simply tipped upside down.
Air Temperature: Liquid-in-glass Thermometer
The maximum thermometer uses mercury and has a
constriction in the stem.
The bulb is mounted slightly higher than the rest of the
column.
As the temperature increases, the mercury in the
thermometer expands and moves freely up the tube past
the constriction. When the air temperature begins to drop,
the constriction prevents the mercury from flowing back
down the tube.
Can be reset by shaking it.
Air Temperature: Liquid-in-glass Thermometer
Met 163: Lecture 4
Chapter 4
Thermometry
Thermoelectric Sensors
The junction of two dissimilar metals forms a
thermocouple.
When the two junctions are at different temperatures, a
voltage is developed across the junction.
By measuring the voltage difference between the two
junctions, the difference in temperature between the two
can be calculated.
Or, If the temperature of one junction is known and the
voltage difference is measured, then the temperature of
the second junction can be calculated.
Thermocouple
Tair
Tref
The temperature of the measuring junction (Tair)
can be calculated if the temperature of the
reference junction (Tref) is known and the
voltage difference is measured.
Thermocouples
Thermocouples provide:
-A wide useful temperature range,
-Are inherently differential,
-Are rugged
-Reliable and inexpensive
-And usually have a fast response.
The main disadvantage of thermocouples:
-Very low output, on the order of 40 μV/ °C.
-Slight nonlinearity
-And need for calibration.
Thermocouples
There are some observed laws of thermocouple behavior
used as a rule-of-thumb guide to thermocouple circuit
design and construction.
Thermocouples
The thermoelectric effects: when one junction has a
different temperature than the other, an electromotive
force is produced in the circuit and current flows.
The magnitude of the force or potential depends on the
temperature difference between the two junctions.
There are three components of the thermoelectric effects:
The Seebeck effect, Peltier effect, and Thomson effect.
Thermocouples: Seebeck effect
The Seebeck effect: the conversion of thermal energy to
electrical energy.
This effect measures the ease at which excess electrons
will circulate in an electrical circuit under the influence of
thermal difference.
The change in the voltage is proportional to the
temperature difference between the junctions when the
ends are connected to form a loop.
Seebeck effect: demo
http://www.youtube.com/watch?v=jlMERuu4IiU
Thermocouple
Tair
Tref
The temperature of the measuring junction (Tair)
can be calculated if the temperature of the
reference junction (Tref) is known and the
voltage difference is measured.
Seebeck Effect
Thermocouples: Peltier effect
The Peltier effect: closely related to the Seebeck effect. It
represents the thermal effect due to a reversible current
through dissimilar materials or through similar metals due
to an external source of current.
A current flow in one direction might warm the junction of
the two dissimilar materials (and release heat to the
surroundings of that junction), whereas if the current was
reversed, the junction would cool (and absorb heat from
its surroundings).
Thermocouples: Peltier effect
Thermocouples: Thomson effect
The Thomson effect: the absorption or liberation of heat
by a homogeneous conductor due to a current flowing
through it.
It is primarily evident in currents introduced from external
sources and those generated by the thermocouple itself.
The ability of a given material to generate heat with
respect to both a unit temperature gradient and a unit
current, is gauged by the Thomson coefficient.
The importance of the Peltier and Thomson effects is
essentially infinitesimal because the heat evolved is
negligible compared to the amount of thermal energy
available from the environment to the junctions of T1 and
T2.
The (Thermocouple) Thermoelectric Laws
The three fundamental empirical laws behind the
accurate measurement of temperature by
thermoelectric means are the:
1. Law of homogeneous materials
2. Law of intermediate materials
3. Law of intermediate temperatures.
Law #1: the voltage across a thermocouple is unaffected
by temperatures elsewhere in the circuit, provided the
two metals used are each homogeneous.
Thus one can use lead wires made of thermocouple
metals.
The (Thermocouple) Thermoelectric Laws
2. Law of intermediate materials
Law #2: If a third metal is inserted in either A or B and if
the two new junctions are at the same temperature, no
effective voltage is generated by the third metal. This
means that a real voltmeter (or amplifier) can be used.
The terminals of a voltmeter are usually made of a third
metal and can be close together. It is important to
make sure the terminals of the voltmeter are at the
same temperature.
The (Thermocouple) Thermoelectric Laws
3. Law of intermediate materials
Law #3: If a metal C is inserted in one of the AB junctions,
then no net voltage is generated so long as junction AC
and BC are at the same temperature.
This means that the two wires or a junction can be
soldered together and the presence of the third metal,
solder, will not affect the voltage if there is no
temperature gradient across the solder junction.
The (Thermocouple) Thermoelectric Laws
T1
V1
A
V3
+
G
A
B
Ref.
T2
Fig. 4-5 (a)
V2
Thermocouples
Common thermocouple types
Type
Metal
T
J
E
K
Copper and constantan
Iron and constantan
Nickel (10% chromium and constantan)
Nickel and Nickel (5% aluminum/silicon)
Thermocouples
A thermocouple is inherently a differential temperature
sensor; it measures the temperature difference
between two junctions.
Absolute temperature measurements can be made only if
one of the junctions is held at a known temperature or
if an electronic reference junction is used.
A block of metal (aluminum, copper, or any highly
conductive metal) can be used for the reference
temperature.
This is done by inserting the reference junction of the
thermocouple in the block and simultaneously
measuring the temperature of the block.
Thermocouples
The Campbell Scientific data loggers have this metal
block underneath the wiring panel.
The CSI data loggers have special instructions in their
programming language that allows for thermocouple
measurements.
See the CR1000 manual.
Thermocouple output voltages
Thermocouples
The most common type of thermocouple used for
meteorology is the copper-constantan (Type-T).
Its range of use varies from -200°C - 350°C, but is mostly
used in the -60°C to 100°C with an accuracy of ±0.5°C.
Another common type used in meteorology is the Type-E.
Electrical Temperature Sensors
One whose resistance varies as a function of temperature.
- Thermistors
- Resistance Temperature Sensors (RTDs)
Thermistors
Thermistor or thermal resistor is a hard, ceramic-like
electronic semi-conductor, commonly made from a
mixture of metallic oxide materials.
Have a very large negative resistance coefficient (i.e., an
increase in T by 1°C yields a decrease of 5% in
resistance).
Thermistors
RTD: Resistance Temperature Detectors
• Platinum is most commonly used for precision
resistance thermometers because it is stable, resists
corrosion, is easily workable, has a high temp melting
point, and can be obtained to a high degree of purity.
• Simple and stable resistance-temperature relationship.
• Platinum is sensitive to strain; bending the sensor can
change the resistance.
RTD: Resistance Temperature Detectors
Resistance of a platinum sensor is given by
RT  R0 (1  aT  bT )
2
with sufficient accuracy for the meteorological
temperature range -50 to 50°C. R0 is resistance at 0°C
RT= resistance of sensor at temperature T°C
Coefficients depend on purity of platinum;
a = 0.00385 or 0.00392°C-1
Because the RTD resistance is fairly low and the change
with temperature is small a bridge circuit is often used.
RTD: Resistance Temperature Detectors
Because the RTD resistance is fairly low and the change
with temperature is small a bridge circuit is often used.
The bridge circuit converts resistance to voltage and can
be amplified to a reasonable level using an instrument
amplifier (CR1000 data loggers have this circuit built
in).
RTD: Resistance Temperature Detectors
RTD: Resistance Temperature Detectors
RTD: Resistance Temperature Detectors
RTD: Resistance Temperature Detectors
Campbell Scientific
Exposure of Temperature Sensors
Unaspirated Radiation Shield
Error in Unaspirated Radiation Shield