Transcript met163_lecture_4

Met 163: Lecture 4
Chapter 4
Thermometry
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.
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.
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=αL0ΔT
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.
Air Temperature: Thermal Expansion
KTL
y
t
2
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. Mercury can be
used on above -39°C, its freezing point, alcohol can be
used down to -62°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.
Two special liquid-in-glass thermometers are used to
measure the minimum and maximum temperature. The
minimum thermometer uses alcohol with a dumbbell in the
stem.
The thermometer is mounted horizontally, 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.
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, and as the temperature increases the volume of
mercury in the bulb increases and mercury is forced
through the constriction.
When the temperature decreases, the column breaks at the
constriction. The remaining column above the constriction
indicates the maximum temperature.
Air Temperature: Liquid-in-glass Thermometer