Geography Reference
In-Depth Information
5.6.4.1 Radiant temperature measurement
to 0.993, but using a single average emissivity of 0.985
will result in calculated temperatures of 29.00-28
69 C
.
There are many types of TIR thermometers available on
the market ranging from under $100 to several thousand
dollars (USA). These thermometers use similar tech-
nology to the satellite- and aircraft-based TIR imaging
sensors, to detect the radiation emitted in the TIR spec-
trum; however, they are typically designed to measure the
temperature of a single location, not to provide spatially
extensive images, and are sensitive to the entire TIR spec-
tral range rather than a narrow spectral range. Increased
price for TIR imaging sensors will generally provide bet-
ter optics, aiming capabilities and insulation. The latter
is important for obtaining accurate temperature, as it
reduces the sensitivity of the device to environmental
conditions such as rapid heating or cooling of the inter-
nal components of the thermometer, which will change
the amount of unintended TIR radiation reaching the
sensor. Experiments with an inexpensive TIR thermome-
ter (e.g., a TempTestr-IR from Oakton, 2005) showed
that even short-term exposure to direct solar radiation
in a climate-controlled building could substantially alter
temperature readings from the thermometer (by several
degrees Celsius). Increasing the insulation around the TIR
thermometer and limiting its direct exposure to extreme
conditions (such as direct solar radiation) can reduce the
uncertainty in the measurement.
Care must also be taken that the TIR radiant ther-
mometer is measuring only water temperature. These
devices have a distance to target/spot size ratio that will
indicate how wide the area being sensed is at different
distances from target. Inexpensive sensors will typically
have a lower ratio (e.g., 6:1, or 9 . 5 ), in which case the
spot diameter increases by 1 cm for every 6 cm greater
distance of the target. The target area for less expensive
devices will expand more rapidly with distance than for
a more expensive device (e.g., with a ratio of 75:1, or
0
.
(
3 K error). Ultimately, the change in temperature
resulting from the emissivity correction is likely to be
small relative to the other sources of error when using
inexpensive devices.
0
5.6.4.2 Kinetic temperature measurement
There are a variety of technologies used for the mea-
surement of T k . These can be divided into analog and
digital technologies and hand-held and self-contained
temperature-logging units. Analog thermometers, most
notably bulb thermometers, have traditionally been less
expensive and therefore easier to supply to many field
staff taking measurements. Digital thermometers may
be easier to read and are potentially more robust. Self-
contained data loggers are still the most expensive option,
but they can be used to monitor water temperature for
extended periods of time and in locations where reg-
ular access may be difficult. A combination of devices
can provide an expanded network of observations in
conjunction with an image acquisition as long as all
devices are calibrated and care is taken in selecting
measurement locations.
Thermometers will generally be used to measure water
temperature near the water surface since the measured
temperature will need to be read from the device and
recorded. Because inserting the thermometer into the
water will break any skin layer, such measurements will
always be of the bulk water, not the skin itself. Measure-
ments in the top 10 cm of water are most appropriate
for comparison with calibrated TIR images, although the
collection of simultaneous measurements can be difficult
due to personnel restrictions. Temperature measured by
data loggers mounted to the streambed tends to be lower
than that measured using near-surface thermometers and
TIR sensors. As with measurements of SST, this tempera-
ture difference is likely the result of thermal stratification
(see Section 5.6.3 for more details).
In the next sections, we apply the knowledge described
in this theoretical background to two examples of using
TIR remote sensing to monitor water temperature.
76 ). The ratio should be smaller if measurements are
made closer to the target in order to reduce the likelihood
of interfering objects (e.g., rocks, logs) being included in
the measurement.
These TIR radiant thermometers require that a single
representative
.
be specified, to ensure that the out-
put temperature is appropriate for the material being
measured. Higher temperature accuracy can be obtained
by using accurate emissivities, but for spectrally vari-
able targets, an accuracy limit is reached because the
target is a selective radiator and not, in fact, a gray-
body. For example, for distilled water at 300 K the
emissivities between 10 and 14
ε
5.6.5 Keypoints
When absolute water temperature is required, it is
necessary to collect validation data. If only relative
temperature differences are required, validation data
requirements can be reduced or eliminated.
μ
m range from 0.965
Search WWH ::




Custom Search