5.2 Response Characteristics of Meteorological Sensors
The
response characteristics of the sensors used in meteorological monitoring
must be known to ensure that data are
appropriate for the intended application. For example, an anemometer
designed to endure the rigors experienced on an ocean buoy would not be
suitable for monitoring fine scale turbulence in a wind tunnel; the latter
application requires a more sensitive instrument with a faster response time
(e.g., a sonic anemometer). On the other hand, a sonic anemometer is
probably unnecessary if the data are to be used only to calculate hourly
averages for use in a dispersion model. Recommended response characteristics
for meteorological sensors used in support of air quality dispersion
modeling are given in Table 5-2. Definitions of terms commonly associated
with instrument response characteristics (including the terms used in Table
5-2) are provided in the following.
Calm.
Any average wind speed below the starting threshold of the wind speed or
direction sensor, whichever is greater [4].
Damping
ratio. The motion of a vane is a damped oscillation and the ratio
in which the amplitude of successive
swings decreases is independent of wind speed. The damping ratio, h, is the
ratio of actual damping to critical damping. If a vane is critically damped,
h=l and there is no overshoot in response to sudden changes in wind
direction [18] [19] [20].
Delay
distance. The length of a column of air that passes a wind vane
such that the vane will respond to 50% of a sudden angular change in wind
direction [19] The delay distance is commonly specified as "50%
recovery" using "10° displacement"
[2, 3].
Distance
constant. The distance constant of a sensor is the length of
fluid flow past the sensor required to cause it to respond to 63.2%, i.e., l
- l/e, of the increasing step-function change in speed [19,20].
Distance constant is a characteristic of cup and propeller (rotational)
anemometers.
Range.
This is a general term which usually identifies the limits of operation of a
sensor, most often within which the accuracy is specified.
Threshold
(starting speed). The wind speed at which an anemometer or vane
first starts to perform within its specifications20.
Time
constant. The time constant is the period that is required for a
(temperature) sensor to respond to 63.2%, i.e., l - l/e, of the step-wise
change (in temperature). The term is applicable to any
"first-order" sensors, those that respond asymptotically to a step
change in the variable being measured, e.g., temperature, pressure, etc.
Several
publications are available that either contain tabulations of reported
sensor response characteristics [18],
[21] or specify, suggest or recommend values for certain applications
[2, 3, 9]. Moreover, many manufacturers are now providing this
information for the instruments they produce [21]. An EPA workshop
report on meteorological instrumentation [3] expands on these
recommendations for certain variables.
Manufacturers
of meteorological instruments should provide evidence that the response
characteristics of their sensors have been determined according to accepted
scientific/technical methods, e.g., ASTM standards [22]. Verifying a
manufacturer’s claims that a meteorological sensor possesses the
recommended response characteristics (Table 5-2) is another matter; such
verification can accurately be accomplished only in a laboratory setting. In
leu of a laboratory test, one must rely on quality assurance performance
audit procedures (Section 8.4) - the latter will normally provide assurance
of satisfactory performance.
5. SYSTEM PERFORMANCE
5.1 System Accuracies
5.2 Response Characteristics of Meteorological Sensors
5.3 Data Recovery
5.3.1 Length of Record
5.3.2 Completeness Requirement
5.4 Recommendations
0.5 m/s