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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 "1displacement" [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.

Table 5-2
Recommended Response Characterization for Meteorological Sensors

Meteorological variable  Sensor Specification
 Wind Speed
   Horizontal Starting Speed
Distance Constant
0.5 m/s
5 m
   Vertical Starting Speed
Distance Constant
0.25 m/s
5 m
Wind Direction Starting Speed
Damping Ratio
Delay Distance
0.5 m/s @ 10 deg.
0.4 to 0.7
5 m
Temperature Time Constant 1 minute
Temperature Difference Time Constant 1 minute
Dew Point Temperature Time Constant
30 minutes
- 30 °C to + 30 °C
Solar Radiation Time Constant:
Operating Range:
Spectral Response:
5 sec.
- 20 °C to + 40 °C
285 nm to 2800 nm

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.

 5.1 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

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