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  • Author or Editor: G. W. THURTELL x
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G. W. Thurtell
and
C. B. Tanner

Abstract

A transistorized, battery-operated integrator suitable for obtaining the time average of fluctuating electrical analogue signals in the field with relative errors of about 0.1 to 0.2 per cent is described.

A solid state operational amplifier is used in an R-C integrator circuit. Continuous integration is accomplished by reversing the polarity of the input signal each time a fixed time-integral has occurred. The addition of a resistor to the ordinary R-C integrating circuit reduces the error resulting from the finite switching time. The integral is obtained by counting the input polarity reversals which are registered on a counter (mechanical or electronic) to be automatically recorded at desired intervals.

The use of transistor converters allows the complete circuit as well as additional equipment to be operated several days from one 12V automobile battery.

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M. J. Heikinheimo
,
G. W. Thurtell
, and
G. E. Kidd

Abstract

A fast response C02 and water vapor (H2O) analyzer was developed in this study for the measurement of atmospheric turbulence fluctuations and, in conjunction with a fast response anemometer, transport of these entities. High speed and high resolution detection of C02 and H2O was accomplished simultaneously for the same air sample with open path IR absorption techniques. The miniature size sensor features a once folded 12.5 cm pathlength. The sensor structure and sampling technique provided direct compensation for undesirable factors (e.g., dust) affecting transmission of IR-radiation in the optical path and for the changes in source intensity and detector response due to sensor temperature fluctuations.

The rms-noise level of the C02 and H2O outputs corresponded to 0.3 ppm and 0.02 g m−2 fluctuations respectively, when absorbing gases were removed from the path. Spectral analysis showed that the noise was well below the signal level for both gases within the frequency range from 0.002 to 5 Hz. In comparison with the commercially available Lyman-α hygrometer the H2O output noise level of the IR-sensor was a factor of two larger, but the simultaneously measured signal spectra and also the latent heat flux values obtained with the two sensors in the field were practically identical. The IR-sensor produced C02 flux values that were comparable to those reported in the literature.

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S. M. McGinn
,
K. M. King
, and
G. W. Thurtell

Abstract

The objective of this study was to improve the accuracy of net radiation (R n ) measurements under conditions conducive to dew or frost deposition. Two nonventilated net pyrradiometers were mounted over grass during November and December 1986. A heating element was located on the supporting arm of each radiometer so that heal would be conducted to the sensing head. Heat was applied to one radiometer for a few days, followed by a period during which no heat was applied. The procedure was repeated, alternating between radiometers throughout the experiment. Heating the radiometers successfully averted the deposition of dew and frost on the domes, which produced errors in R n as high as 54 W m−2. The effect of heating alone was slightly asymmetric and resulted in a significant decrease in the mean R n of 8 W m−2 relative to the unheated radiometer. This effect can be compensated for in the calibration of the radiometer.

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M. L. Wesely
,
G. W. Thurtell
, and
C. B. Tanner

Abstract

A three-dimensional pressure-sphere anemometer and fast thermometer system (P.S.A.T.) was used to measure vertical heat flux density in the atmospheric surface layer at 1–4 m above alta fescue and snap beans. Good agreement with independent measurements was obtained, which shows the the P.S.A.T. is sufficiently small and has adequate high-frequency response and accuracy for eddy correlation measurements within 1 m of the surface. Also obtained with the P.S.A.T. were (uT )/( wT ), r u,T , r w,T and σ T /T* , and their dependence upon stability. When the atmosphere was thermally stable, slow wave motions frequently increased σ T even though turbulent mixing was lacking.

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G. W. Thurtell
,
C. B. Tanner
, and
M. L. Wesely

Abstract

A rugged and stable pressure-sphere anemometer system is described which Provides an accurate measurement of wind velocity and direction within a meter of the ground. The horizontal wind velocity, (u 2 + v 2)½, agreed very closely with cup anemometer measurements, indicating good accuracy in the measurement of the dominant term u. Eddy correlation measurements of shear stress with the pressure sphere agreed very well with Davis shear-stress meter measurements and satisfactory agreement was found with data obtained from wind velocity profiles and from wind measurements using a drag coefficient. Ratios of σ w /u * during neutral periods were found to be in excellent agreement with values derived by Panofsky and Lettau, providing further indication of the accuracy obtainable with the pressure-sphere system.

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J. P. Kerr
,
G. W. Thurtell
, and
C. B. Tanner

Abstract

The silicon photovoltaic solar cell has made possible the construction of simple pyranometers of reasonable accuracy. Cell response is linear, temperature sensitivity is low, and spectral response does not cause serious error, provided the cell is used in open sunlight.

The solar cell has been mounted beneath a special diffusing unit to obtain a rugged pyranometer with excellent cosine response. This pyranometer has been coupled with a solid state integrator developed for this purpose. The integral is recorded with either visual or printing counters.

Tests were made during September 1965 through February 1966 when low solar altitude and severe operating conditions would cause greatest error; and again during March 1966 through July 1966 when solar radiation intensities were high. For the first period the standard error of estimate and the solar radiation means were, respectively, 84 and 2200 Wh m−2 day−1. For the second period the corresponding values were 158 and 5630 Wh m−2 day−1.

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J. P. KERR
,
G. W. THURTELL
, and
C. B. TANNER

Abstract

The time and space variability of global radiation have been studied using data collected from a mesoscale network of integrating pyranometers established in Wisconsin, for the period December 1966 through June 1967. The data have been normalized so that they are expressed as a percent of the clear day global radiation. The atmospheric transmission coefficient over the State changes from about 0.75 in winter to 0.60 in summer. For a typical month, the standard deviations of the State daily mean varied from a few percent up to 50 percent of the State mean. Mean day-to-day changes of approximately ± 18 percent-radiation were recorded. From use of records for any one site in the State, the global radiation elsewhere in the State can be estimated with an approximate standard error of ±25 percent or less of the clear day radiation on a daily basis, ± 15 percent or less on a 5-day basis, and ± 10 percent or less on a monthly basis. Alternatively, if the network data from the sites surrounding the unknown point can be used for interpolation, the global radiation anywhere in the State can be estimated with an approximate standard error of ± 20 percent or less of the clear day radiation on a daily basis, ± 10 percent or less on a 5-day basis, and ± 6 percent or less on a monthly basis.

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