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Calibration and Field Testing of a Two-probe Gamma Gauge¹

ABSTRACT

A series of laboratory and field calibration tests was carried out with a commercial double-probe gamma gauge in an attempt to resolve difficulties encountered in routine field use of the equipment. The laboratory tests showed that transmitted count rate is affected by the extent of the medium surrounding the beam path, indicating that linearity between log of count rate and sample density is not sufficient proof of monochromatic radiation. The equipment was also found to be sufficiently sensitive to require the use of separate attenuation coefficients for soil solids and for water. A universal calibration equation for wet density can therefore not be used with this equipment. Due to the above two phenomena, the calibration stand supplied with the instrument does not yield a calibration curve adequate for accurate work in the field range of soil moisture. A valid calibration therefore requires measurements of moisture and density in a large soil volume over a range of conditions. In view of the technical difficulties in meeting these conditions in the laboratory, empirical field determination of a multipoint calibration curve appears to be the best choice. Once such a curve has been developed, simple two-point calibration checks of unknown soils could be performed in the laboratory, provided the assumptions of linearity between log of count rate and medium density, and of the effective universality of the attenuation coefficient of soil may be taken as valid. When properly calibrated, the equipment tested is sensitive and reliable for the determination of soil bulk density in the field.

¹ Contribution from the Seagram Centre for Soil and Water Sciences, The Hebrew University of Jerusalem, The Faculty of Agriculture, P.O. Box 12, Rehovot 76-100, Israel. This investigation was funded in part by the International Atomic Energy Agency and by the Government of the Netherlands as part of a cooperative project with the Tillage Laboratory of the State Agricultural University, Wageningen.

² Soil Physicist, Research Assistant, and Irrigation Extension Specialist, respectively.

Received for publication June 30, 1980. Accepted for publication December 23, 1981.