Should the Standard Count be Excluded from Neutron Probe Calibration?.

After 6 decades of use, the neutron probe remains one of the most accurate tools for indirectly measuring soil moisture content. Usually the ratio of the neutron count to a standard count is used in the calibration of a neutron probe. The standard count is typically a count in the probe shield while it rests atop the carrying case or a count inside a large invariant calibration tank. Occasionally, standard counts are determined as the probe sits atop each access tube in the field. In this case, the count ratio-based calibration carries the error in the standard count through to all the loggings. An alternative calibration is to use the neutron count-based calibration, with proper correction of radioactive decay, counting time, and instrument difference. To evaluate both approaches, the standard counts and shield counts of a neutron probe used for three decades are analyzed. The results show that the impact of the conditions around each access tube on the standard count is nontrivial. The error in the standard count cumulates in the calculation of water storage and could indicate false consistency among replicates. The analysis of the on-the-tube shield counts indicated negligible aging effect of the instrument during a period of 26 years. It is concluded that the use of the count-based calibration is appropriate. The count-based calibration is especially useful for historical data when the standard count was questionable or absent.