Should the Standard Count Be Excluded from Neutron Probe Calibration?

Core Ideas Traditional count‐ratio‐based neutron probe calibration introduces unnecessary error. Count‐based calibration eliminates this error in water content measurement. Count‐based calibration is useful when the standard count is questionable or absent. The aging effect of a neutron probe over a period of 26 yr was negligible. About 6 decades after its introduction, the neutron probe (NP) remains one of the most accurate methods for indirect measurement of soil moisture content. Traditionally, the calibration of a NP involves the ratio of the neutron count in the soil to a standard count, which is the neutron count in the fixed environment such as the probe shield or a specially designed calibration tank. The drawback of this count‐ratio‐based calibration is that the error in the standard count is carried through to all the measurements. An alternative calibration is to use the neutron counts only, not the ratio, with proper correction for radioactive decay and counting time. To evaluate both approaches, the shield counts of a NP used for three decades were analyzed. The results show that the ground surface conditions have a substantial effect on the standard count. The error in the standard count also impacts the calculation of water storage and could indicate false consistency among replicates. The analysis of the shield counts indicates negligible aging effect of the instrument over a period of 26 yr. It is concluded that, by excluding the standard count, the use of the count‐based calibration is appropriate and sometimes even better than ratio‐based calibration. The count‐based calibration is especially useful for historical data when the standard count was questionable or absent.