The calibration of radon measuring instruments is the most important and key technical link to ensure the reliability and accuracy of radon observation data and plays a vital role in seismic monitoring and prediction.

This study aims to obtain a calibration method suitable for the calibration of newly connected seismic water radon monitor according to the National Metrological Verification Regulations for Radon Meters (JJG825-2013), the calibration experiment is carried out for the analog radon observatory (manual), which is newly adopted by the seismic network, and the calibration method suitable for the seismic water radon observation is obtained.

Firstly, the FD-125 radon-thorium analyzer was used as the experimental instrument, the calibration experiments were carried out in the standard radon chamber by three calibration methods: circulation method, vacuum method, and flow-gas method. Then, the volume response coefficients of three methods were calculated, and the uncertainty sources and mathematical models were analyzed according to JJG825—2013 regulation. Measured values by internationally recognized standard AlphaGUARDPQ2000Pro radon meter were taken as theoretical references of radon concentration for radon chambers, hence the uncertainties of three calibration methods were obtained.

The results show that the circulation calibration method has the highest detection efficiency and fewer influencing factors, and is suitable for manual and digital radon detector calibration. The vacuum calibration method is affected by the standard radon gas diluted by pipeline gas and pressure balance, resulting in low negative pressure sampling efficiency and high volume activity response coefficient R. As for the flow-gas calibration method, the continuous injecting of radon gas into the scintillation chamber affects the dynamic stability of radon gas in the scintillation chamber, and radon gas is not discharged to the background during calibration, resulting in the high volume activity response coefficient R and high intrinsic error.

The cyclic method is more suitable for the calibration of new seismic water radon observatories, and this study provides a reference for the calibration of other new water radon observatories and gas radon observatories.