Some ways for measurement accuracy improvement of microwave device UWSNP are considered. Basic of them are: taking into account errors under registration of frequency-dependent relative reflected power, choice of operation part of frequency range, careful plant calibration of each sample of the device. Using the device modification UWSNP-2M as an example under its on-site exploitation, an approach for optimal tuning of the device is suggested that takes into account factors jointly influencing on measuring accuracy.