The 21839, manufactured by Sentek, provides a 1 m EnviroSCAN probe without the clip-on sensors or seals. It consists of an SEN21010 1.5 m Access Tube, a 1 m probe rod, SEN22400 Top Cap Assembly, and a handle set. Up to ten click-on water-content sensors can be added. Each sensor measures water content at one position on the probe; one sensor per measurement depth is required.