Abstract: A valve system (1, 100) for a vessel has a valve body (2, 101), and a valve stem (26) extending through the valve body (2, 101) and terminating in a plug (20, 102) for axial movement between open and closed position as the stem moves through the valve body. There is at least one probe passageway (12, 13, 108) through the valve stem (26, 121), at least one of which has a circumferential seal (9-11, 105-106) for engaging a probe inserted in the passageway and being exposed in a vessel The circumferential seal (9-11, 105-106) seals the inside of a vessel from atmosphere while allowing a probe to be inserted and removed by moving axially in the passageway. This avoids need to remove the valve system to insert or remove a probe, and indeed a single expensive probe can be used at different times with different valve systems. When there is no exposed probe required, the seal may be completed by simply inserting a dummy probe in the passageway.