Made of rubber, this stopper is mainly used on the STM 2150
There are two types of rubber stoppers :
Not pierced, ref. STM 21303 used to close the tensiometer tubes, STM type during measures.
Pierced (with a hole), ref. STM 213031t which will be placed by the operator on the tip of the syringe used to prepare the tensiometer tubes, STM 21750 type or STM 2000S.