When the Sample Thief strikes the bottom of the tank, a plunger assembly opens to admit the sample. The plunger closes again when the sampler is withdrawn, forming a tight seal. Samples can be taken at any depth with the use of a secondary trip line. The thief is equipped with a plunger locking cam for tight closure during transport. The FS 220 takes a 16 oz. (473 ml) fluid sample, and is made of plated brass. The FS 225 option is the same unit as the FS 220, but made from stainless steel.