The Model GV-100-S-TR Gas Sampling Pump can precisely collect a sample volume for a detector tube. The full-stroke (100 mL) and the half stroke (50 mL) positions are marked exactly by the red line on the pump shaft, and the handle is precisely locked at those positions. If you pump fully "n" times by allowing for sampling time intervals, a volume of 100 mL x "n" can be sampled. Each detector tube is calibrated based upon a prescribed (standard) volume of sample. Also, the pump piston has been designed with a smaller diameter so that the handle can be pulled out with even less effort.