A syringe is filled with air by pulling the plunger to exactly 50 mL.
A nail with pliers is heated over a tea light to melt a hole into the syringe plunger in line with the top of the syringe barrel. (This is to make sure that it does not slip back).
The nail is removed again, the plunger is pushed and the syringe locked with a sealed connector.
The syringe plunger is pulled to the 50 mL mark in the sealed syringe and fixed in that position by inserting the nail.
The empty syringe is weighed, its mass recorded.
The syringe closure is removed,.The syringe together with the closure and the nail are weighed together, their mass recorded..