system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When a tank is filled with helium does the tank become lighter or heavier?

I just picked up a small tank of Helium from the store for work and was suprised by how light it was. Is the tank filled with Helium heavier or lighter than it was before the helium was added?
gpt
The helium in your tank is compressed. If it weren't, you couldn't use it to blow up balloons: if the pressure on both sides is the same, there isn't any flow. A side effect of this is that you can get more mass in the same volume. And as a result, your helium tank will be heavier than the tank's volume of helium at standard pressure and temperature.