ELI5. If I exhale before going on a scale, then inhale deeply afterwards, will the reading on the scale increase, decrease, or not change?

Question

Of course the difference is quite miniscule but I became curious while standing on my scale exhaling to look leaner than I was. Unfortunately my scale isn’t precise enough to be able to test this so here I am. Any explanation would be greatly appreciated 🙂

In: Physics

Answer 1

You will gain mass by inhaling, but the reading on the scale won’t show any difference. That’s because the scale never actually measures your weight. It measures your weight plus the weight of the air above the scale. It’s just calibrated to show zero if there is nothing but air on top of it. (I.e. you are not standing on it).
Now when you stand on it, you are replacing some of the air with your body (which is heavier than air).
When you inhale, you are replacing some more of the ambient air, but this time only with air (which weighs the same).