What are the errors introduced in mass determination if the object being weighed is warm?

Answer 1

Anything that displaces a fluid feels an upward pressure equal to the weight of that volume of fluid. That is called buoyancy, or floating. It's hard to imagine a piece of steel floating, but the upward force of the displaced air reduces the measured weight. A warm object expands, so it displaces more air and is lighter.

Most materials, and all metals, absorb air and that increases their mass. A warm material might expel some of the air, removing some mass.

Air around the object rises when heated and the turbulence of moving air can cause random readings of the weight. Precision scales have doors to exclude air movements, and some scales use a vacuum chamber.

Antoin Lavoisier, the French guy who invented chemistry, wrote a book explaining how to reproduce his experiments. He devoted an entire section to describing his very precise scales and other instruments. One of them was in the basement of a building: the entire basement. The house was empty, existing only to cover the basement, and the basement was insulated with bark chips to prevent outside heat from causing inside air currents. There was a tiny door to load the scale, and a tiny window to observe the measurement. You can download his book "Elements Of Chemistry" Kindle edition.