I know that gauge pressure takes the atmospheric pressure as a zero point whereas absolute pressure measures the pressure in the open air or something along those lines. Does a barometer measure pressure within the tube itself? By that logic it is not measuring pressure in the open air but gauge pressure instead. If that is truly the case, then how are we able to obtain the atmospheric pressure using the gauge pressure?
A mercury barometer measures absolute pressure. There is a vacuum above the mercury in the top of the tube. If there were no atmospheric pressure, a vacuum, surrounding the barometer, it would be at 0 inches where the mercury levels inside and outside of the tube would be level because there would be zero pressure inside and zero pressure outside of the tube. Any atmospheric pressure will push the mercury up inside the tube. The weight of the raised mercury per square inch will equal the external atmospheric pressure per square inch.