The pressure of a liquid measured with the help of a piezometer tube is?
Either #2 or #3 it depends upon the type of piezometer used (sealed or unsealed). If sealed with a vacuum as the reference pressure it is an absolute pressure. Zero is referenced against a vacuum so the device will read 14.7 psi at seal level. If vented it is a gauge pressure and will reference zero to the roughly 14.7psi of the atmosphere the device will read zero at sea level. Edit: because of the venting the effect of the surrounding atmosphere is removed so at ground level (0 depth) the reading is always zero even if there are barometric changes in air pressure. A sealed piezometer at ground level will pickup variations in atmospheric barometric pressure. I suspect that the MOST CORRECT answer is #2 gauge because of the application to monitor liquid level. Liquid level is determined by density and the pressure of just the liquid the 14.7 psi of the atmosphere is not considered so at the surface the level would be zero.