So say I've got a pipe, and it's got some kind of an internal pressure. Now, it's completely enclosed, so atmospheric pressure cannot affect it. Then, if I wanted to measure the pipe's pressure by drilling a hole and attaching a piezometric tube, like in the picture, wouldn't I be providing an entrance to the atmospheric pressure, and thus the pressure inside the pipe would increase accordingly, according to the Pascal's law, which states that any increase in a fluid's pressure (pipe's contents are affected through the piezo tube) is transmitted undiminished? So the actual pressure inside the tube, P, would become P+Pa, which would mean to get the actual pressure we'd have to subtract Pa? I know I'm not making much sense here, but imagine a container with fluid in it. If we opened the container and allowed the fluid's surface to be in contact with it, the atmospheric pressure would increase the amount of pressure already present in the liquid,so the total pressure would be higher for Pa.