Abstract: Disclosed herein is a process simulator for substituting a vessel that is to be measured. In any process where the liquid level is to be measured by a pressure instrument and where a bubble system is used to obtain the measurement, the process simulator provides an accurate comparative apparatus. It includes a tank containing the liquid which communicates with a bubble pipe removeably inserted into the tank in which the bubble pipe communicates with an air line. Seals are provided at the intersection of the bubble pipe and the tank to provide a sure fit and a scale indicator is disposed upon the tank to provide a technique for aligning comparative levels on the bubble pipe with their marks thereon.