Trip Tanks are often circulated over the hole while tripping and loss is monitored usually electronically from the drillers console. Every 5 stands the calculations have to be quickly made to make sure the hole is taking the proper amount of fluid. Not filling properly would indicate a gas kick or swabbing the hole as well as a few other possibilities. A small tank (trip tank) is used because it is easier to get a more accurate reading than if you left the flow line lined up on the pit system. We run a 90 bbl trip tank