The difference of pressure between the outside air and the smoke-box gases may be measured by the difference of the water levels in the limbs of a U tube, one limb being in communication with the smokebox, the other with the atmosphere.
Each ejector is fitted with a non-return valve to prevent smokebox gases being drawn back through the ejector exhaust.
No one else had rectangular headboards carried across the center of the smokebox door.
Smaller, standard boilers were fitted, which resulted in the fitting of a distance piece on top of the smokebox saddle.
Back pressure and smokebox vacuum could be altered by raising and lower the brass choke positioned between the nozzles.