Generally it is more convenient to use an interior mounted pressure chamber from which air is exhausted to obtain a lower pressure on the interior surface of the specimen.
A calibrated rack of nozzles is then used to spray water at the proper rate on the exterior surface. Under circumstances where it is desirable to use an exterior-mounted pressure chamber, the spray rack must be located in the pressure chamber and air supplied to maintain a higher pressure on the exterior surface. Exterior chambers are difficult to attach readily and seal to exterior surfaces.