A method of estimating cerebral blood flow includes obtaining a measure of time-varying blood volume in the head, using an impedance plethysmography, obtaining a measure of time-varying blood volume in the scalp, and using the time-varying blood volume in the head and scalp to estimate cerebral blood flow. A steam tunnel sub-enclosure includes exhaust ducts collecting steam at the conveyor’s entrance and exit thereof to prevent steam from being released into the rest of the oven where it might interfere with ignition control equipment. A drift region is formed extending across the isolation region, the IGBT region, and the diode region, inside the semiconductor substrate. Additionally, a third pressure responsive surface area is defined on the valve spool and is selectively subject to pressurized fluid from one of the main pressure source and the control source. This makes it possible to accomplish miniaturization of the aeration tank 7, reduction in scale of the whole equipment and therefore reduction in initial cost.