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If a metal is sufficiently hot, it may be quenched in a coolant with the metal initially surrounded by a vapour film. Estimates of these vapour film thicknesses can be obtained using heat transfer data and photography. However. with both these techniques, the accuracy of the estimates can be poor.A new method is developed in which the boiling system is incorporated into an electrical circuit and the capacitance of the vapour film is continuously recorded using a capacitance-resistance measuring instrument. The instrument operates by incorporating the boiling system into a 100 kHz 4c circuit. The voltage across the system is multiplied by unit square waves in phase and 90" out of phase with the alternating current. From the multiplied signals. the capacitive and resistive components of the boiling system are deduced. With this instrument. vapour film capacitances may be accurately and conveniently monitored with high time resolution. ln these quenching experiments, the vapour film thicknesses (-100pm) are small compared with the dimensions of the metal undergoing film boiling (area-104 mm'). The vapour film thicknesses are therefore estimated from the capacitance data using the standard equation for a parallel-plate capacitor. i . Present addresses: Courtaulds Research, 72 Lockhurst Lane, Coventry CVh SRS, U K and Chemical Engineering Dcpartment, The Unhersity of Birmingham. PO Box 363, Birmingham Bl.5 2TT. UK. 766

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