The cavity used in the present work is 74 cm long, thus giving an aspect ratio of 15 to 20. As stated earlier, the resulting flow is expected to be in the form of cells with an axis parallel to the longer side. The light beam of the interferometer averages the temperature field along the length of the cavity. The temperatures of the hot and cold walls are measured using ten 18 gage chromel-alumel thermocouples on each surface. Temperature on each surface is found to be constant within along the path of the light beam. The hot and the cold walls of the cavity reach steady state in about two hours. Environmental conditions in the laboratory are stable for three hours and data is colected over this duration.

At the Rayleigh numbers considered in the present investigation the fringes in the core of the cavity are not strictly stationary even after a sufficiently long time is allowed to elapse. To estimate the extent of this uncertainty  in the heat transfer data the following procedure has been adopted. Five interferometric image are recorded at the camera snapping speed of one picture every 20 minutes at predetermined instants of time. This time interval is requied to transfer light intensity data over pixels from the camera to the PC. The Nusselt number evaluated at 20 minute intervals. The uncertainty in this average Nusselt number in relation value is as high as with confidence.

The time required to collect five images consecutively is 100 minutes. This time is, however, small in comparison to the time scale of evolution of flow. The first image is typically collected after 8 to 10 minutes and the flow reaches steady state after 2 to 3 hours. Hence, the response time of the measurement system can be considered to be negligible.