3 Instrumental set up and data analysis

The details of the present experiment have earlier been described by Pant & Sagar (1998). For recording the microthermal fluctuations at a level, pair of thermal sensors made of thin (25 $\mu$ in diameter) Nickel wire is used. The horizontal separation between the two sensors is one metre. They are mounted at heights of 6, 12 and 18 m above the ground on the mast. Thus, the inter-spacing between the microthermal sensors is $\sim\! 6$ m. The length of a sensor-wire is chosen in such a way that its effective resistance comes out to be $\sim\! 250$ ohm at room temperature. This yields a temperature resolution of $\sim\! 0.01\hbox{$^\circ$}$C for the system. For recording the instantaneous ambient temperature around the microthermal sensors with an accuracy of $0.1\hbox{$^\circ$}$C, Pt-100 RTD temperature sensor and ADAM-4013 analog input module are used. Analog input module ADAM-4013 uses a microprocessor controlled integrating A/D converter to convert RTD signals into digital format. The data are recorded in a PC with the help of the ADAM series of analog input modules and immediately processed in the PC. The ADAM series is a set of intelligent sensor-to-computer interface modules, containing built in micro-controller yielding 16-bit resolution in A/D conversion. They are remotely controlled. The commands are transmitted in RS-485 protocol. The signals from the sensors and RTDs are connected to the electronic circuit inside the observing room by cables. A DC current is fed through these sensors whose resistance changes according to microthermal fluctuations and consequently, produces variable voltage signal for differential amplifier. The amplifier output is sent to a signal processing equipment. The GENIE software package was used for data acquisition, processing and control. Atmospheric pressure is almost the same within a height of 18 m. Its observed value at Devasthal is $\sim\! 810\pm 2$ mb which has been used in our further data analysis.