Proceedings of the Technical Session of Institute of Physics, Sri Lanka 11 (1995) c1

Some Characteristic Features of Radiation Field Signatures from Tropical Cloud Flashes
K.P.S.C. Jayaratne and W.J.M. Samaranayake
Department of Physics,University of Colombo, Colombo 3.

Although the majority of lighting discharges occurs within the confines of a thundercloud, a very few studies have been made on the characteristics of cloud flashes compared to the more easily measurable and deleterious ground flashes.

Measurement of fast electric field variations due to cloud flashes were made in Colombo, Sri Lanka during the April-May, 1993 lighting period. The lighting electromagnetic radiation field picked up by a flat plate antenna was fed to a Gould 3040, 20MHz transient recorder through an ultra-fast buffer amplifier with 10 ns rise time and I ms time constant. Waveforms appeared on the screen were recorded on VHS video tapes using a video camera with a macro lens.

Bipolar pulses were found to be the most common type of could flashes involve with the highest current moment. The mean value of zero crossing time of 161 bipolar flashes recorded in 8 thunderstorms was 4.76 ms (S. D. = 5.63 ms) with maximum and minimum values being 28.20 ms and 0.80 ms, respectively. The mean full-wave-cycle time was found to be 9.76 ms (S. D. =5.63 ms) with maximum and minimum values being 47.60 ms and 2.10 ms, respectively. It was observed that there exits more cloud flashes producing negative type bipolar pulses consisting with the raising of negative charge or the lowering of positive charge. The ratio of negative bipolar pulses to that of positive bipolar pulses was 4.19.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 11 (1995) c2

Damage to Structures from Ground Vibration Caused by Piling Operation
A.S. Pannila and H.P.N.J. Gunasekara
Calibration & Measurement Unit, CISIR, Colombo 7.

Ground-Borne vibration from piling operations often cause noticeable vibration in nearly buildings. Building vibration could cause discomfort or annoyance to occupants and damage to sensitive equipment or buildings. Damage to structures could be ranging from hairline cracks in plaster to major impairment to the building superstructure. In extreme causes vibration can be the cause of building collapse.

During last two years several piling operation with different types of piling method were carried out in the city of Colombo. It has been now realized that, the control of ground vibration due piling operation is an urgent need due mainly to an increasing number of complaints from the public. National standards with respect of building vibration is not available in Sri Lanka. However, as a result of several measurements carried out by CISIR during different types of piling operations revealed that a combination of British and German standards could be used in Sri Lanka in assessment of building vibration. A set of guidelines proposed by the CISIR in this paper was submitted to the Central Environmental Authority to be gazetted under National Environment act (NEA). Damage to building can occur as a result of dynamic stresses or strains on the structure. Dynamic strains are directly related to the "peak particle Velocity", which is normally measured in case if assessment of building vibration. The natural frequency and the degree of damping of the building can also have an effect. However, it is not possible to define universal criteria that could be used to predict structural damage to buildings because of many variables that are involved.

The paper provides some guidance abut what parameters should be measured, location of measurement and interpretation of data, with respect to damage criteria of buildings. A comparison of vibration data, with different types of piling operations and practical methods to reduce the vibration is also discussed.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 11 (1995) c3

Power Frequency Characteristics and Over Voltage Disturbances (impulses) in 230 VAG Electric Power Distribution System in Sri Lanka
K.P.S.C. Jayaratne and A.G. Dayananda
Department of Physics, University of Colombo, Colombo 3

Delicate electric and electronic equipment in the modern world demand a considerably stabilized power supply system with minimum disturbance. The normal voltage fluctuations (slow variations) in a power network are mainly caused by unbalanced supply and demand of power. However, disturbances of electric power transmission and distribution system are frequently caused by two kinds of transient voltages whose amplitudes may greatly exceed the peak value of the normal a.c. operating voltage. The first kind is lightning over voltages and the second kind is caused by switching phenomena.

This paper describes the variation of power frequency voltages, and over voltage disturbances due to switching and lighting impulses observed in a low voltage power installation network in Colombo-07.

Measurements were made at 15A/230 VAC service outlet to a AC power mains at the Department of Physics, University of Colombo. A bridge rectifier coupled to a step-down transformer was used for voltage measurements with an accuracy of ±0.5 V.A frequency to voltage converter was used to monitor the frequency variations with an accuracy of ±0.4 Hz. Recordings were made using a dual channel flatbed pen recorder. In order to distinguish lighting and switching impulses, surge voltages in the low voltage line were measured simultaneously with the lightning electromagnetic signal picked-up by an out-door broad band plate antenna set-up coupled to a 4 channel 300 MHz LeCroy transient recorder.

Measurements made during the period May-June, 1994 indicated an average line voltage of 235 V with a maximum peak value 245 V and a bear minimum 214 V. The diurnal variation of ac voltage averaged for 30 days during the test period was found be triple period oscillatory type. The three maximum were observed around mid-night, 1230 hours with half hour average voltages 238.5 V, 234.7 V, and 236.7 V, respectively. Those maxima are tallyed with domestic/industrial non-working hours. The three voltage minima obseved were at around 1030 hours, 1500 hours. and 1845 hours with half hour average voltages 231.5 V, 232.5 V, respectively. The first two minima are corresponding to the maximum power consumption at industries/offices during day-time, while the last is due to after-dark lighting. The average frequency during this period was 49.8 ±0.4Hz and it indicated no significant variation during the test period. The total number of surge occurrence measured in 30 days during the test period was 228. It was found that 71 of them were due to lighting induced overvoltages and 157 due to switching impulses.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 11 (1995) c4

Level of Exhaust Emission from Diesel Vehicles in Sri Lanka
H.P.N.J. Gunasekara, A.S. Pannila and S.I. Perera
CISIR, Colombo 7.

With the increasing volume of traffic in Colombo and suburbs it was observed that pollution of air has been getting worse during the last few years as indicated by measurements taken by several institutions.

As the first step the Central Environmental Authority (CEA) has imported 65 smoke meters from Sweden on the recommendation of former Ministry of Environment Parliamentary Affairs to monitor the level of smoke emitted by diesel vehicles with a view to get the owners to reduce excessive emissions from their vehicles. Though excessive emissions could be visually it would not be possible to prosecute the owners in the basis of visual observation alone.

Prior to gazetting regulations specifying the minimum smoke level permissible, CISIR carried out a survey on the level of smoke emitted by diesel vehicles. The measurements were carried out in free acceleration mode on about 75 vehicles of different types (cars, lorries, buses, trucks etc.)

This survey revealed that about 50% of the vehicles checked has smoke levels measured in terms of opacity above 90%. However the maximum allowable limit of smoke varies from country to country some of these values are India-75%. Thailand-52%, Malayasia-50% and Sweden 45%. None of these values can be directly implemented without any idea of emission levels of diesel vehicles in Sri Lanka. In this paper we present the levels of smoke emitted from different types of vehicles and the suitable recommendation to implement this in Sri Lanka.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 11 (1995) c5

A Study of Variation of Track Registration Properties of Cr-39 with Humidity
A.D. Chithrani¹ and P. Mahawattha^2
1Department of Physics,University of Colombo, Colombo 3.,²Radio Isotope centre, University of Colombo, Sri Lanka

Solid state Nuclear track detection techniques can be successfully applied in many areas in nuclear science. Radiation dosimetry, Uranium exploration and earth quake prediction are some examples.

CR-39 is a solid state Nuclear track detector which is widely being used in Randon measurements in different environmental conditions. The main aim of this study was to study the effect of humidity on alpha track registration properties of CR-39. Environments of different Relative Humidities were created using saturated chemical solutions.

Proceedings of the Technical Session of Institute of Physics, Sri Lanka 11 (1995) c6

Long-term trends of rainfall and temperature in Sri Lanka
Lalith Chandrapala
Department of Meteorology, Colombo 7

Rainfall fluctuations in Sri Lanka for the 125 year period from 1871-1994 and the fluctuations of the annual air temperature during the period 1931-1990 are studied using trend analysis. Long term rainfall data of 34 meteorological stations and temperature data of 17 meteorological stations are used in the analysis. Fluctuations are studied within the five homogeneous rainfall fluctuation regions proposed by Suppiah and Yoshino (1984) using empirical function analysis.

Generally decreasing trends of rainfall during the Northeast monsoon, and the First intermonsoon seasons and increasing trends during the southwest monsoon and the second intermonsoon seasons during the recent period are observed. Annual rainfall also show a generally decreasing trend since mid-1960s.

Warming trend during the most recent period is observed over all regions. A liner regression analysis performed for the 30-year period 1961 to 1990 show an increasing trend of temperature between 0.009 to 0.026 oC/year over Sri Lanka.