A Comparison of Fibreglass and Class 'a' Evaporation Pans

on the meteorological site at the Experiment station, evapoEvaporation from fibreglass pans painted silver was less than ration from the metal pan was greater. from Class 'A' (metal) pans. Painting the fibreglass pans with dark paint decreased the difference in evaporation but the seasonal differences were not uniform. It was also shown that placing a screen over both types of pans decreased evaporation while lowering the water level in a Class 'A'. pan increased evaporation. It was concluded that fibreglass pans should not be used in the sugar industry. Introduction n The South African sugar industry maintains a network of 29 Grade A meteorological stations throughout the cane belt. One w of the standard items of equipment which has been installed at each station is the United States Weather'Bureau (USWB) Class 'A' evaporation pan. It is constructed from 22 gge galvanised iron. The pan is 1,22 m in diameter, 0,25 m deep, and has a hook gauge situated in a stilling well. The water in the pan is replenished to the level of the hook gauge each day. ~~~~~~~~i~~ from the class 'A' pan is used to estimate the FIGURE 1 Successive positions of seven fibreglass pans and one standard Class 'A' pan in Experiment 2. amount of water lost as a result of evauotransuiration in the field. The pan is therefore particularl; valuable in irrigated areas where a soil moisture profit and loss account can be used to determine how much irrigation water is required. The USWB Class 'A' pans usually last between 36 and 48 months, depending on the standard of maintenance. Because the cost of metal pans has increased in recent years, consideration was given to making the pans out of fibreglass. One modification was necessary and that was the upper edge which had to be strengthened by a small brace facing outwards. The pan would otherwise have been too flimsy and would probably have cracked during cleaning. Fibreglass was considered to be a suitable alternative because very little maintenance would be required, it lasts longer than metal and it costs slightly less than metal. Methods and Materials Experiment I A fibreglass pan was painted bituminous silver and installed in the meteorological station on a raised pallet at the Experiment Station at Mount Edgecombe. Next to it was the conventional metal pan. Readings were taken for fifty days after which the experiment was terminated and it was found that water evaporated from the fibreglass pan at a rate significantly lower than from the metal pan. Experiment 2 Seven fibreglass pans and one metal pan, all painted bituminous silver, were mounted on concrete pallets on a level cement-covered strip, and not on a grass sward, which is normal meteorological practice. The experiment was conducted for four weeks and the pans were moved from one position to another every seven days as shown in Figure 1. Readings for the eight pans were taken daily with a single dipper gauge. After four weeks the differences between the readings of the seven fibreglass pans were negligible. There was however, a consistent difference in the amount of evaporation from the two types of pans. As was observed in Experiment 1 carried out Experiment 3 It was thought that the lower readings obtained from the fibreglass pans were due to the lower thermal conductivity of the fibreglass. Pans were therefore painted two different dark colours to determine whether heat absorption of the fibreglass could be increased. In addition, two pans were covered with a 25 mm chicken wire screen attached to a 10 mm round bar hoop. The screens were laid on top of the pans, following the procedure used by the Department of Water Affairs. Pans need to be screened in areas where game, stock or birds can gain access. The pans were painted, rotated as in Experiment 2 and treated as follows: control, standard metal pan, painted silver fibreglass pan painted silver fibreglass pan painted silver, covered with Department of Water Affairs wire screen fibreglass pan painted dark green outside, silver inside fibreglass . . . . . . . pan . . painted . . dark green inside and outside fibreglass pan painted black outside, silver inside fibreglass pan painted black outside, silver inside with Department of Water Affairs wire screen fibreglass pan painted black inside and outside Pans were repainted at six-weekly intervals if the colour or sheen appeared to have changed. The experiment was conducted for twelve months, readings being taken daily. The total evaporation per month from each pan was used to compare pans and in all cases it was found that the paint increased the amount of evaporation, some more than others, and that screening reduced evaporation. Experiment 4 Evaporation from the fibreglass pan which was painted black inside and outside was increased to almost the same extent as it was reduced by the screen during the twelve month period. A screen was placed over a pan which was painted black both inside and outside and evaporation was measured and compared with that of two metal pans, one of which had the hook gauge at the normal level and the other where it was 25 mm 160 Proceedings of The South African Sugar Technologists' Association June 1983 lower than usual. After seven months the experiment was terminated because evaporation from the screened fibreglass pan, differed from the amount which evaporated from the standard pan according to the season. In the metal pan in which the water level was maintained at a shallower depth, evaporation was greater than that from the standard pan. Results and Discussion Experiments 1 and 2 In Experiment 1 where the fibreglass pan was painted silver and installed in the meteorological site at the Experiment Station during April and May, the results showed that evaporation from this fibreglass pan was 8,2% lower than the evaporation from the standard pan. This can be seen in Table 1.