h2. Surface Tension
The affect of surface tension has the potential to seriously hinder the effective and accurate dosing of alum in AguaClara plants. Surface tension is caused by the cohesive forces of liquid molecules at the water surface. The concept of surface tension can be visualized by the figure below.
!surface tension.png|align=centre,width=650,height=320!
h5. Figure 1: The affect of surface tension
As can be seen in the figure above, a certain head, h, is required to overcome surface tension and form a water droplet. The graph above indicates this relationship between the height required to form the water droplet and the diameter of an orifice subject to the water head. Any value beneath of the line indicates that there is not sufficient head to form a water droplet, and no flow will occur. If the head subject to a set orifice size is above the line, there is adequate water height to cause flow.
The relationship shown in the graph above is represented by the formula:
{latex}$$
\Delta h = {{2\sigma } \over {\rho g{d \over 2}}} - {{2{d \over 2}} \over 3}
$${latex}
where:
{latex}$$\Delta h$${latex}= the height of water above the orifice
{latex}$$\sigma $${latex}= Surface tension, force/length
{latex}$$\rho $${latex}= density of alum, mass/volume
{latex}$$d$${latex}= Diameter of the orifice, length
The affect of surface tension can manifest itself in errors in the calculated head values in the dosing system. A certain headloss on the lever arm corresponds to a certain flow rate of alum. If surface tension increases the necessary head required to achieve this flow rate, underdosing will result. This concept can be shown in the figure below:
!surface tension1.png|align=centre,width=550,height=320!
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