Surface Tension Measurements

i am measuring pure PP surface tension by datapysic device,(plate prob). As the heating system of the machine is very weak, i melt the PP externally and take it in the molten form to measure it, but after on or two measurements the curve of the surface tension just goes up indicating increase in the force on the prob as if the pp is climbing the prob, actually the melt become alittle bit viscous when it is in the measuring chamber,

so can i use the max value of the graph as the vlue of the surface tension?

and if any one measured using this device DCAT21 can help me

thanx all