Verifying Polyethylene Hydrostatic Balance Test Calibration for Entry
Polyethylene hydrostatic testing requires degassed, temperature-corrected fluid baths and void-free plaques to prevent false 0.002 g/cm³ receiving rejections.
Surface Tension Drag
Meaning
Precision weighing of a solid sample suspended in a liquid is affected by the forces acting at the boundary where the suspension wire enters the fluid. The surface tension drag is the vertical force exerted by the liquid meniscus on the suspension wire as it crosses the liquid-gas interface. This force governs the apparent weight of the suspended sample and introduces a systematic offset in hydrostatic density measurements.
The effect of this force is limited to the region of the wire that directly intersects the liquid surface.
Physical Mechanism
Cohesive forces between the molecules of the immersion fluid generate a surface skin that resists penetration. As the suspension wire of the weighing pan passes through this skin, the surface tension drag pulls downward or pushes upward depending on the contact angle of the liquid on the wire. This force is directly proportional to the perimeter of the wire and the surface tension of the fluid.
Error Source
Changes in the height of the liquid meniscus due to evaporation can alter this force during a measurement.
Mitigation Action
Minimizing the surface tension drag is achieved by using extremely thin wires, often made of platinum or tungsten, and adding trace amounts of wetting agents to the water. This reduction is vital when weighing small polymer samples, where the surface tension force can be of similar magnitude to the buoyant force of the sample itself. Controlling this variable ensures that density measurements remain accurate and repeatable.