Why is temperature and pressure compensation necessary for TOV viscometers?

Temperature and pressure affect viscosity!

The effect of shear rate and shear stress on viscosity: Generally, shear stress increases as the shear rate increases, whereas viscosity decreases as either the shear rate or shear stress increases. The stronger the dependence of shear viscosity on shear rate, the more rapidly the viscosity decreases as the shear rate increases. Such polymers are known as shear-thinning polymers. This phenomenon of shear thinning is an inherent characteristic of polymers, but the degree of shear thinning varies among different polymers ； The effect of temperature on viscosity: The mechanism by which viscosity depends on temperature lies in the relationship between molecular chains, \"free volume\", and temperature. Below the glass transition temperature, the free volume remains constant, and as the temperature increases and the macromolecular chains begin to vibrate, the volume increases. When the temperature exceeds the glass transition temperature, the large chain segments begin to move, the free volume between the segments increases, the forces acting between the segments decrease, and the viscosity drops. Different polymers have varying sensitivities to temperature in terms of viscosity. The effect of pressure on viscosity: When a polymer melt is injected, it is subjected to both internal static pressure and external dynamic pressure, during both the pre-plasticization stage and the injection stage. During the pressure holding and feeding phase, polymers are generally subjected to a pressure of 1500–2000 kgf/cm2; in the case of precision molding, this pressure can reach up to 4000 kgf/cm2. Under such high pressures, the free volume between molecular chains is compressed. As the free volume between molecular chains decreases, the approach of macromolecular segments strengthens the intermolecular forces, resulting in an increase in viscosity. At a constant processing temperature, the compressibility of polymer melts is greater than that of ordinary liquids, and it also has a significant impact on viscosity. Due to the different compressibility of polymers, the sensitivity of viscosity to pressure also varies ； A higher compression ratio results in greater sensitivity. Polymers also experience an increase in viscosity due to increased pressure, which has an equivalent effect to lowering the melt temperature.