Capillary Method for Measuring Kinematic Viscosity: Complete Calculation Logic from Flow Time to Viscosity Value

I have learned to operate the kinematic viscometer, but every time I see the instrument automatically calculate a result, I always feel like it's a black box. In fact, the calculation logic of the capillary method is very simple, and it is based on a classic fluid mechanics principle that has been mature for decades - Hagen-Poiseuille's law.

The basic principle of the capillary method

A certain volume of liquid is placed into a calibrated glass capillary viscometer, and the liquid is allowed to flow through the capillary under its own gravity at a constant temperature. The time (in seconds) required for the liquid level to flow from the upper mark to the lower mark is measured. This time is multiplied by the calibration constant of the capillary to obtain the kinematic viscosity at that temperature.

Calculation formula: ν = C × t

among which

ν: Kinematic viscosity, unit: mm²/s

C: Capillary viscometer constant, with the unit of mm²/s², is calibrated and stamped on each viscometer tube by the manufacturer

t: Liquid flow time, unit: s (second)

The significance of the capillary constant

The capillary constant C is determined by the geometric parameters of the viscometer tube (inner diameter, capillary length, liquid level height). Each viscometer tube has a slightly different C value, so it is necessary to input the actual C value of the viscometer tube being used, and they cannot be mixed. In the parameter setting interface of HZYN-1301, different constant values can be input for the four capillary tubes respectively.

Why must the flow time be ≥ 200s

This is a clear requirement of the national standard GB/T 265-88, and the instruction manual for HZYN-1301 also emphasizes this point. The reason is that when the flow time is too short, the flow of liquid in the capillary is disturbed by kinetic energy (inertial effect), and the flow is not purely determined by gravity and viscosity, resulting in a lower calculation result. Only when the flow time is long enough (≥200s) and the inertial effect can be neglected, can the result be reliable.

Due to its extremely thin diameter, the viscosity tube with an inner diameter of 0.4mm has stricter requirements: flow time ≥350s.

Selection of capillary inner diameter

The HZYN-1301 is equipped with capillaries of four different inner diameters: 0.8/1.0/1.2/1.5mm. Different inner diameters are suitable for samples with different viscosity ranges: the lower the viscosity, the thinner the capillary should be selected to ensure a sufficient flow time; the higher the viscosity, the thicker the capillary should be selected to avoid an excessively long flow time. When selecting, make a preliminary judgment first. If the flow time is significantly too short (<200s), switch to a smaller diameter capillary.