The capillary viscometer is the core component in the determination of kinematic viscosity and also the most prone to issues. A contaminated capillary not only leads to inaccurate data but also leaves contaminants on the wall of the tube, affecting the testing of all subsequent samples.

The necessity of cleaning

The inner diameter of the capillary is extremely thin (0.8-1.5mm), and any trace of grease, dirt, or residue from previous samples on the wall of the capillary will change the contact angle (wettability) between the liquid and the wall, thereby affecting the flow speed of the liquid in the capillary. This effect, when converted to flow time, may only result in a difference of a few seconds. However, according to the formula viscosity = C × t, a difference of a few seconds multiplied by the capillary constant may cause a viscosity error of 1% to 2%. For high-precision tests, this is already an unacceptable systematic error.

Regular cleaning (after each use)

Wash the inner wall of the viscosity tube repeatedly with solvent oil (petroleum ether) or gasoline for at least 3 times, and then dry it with clean air (hot air filtered through cotton).

Key points of operation: The solvent must be able to dissolve the tested oil sample, and it must be completely evaporated after cleaning, otherwise the residual solvent will alter the viscosity of the next sample. Petroleum ether, with good volatility, is the preferred cleaning solvent.

Cleaning for severe pollution (when there is dirt)

The instruction manual provides a cleaning procedure for removing dirt, which involves using the following in sequence:

Chromic acid cleaning solution (potassium dichromate + concentrated sulfuric acid, strong oxidizing property, used to remove organic dirt)

Water (for rinsing chromic acid solution)

Distilled water (with ionic impurities removed)

95% alcohol (replacing water to accelerate drying)

Dry in an oven or blow-dry with hot air filtered through cotton

Warning: Chromic acid cleaning solution is highly corrosive and toxic. Wear protective gloves and goggles during operation. Waste liquid cannot be directly discharged and requires specialized treatment.

Vertical holding of viscosity tube

Special instructions in the manual: The capillary tube must be kept vertical after being placed in the bathtub. Tilt of the viscometer tube will change the driving liquid level difference of liquid flowing in the capillary tube, resulting in a change in flow time (the greater the tilt, the greater the deviation). When securing the viscometer tube, check the position of the support to ensure that the viscometer tube is hung vertically and does not touch the inner wall of the bathtub.

Storage precautions

The cleaned and dried capillary tubes should be stored upright, with the ends sealed with filter paper or plugs to prevent dust from entering, and kept in a dry and clean environment. Do not mix viscosity tubes of different models (with different inner diameters) to avoid using the wrong one.