There are two types of viscosity that we usually deal with; kinematic and dynamic viscosity. Kinematic viscosity is a measure of a fluid’s internal resistance to flow under the force of gravity, and is usually given in the units of centistokes, cSt. Dynamic viscosity is a measure of a fluid’s resistance to flow when there is an external force, and has the units of centipoise, cP. For pump analysis dynamic viscosity is generally more useful, but kinematic viscosity does have application – especially for Newtonian liquids, and if you find yourself needing to measure the kinematic viscosity then a glass capillary is the simplest way.
The glass capillary is usually several hundred millimeters tall, and is made with precise length and inside diameter dimensions. It will have show two marks on the capillary, and the kinematic viscosity is determined by simply measuring the time it takes for a known quantity of the liquid to flow through the capillary between the two marks, and multiplying it by a capillary constant. The capillary constant will usually be provided by the manufacturer of the capillary, but if you have lost it you can work it out by calibrating the capillary with some fluid of known viscosity.
Although they are simple instruments, capillaries can be overwhelming to use because of the huge variety in styles. Each style serves a purpose, so this article is a primer on how to pick the most appropriate capillary for your application, how best to use it, and followed by a few cleaning tips that we’ve picked up along the way.
How to pick a glass capillary tube
First you should pick the style of capillary tube based on the transparency of the liquid, and then select the right size that will suit the viscosity that you expect.
Transparent or translucent liquids
If the liquid to be measured is transparent or translucent, and the meniscus can be readily observed through a ~3 mm diameter column, the following types of viscometers can be used:
If viscosity needs to be measured at a number of different temperatures and the liquid is transparent then a suspended level viscometer can be used:
If the liquid is opaque then a reverse flow viscometer can be used:
These wet the timing section of the viscometer capillary only during the actual measurement. Note that reverse flow viscometers can’t readily make repeat measurements for verification – the glass needs to be cleaned, dried and refilled prior to repeating a measurement.
Extra viscous liquids
If the liquid is too viscous to easily flow under gravity then a vacuum viscometer can be used:
In this style a very accurately-controlled vacuum is applied to one end of the viscometer to pull the liquid through the capillary into the timing bulb. These are all reverse flow viscometers, and they require vacuum regulators to maintain a constant vacuum.
Sizing the glass capillary
A glass capillary will only be useful for a certain range of viscosities. This is because the flow needs to be laminar, and if the liquid passes through the capillary is too fast this condition will not be met.
Calculating kinematic viscosity
The formula for working out kinematic viscosity (mm2/s) is given in ASTM D445-12 and especially in D446-07 s 7. This standard also gives the correction for kinetic energy and surface tension. Another useful reference is BS 188:1977.
In terms of equipment, to measure the viscosity a simple stopwatch is sufficient, but depending on the fluid, it may be necessary to sit the capillary in a temperature-controlled bath to ensure consistent results.
Cleaning the viscometer
The viscometer will only give accurate measurements if it is clean and dry. A suitable solvent needs to be flushed through the capillary (possibly at elevated temperature), and then the solvent itself needs to be flushed with a highly volatile solvent to ensure any remnant is evaporated quickly. Acetone is recommended for this function.
Clean air can be blown through at low velocity, but be careful that if the solvent rapidly evaporates, the surface can cool the surface of the glass to the point where humid air may condense. To avoid this try and warm the air.
If there are deposits of insoluble material, fill the viscometer with a chromic acid and leave it to soak for the day. Chromic acid is strongly oxidizing and will convert many materials to a soluble form without attacking borosilicate glass. It will not affect the calibration constant.
Don’t use highly alkaline cleaning solutions. Repeated use of a solution with a pH greater than 10 can affect the calibration.
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