## What is Viscosity?

Viscosity is a measurement of resistance, or internal friction, to flow by external forces. Viscosity measures a fluid's flow resistance. Viscosity describes a moving fluid's resistance. A fluid with large viscosity does not flow easily because its specific makeup gives it a lot of internal friction. A fluid with a low viscosity flows easily because its molecular makeup has little friction while in motion.

## Measuring Viscosity: Rising Bubble Principle and Technique

The time it takes for an air bubble in liquid to rise is related to its viscosity. The inside diameter of a viscosity tube affects bubble rise rates; therefore, viscosity tubes must be precision manufactured to a tolerance that will keep the bubble rise rate variations below the level of visual detection. Variations in viscosity readings or comparisons are visually not perceivable from viscosity tubes with inside diameter tolerances closer than +/- 0.025mm. Differences in viscosity cause the readings to vary.

Viscosity can be determined by either measuring the time it takes an air bubble to pass between two marks on a viscosity tube, or by comparison to the bubble rise rate of calibrated viscosity standards. In order to obtain precise results, it is important to pay attention to uniformity of temperature and making sure viscosity tubes are vertical.

## How to use Viscosity Tubes

1. Fill the viscosity tube to the 100mm score level.
2. Insert a cork to the 108mm score which will create an air bubble.
3. Place the sample viscosity tube in a rack immersed in a water bath until the uniform temp of 25ºC / 77ºF is obtained.
4. If viscosity is going to be determined using a comparable sample, it also needs to be the same temperature.
5. Determine viscosity by Comparison of Bubble Rise Rate or Time-Method (ASTM D-1545). Both methods are explained below.

## Determine Viscosity: Comparison Method Bubble Rise Rate

This determination of viscosity is performed using the bubble rise rate comparison of the sample as compared to samples with a known viscosity. Remove the rack containing both the samples with unknown viscosity and those with a known viscosity standard from the water bath. Hold the rack vertically to stabilize the air bubbles, and then quickly invert. Compare the air bubble speed of the samples to those with known viscosity. The faster bubble rate rise, the lower the viscosity.

## Determine Viscosity: Time-Method ASTM D-1545

Using the Time-Method to determine viscosity requires the use of time-test viscosity tubes. Measure the length of time (in seconds) that it takes an air bubble to move from the 27mm score to the 100mm score on the time-test viscosity tube. The distance of 73mm is equal to the viscosity of the liquid in "Stoke" units. Alternatively, viscosity can be measured in "Bubble Seconds". The position of the air bubble related to the 27mm score should be similar at the end of the test at the 100mm score.

## Selecting the Correct Viscosity Tube

Viscosity tubes are separated into four types based on bore size, # of score marks, and type of testing.

Time-Test Viscosity Tubes (10.65mm Bore Size)
ASTM D-1545 test tubes are also referred to as time-test viscosity tubes. Time-test viscosity tubes are scored at 27mm, 100mm, and 108mm. These time-test viscosity tubes are subjected to additional testing to ensure the 10.65mm bore size that is extremely important for precise measuring.

Class G-10.65 and Class V-10.75 Viscosity Tubes
Both Class G-10.65 viscosity tubes and Class V-10.75 viscosity tubes are used mainly for comparison to Gardner Viscosity Standards. Class G-10.65 and Class V-10.75 viscosity tubes are scored at 100mm and 108mm. Below is an equivalent chart for Bubble Rise Rate to Gardner Viscosity Standards.

 Bubble Rate Rise Gardner Viscosity Standard Class G-10.65 Viscosity Tubes Gardner Grade A Tubes Class V-10.75 Viscosity Tubes Gardner Grade B Tubes

Class V Viscosity Tubes
Class V Viscosity Tubes are made to compare other user-prepared samples or standards with a known viscosity and are the least expensive of the viscosity tubes. Class V viscosity tubes are scored at 100mm and 108mm.

 Viscosity Tubes Viscosity Tube Type Score Marks Tube Inside Diameter Testing Method Time-Test Viscosity Tubes 27mm, 100mm, 108mm 10.65mm ASTM D-1545 Bubble rise or comparison Class G-10.65 Viscosity Tubes 100mm, 108mm 10.65mm (Equal to Gardner A Tubes) Bubble comparison to Gardner A Standards Class V-10.75 Viscosity Tubes 100mm, 108mm 10.75mm (Equal to Gardner B Tubes) Bubble comparison to Gardner B Standards Class V Viscosity Tubes 100mm, 108mm 10.5mm - 11mm Bubble comparison to user-prepared standards