Viscosity Measuremant

7/30/2019 Viscosity Measuremant

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Introduction

The purpose of this experiment is to provide an introduction to the use of the Cannon

Fenske viscometer. This viscometer requires calibration in order to relate the measured

values to the actual values. Thus, another purpose of this experiment is to introduce the

concept of calibration. The viscosity of liquids is quite sensitive to changes in temperature.

Data will be taken at different temperatures to investigate the effect of temperature on

viscosity.

The Cannon Fenske viscometer is a glass capillary viscometer. A diagram of the viscometer

is shown on Figure 1. At the beginning of the measurement, capillary tube R and bulb C

'are filled with the fluid to be tested. The liquid level in C is held slightly: above line E by

applying a slight vacuum to tube N and quickly placing your finger over the end of the

tube. When the vacuum is removed (remove finger from end of tube),

The fluid flows from bulb C through capillary tube R into bulb A. The time required for the

liquid to flow between marks E and F is related to the kinematics viscosity of the liquid by

Equation 1. The viscometer selected must have a minimum flow time of 200 seconds.

Where:

=Ct

=/ , kinematics viscosity, centistokes (ct)

C= viscometer constant

t= flow time, sec.

= viscosity, cp

= density, gm/cm3


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The viscometer coefficient, C, is different for each viscometer and must be determined by

calibration. Calibration may be done with viscosity oil standards or using standard

viscometers.

Viscosity oil standards are oils which have known viscosities at specified temperatures. To

calibrate a viscometer, standard oil which has a flow time of at least 200 seconds is used.

Flow time is determined as described previously. Thus, from Equation (1) .

SSSStC /)/( =

(2)

A standard viscometer is one which has a known calibration C. This constant may have

been determined previously by using viscosity oil standards. Calibration of an uncelebrated

viscometer is done by determining the flow time for the same oil in the ca1ibrated and the

uncelebrated viscometers. .The viscometer constant for the uncelebrated viscometer can be

computed from Equation (3).

Cl = 1/)*( ttC

SS (3)

Where: C1 = constant of uncelebrated viscometer

t1 = efflux time at A fixed temperature for liquid in uncelebrated viscometer

SC

= Calibration constant of standard viscometer

ts = efflux time for same oil as used for t1 and at same temperature but in calibrated

viscometer.

S

= Viscosity, CP, of standard oil, centipoises S

= density of standard oil, gm/3

cm .

S

= density of standard oil, gm/3

cm .

Viscosity measurement


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Cleaning the viscometer between successive determinations, clean the viscometer

thoroughly by several rinsing with toluene, followed by acetone. Dry the viscometer by

passing a slow stream of filtered dry air through the viscometer for 2 minutes or until the

last trace of solvent is removed.

Measurement of flow time

Charge the viscometer with the oil sample and mount in a constant temperature water bath

as described previously. When thermal equilibrium has been reached (15 min.). determine

the flow times for three trial runs.

Compute the kinematics viscosity from the viscometer constant and the three flow times.

Note: viscosities which differ by more than 0.35% from their mean are considered suspect.

Experimental Apparatus and Procedure


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1. The viscometer should be cleaned with a suitable solvent and dry, clean, filtered air

should be blown through the viscometer to remove any remaining traces of solvent.

2. The instrument should be periodically cleaned 'with chromic acid to remove any possible

traces of organic deposits.

3. Jf a possibility of lint, dust, or other solid material is present In liquid sample, this may

be removed by filtering through a sintered glass filter or fine mesh screen. .

4. To introduce sample into the viscometer, Invert viscometer Immerse tube "A" into liquid

and apply suction to "J" which causes the sample to rise to etched line "E", Turn the

viscometer to normal position and wipe tube "A" clean.

5. Insert the viscometer into a holder and place in Constant. Temperature Bath, Allow 10

minutes for viscometer to reach equilation at 100 degrees F.(9 8.89C,) and 15 minutes at

210 degrees F. (98.89c), Or that time required to reach equilibrium.

6. Vertical alignment may be accompanied in bath by suspending a plumb bob In tube"J".

7. Apply suction to tube "A" and bring sample into bulb "D", a short distance above mark

"C".

8. The efflux time is measured by allowing the sample to flow freely through mark "C",

measuring the time for the meniscus to pass C" to E".

9. To repeat efflux time. repeat steps 7 and 8.

10. The Viscosity is calculated by multiplying the efflux time by the viscometer constant.

1. Constant temperature water baths (30 C, 40 C, 50 C)

2. Oil samples


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3. Cannon Fenske viscometers and holding clamps

4. Electronic timer or stopwatch capable of measuring time to the nearest 0.1 sec.

5. Rubber bulb to supply vacuum and rubber hose

6. Thermometer

7. Standard oil for viscosity calibration.

8. Cleaning liquids such as toluene and acetone.

Procedure

You will measure the viscosity of two different oils at three different temperatures.

Temperature variations will be minimized by conducting the measurements in constant

temperature baths. Three constants temperature baths (30C,40C,and 50C) will be set up

and operating in the laboratory.

Select a viscometer which has a minimum flow time of 200 seconds at the calibration

temperature. Each viscometer has an identification number. Record this identification

number. Table 1 contains "approximate" viscometer constants for standard ASTM

viscometers. Note: we have the sizes identified with an asterisk. Clean the viscometer by

rinsing it with toluene followed by acetone. See cleaning procedure described later in this

section.

Calibration of viscometer

Bulb to the large tube (L) on the Cannon Fenske viscometer. Invert the viscometer and

apply suction to tube L with tube N immersed in the liquid sample. Draw the sample to


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timing mark F and quickly turn the viscometer so that it is upright. Mount the viscometer in

the constant temperature bath by attaching a small water bath clamp to tube L. Tube L

should be mounted in a vertical position.

Allow 15 minutes for the sample to attain bath temperature. Record the bath temperature.

Attach the hose .on the rubber bulb to tube N and draw the oil through bulb C to a position

about 5 mm above the first timing mark (E). The liquid level can be temporarily held at this

point by holding your finger over the Choose standard oil. Supplied by the laboratory

assistant. The viscosities of the standard oils are on the labels of the bottles. Distilled water

may be used as a standard when its flow time is 200 seconds or more (Sizes 25, 50).

The quantity of standard oil is limited. It will be necessary for other students to use the

same oil in their calibration experiments, so return the oil to the bottle after use. Be careful

not to contaminate the standard oil with water or any other solvent. In normal laboratory

operations, the oil would be discarded after removal from the viscometer.

Charge the viscometer with the standard oil as follows. Connect a rubber end of tube N.

Remove the suction on tube N and allow the sample to flow freely through the viscometer.

Measure to within 0.2 sec. the time required for the meniscus to pass from the first timing

mark to the second. Measure the flow time for three independent runs and use the average

for the flow time for the sample.

Compute the viscometer constant C from the known kinematics viscosity and flow time

using Equation 2. This will be the constant for this viscometer when determining

kinematics viscosities for all oils which have a minimum flow time of 200 seconds.

There are corrections which can be applied to account for kinetic energy changes, surface

tension effects and thermal expansion of the viscometer due to use at a temperature above

the fill temperature.


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The first correction is usually small. The surface tension correction is not needed when the

calibrating fluid is a standard" oil and the fluid to be tested is oil. You can investigate the

temperature variation of the viscometer constant by transferring the viscometer containing

the standard oil to one of the other temperature baths and detonating the flow time of the

standard oil as described previously. Do this for one viscometer and compare the

viscometer constants.

Return the standard oil to the appropriate bottle when calibration runs are finished.

Discussion and Recommendation

1. Try to use the correct viscometer size. Measuring ranges overlap, but the best

results will be obtained if you are operating near the center of the viscometer range.

If your sample has a viscosity of about 100 cSt, for example, the size 300 (range 50-

250 cSt) viscometer will give more accurate results than either size 200 (range 20-

100 cSt) or size 350 (range 100-500 cSt).

2. Some time is required to allow the sample to equilibrate at the temperature of the

water bath and to allow possibly occurring air bubbles to segregate. Therefore, after

loading the sample, wait at least 10 minutes before making measurements.

3. Avoid overloading the viscometer. Using either too much or too little sample will

produce inaccurate values of viscosity. The volume that fills the tube from E to the

inlet of the A tube will approximately half fill the bulb marked H.


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References:

1. Amyx J.W., M.D Bass, and R.L. Whting, Petroleum Reservoir Engineering Mc

Graw Hill Book Co., (1960) p.4 and 7.

2. Product Safety Bureau, Reference Manual, Book 5, Laboratory Policies and

Procedures, p.C27-2.

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Viscosity Measuremant