These are Couette coaxial cylinder rotational viscometer since the test fluid is contained in the annular space (shear gap) between an outer cylinder and the bob. Viscosity measurements are made when the outer cylinder, rotating at a known velocity, causes a viscous drag to be exerted by the fluid. This drag creates a torque on the bob, which is transmitted to a precision spring where its deflection is measured.

Viscosity as measured by a Couette type viscometer such as the Model 35 is a measure of the shear stress caused by a given shear rate. This relationship is a linear function for Newtonian Fluids, i.e. a plot of shear stress vs. shear rate is a straight line.

These instruments have been designed so that viscosity in centipoise (or milli-Pascal seconds) of a Newtonian fluid is indicated on the dial with the standard rotor, bob, and torsion spring operating at 300 rpm. Viscosities at other test speeds may be measured by using multipliers of the dial reading. A simple method of close approximation of viscosity in a plastic fluid, such as a drilling fluid is described in Section 6B.

The range of shear rates may be changed by selecting rotor speed and using various rotor-bob combinations. A variety of torsion springs are available and designed to be easily interchanged in order to broaden shear stress ranges and allow the measuring of viscosity in a wide variety of fluids.

Figure 1
Model 35 Viscometer

The optional heated sample cups and recirculating sample cups are electrically heated. Make sure the power cord and other wiring associated with these cups is no good condition and properly grounded.

Make sure the viscometer is turned off and unplugged from the source before cleaning or other repair or maintenance. Do not allow the Viscometer Base to get wet. If samples have been spilled or splattered, wipe clean with a damp cloth. Do not allow water to run into the base, as exces-sive water could cause damage to the electrical components.

When heated sample cups are being used, do not exceed 200°F

WARNING
THE STANDARD B1 BOB IS HOLLOW AND SHOULD NEVER BE USED TO TEST SAMPLES HOTTER THAN 200°F. (93°C).

Table 1 lists the proper positions for the viscometer switch and the gear knob combinations to obtain the desired speed. The viscometer gear shift knob may be engaged while the motor is running. Read the dial for shear stress values.

CAUTION

After preparing the instrument for 12-speed testing by setting the gear box shift lever, select the proper speed range with the speed shift switch on the right side of the base, then turn on the motor and set the viscometer gear knob on the top of the instrument. Refer to Table 2 for the correct combination of gear box shift lever setting; speed switch selection; and viscometer gear knob placement. The stress values will appear on the dial.

Figure 2
Gear Box Lever

TABLE 2
TWELVE-SPEED TESTING COMBINATIONS
MODEL 35A/SR12 AND MODEL 35SA/SR12

RPM	Gear Box Lever	Speed Switch	Viscometer Gear Knob
600	Left	High	Down
300	Left	Low	Down
200	Left	High	Up
180	Right	High	Down
100	Left	Low	Up
90	Right	Low	Down
60	Right	High	Up
30	Right	Low	Up
6	Left	High	Center
3	Left	Low	Center
1.8	Right	High	Center
0.9	Right	Low	Center

The rotor may be replaced by aligning the rotor slot and groove with the lock pin in the main shaft socket. Push the rotor upward and lock it into position by turning it clockwise.

1. Remove the dust cap [A] and plug screw [B].
2. Loosen set screws [C] and [D] about 1/2 turn. The spring can now be lifted out. Be careful not to stretch the spring.
3. Insert the new spring, making sure the bottom mandrel is properly oriented and seated. Set screw [D] should line up with the point at which the spring leaves the bottom mandrel. A notch cut into the upper end of the bottom mandrel will help locate this point. Tighten set screw [D], so that it presses against the split ring to hold the bottom mandrel of the spring.
   
   NOTE: Before tightening set screw [C] be positive that the top of the adjustable mandrel is flush with the top of clamp [E]. It may be necessary to slightly compress or stretch the spring to accomplish this.
   
   
4. Tighten set screw [C]. The slot in the top of the adjustable mandrel should line up with clamping set screw [C].
5. Loosen set screw [F] to zero dial under index, then rotate knob [G] as required for alignment, then adjust knob [G] vertically to allow the spring to be clamped in a "free" position, neither stretched or compressed.
6. Tighten set screw [F] and replace the dust cap [A].

Figure No. 4
Torsion Spring Removal and Replacement

The Dead Weight Calibration is easier to perform and if the spring requires adjustment, the proper setting can easily be verified. The Standard Fluid Calibration check verifies the complete instrument is operating properly. It will determine problems of bent bob shaft, rotor eccentricity, and/or runout of the rotor or bob more effectively than the Dead Weight method. Refer to Section 5-B.

1. Remove rotor and bob. Refer to Section 3-A and 3-B. Be sure that the tapered end of the bob shaft is clean, then install the calibrating spool.
2. Install the DW-3 calibrating fixture by clamping it onto the upper portion of the viscometer support legs.
3. Select a weight according to Table 3. Insert the bead at the end of the thread into the recess in the top of the calibrating spool. Wrap the thread a little more than once around the spool and then drape the thread over the pulley.
4. Hang the selected weight on the thread and adjust the calibrating fixture up or down until the thread from the spool to the pulley is horizontal. Compare the dial reading with the reading on Table 3.
5. If necessary, adjust the torsion spring. Refer to Section 5-C, "Adjusting Torsion Spring".

Figure No. 5
Dead Weight Calibration
DW-3 Calibration Fixture

TABLE 3
Dial Deflection for Calibration Weights And Torsion Spring Assemblies

Torsion Spring Assembly (with R1-B1 combination)	Torsion Spring Constant, K1 Dynes/cm/° def	Weight in Grams
		10	20	50	100	200
		Dial Reading
F-0.2	77.2	127.0	254.0	-	-	-
F-0.5	193.0	50.8	101.6	254.0	-	-
F-1	386.0	25.4	50.8	127.0	254.0	-
F-2	772.0	-	25.4	63.5	127.0	254.0
F-3	1158.0	-	-	43.0	84.7	169.4
F-4	1544.0	-	-	-	63.5	127.0
F-5	1930.0	-	-	-	50.8	101.6
F-10	3860.0	-	-	-	-	50.8

1. The instrument being checked must be clean before immersing the rotor and bob into the calibration fluid. If necessary, remove the rotor and thoroughly clean the bob, bobshaft, and rotor. Make sure the bob shaft and rotor are straight and have not been damaged.

   CAUTION:

   The batch number on the label of the calibration fluid must match the number on the viscosity/temperature chart.

2. Fill the sample cup to the scribed line with calibration fluid and place it on the instrument stage. Elevate the stage so that the rotor is immersed to the proper immersion depth. Refer to Fig. 1.
3. Place a thermometer into the sample until the bulb touches the bottom and then secure it to the side of the viscometer to prevent breakage.
4. Operate the instrument at 300 rpm for three (3) minutes. This will equalize the temperature of the bob, rotor and the fluid.
5. Read the dial at 300 rpm and 600 rpm. Record these numbers, and the temperature from the thermometer to the nearest 0.1° C,(0.15° F).

1. Remove dust cap [A], then loosen set screw [C] about 1/2 turn.
2. Insert the calibration tool into the spring and rotate the adjustable mandrel (inside the spring) slightly. Turn the mandrel counterclockwise if the dial reading is too low or turn the mandrel clockwise if the dial reading is too high.
   
   NOTE: Before tightening set screw [C] check the top of the upper threaded mandrel and be positive that it is flush with the top of the clamp [E]. To accomplish this, it may be necessary to adjust the spring by slightly compressing or stretching the spring.
3. Tighten set screw [C]. The slot in the top of the adjustable mandrel should line up with clamping set screw [C].
4. Loosen set screw [F] to zero dial under index, then rotate knob [G] as required for alignment, then adjust knob [G] vertically to allow the spring to be clamped in a "free" position, neither stretched or compressed.
5. Tighten set screw [F] and replace the dust cap [A].

Figure No. 4
Dead Weight Calibration
DW-3 Calibration Fixture

To rapidly determine Newtonian viscosities in cP with FANN viscometer, use the following formula:

nN = S x  x f x C

where,

S = Speed factor (Refer to Table 5)

0 = Dial reading

f = Spring factor (Refer to Table 3)

C = Rotor-bob factor (Refer to Table 4)

nN = Newtonian viscosity - cP

Example: Using an R2-B1 combination at a speed of 600 rpm with an F5.0 spring, and a dial deflects to 189.

nN = 0.5 x 189 x 5 x .315 = 149 cP.

Combinations with the larger gaps are likely to give results that differ from these figures. For best accuracy, calibrate with a standard fluid having a viscosity near the range of interest and using the R-B-F combination to be used in the test.

PV (plastic viscosity, (lbs/100 ft2)/300 rpm) = 600 - 300

YP (yield point in lbs/100 ft2) = 300 - PV

A spring other than F1 may be used if the dial readings are multiplied by the proper "f" factor, but the other rotor-bob combinations can not be used for this rapid, two point method.

K1 = Grg /

G = Load in grams

g = 981 = gravitational constant (cm/sec2)

r = Radius arm = 1 cm

 = Dial reading in degrees

Example: The required setting for the F1 spring is 386 dynes/cm/degree deflection with the R1-B1 combination. Using the 50 gm weight supplied with the fixture, the formula is:

K1 =

Notes: Computed for standard Torsion Spring (f = 1) For other torsion springs multiply viscosity range by f factor Minimum viscosity is computed for minimum shear stress and maximum shear rate For practical purposes the minimum viscosity is limited to 0.5 cP because of Taylor Vortices Maximum viscosity is computed for maximum shear stress and minimum shear rate

Periodically test the bob shaft bearings. Operate the instrument at 3 or 6 rpm with no sample around the rotor and bob. Observe movement of the dial. It should not move more that +/- 1 division. Rough bob shaft bearings should be replaced.

Instrument should be serviced by qualified personnel only. If factory service is required, contact Fann for return authorization.