Digital Viscometry

Primary Equipment:
Created: February 24, 2010 by Bob Cox

Revised: August 10, 2010 by Bob Cox

University of Utah, Chemical Engineering

The Wells-Brookfield viscometer rotates a spindle at a precise speed and measures the torque necessary to overcome the viscous resistance to the induced movement caused by the presence of sample fluid between the spindle and a stationary plate. This is accomplished by driving the spindle through a calibrated beryllium copper spring. The degree to which the spindle is wound can be indicted by a pointer and dial or translated into a digital reading which is proportional to the viscosity of the fluid.

LVTDV-II with CP-52, CP-41 Cone/Plate, Operational Procedure:

Note: The CP-41 requires twice the sample volume as the CP-52.

  1. Check fluid level in circulating bath, turn on power, and set bath to desired temperature.
  2. Check the bubble window on the top of the viscometer to verify that the instrument is level, if not adjust the viscometer stand until the bubble is centered.
  3. Swing sample cup clip to one side while holding cup and remove sample cup. With cone spindle not installed, turn on power then motor using switches on the front panel. Using the motor speed control on the left side, adjust the motor speed to 12 RPM. Press auto zero on the front pad, when display reads zero then turn off motor but NOT the power. If you turn off or loose power you must go back and repeat the auto zero step above.
  4. Using the wrench supplied with the instrument, hold the viscometer shaft steady and screw on the CP-52 cone spindle (finger tight only). Threads are reverse. Note: Lift shaft upwards slightly while screwing on spindle, but avoid applying sideways force on viscometer shaft.
  5. Replace sample cup and secure it with sample cup clip. Avoid hitting the spindle when reattaching the sample cup.
  6. Turn the motor switch located on the front of the viscometer to the ON position (watch motor torque closely, if it climbs fast and high shut off motor immediately!), and adjust the viscometer speed to 12 RPM. Press the SPDL key and enter the spindle number (Ex. 52 for CP-52) and then press % (or CPS, or SS) key. (See instrument manual)
  7. If the digital reading regularly fluctuates or will not settle to a value near zero (indicating that the pins on the spindle and sample cup are contacting) turn the adjusting ring to the left (clockwise when looking from above) until the reading stabilizes at or near zero. If the digital reading remains at or near zero, continue to the next step. Allow sufficient time for readings to settle down whenever making adjustments.
  8. Turn the adjusting ring to the right (counterclockwise when looking from above) in small increments of approximately one or two division markings on the ring while watching the digital readout. Continue turning the ring to the right until the readout fluctuates upscale, indicating that the pins have made contact.
  9. Once contact has been made, turn the adjusting ring back to the left in small increments until the dial reading stabilizes at or near zero, indicating that the pins are no longer in contact.
  10. Again turn the adjusting ring to the right, this time in very small increments (about 1/64 per adjustment). Continue adjustments until the readout fluctuates regularly by a small amount. This determines the point at which the pins are just making contact.
  11. Make a small pencil mark on the adjusting ring directly under the index mark on the pivot housing. Turn the adjusting ring to the left the width of one division marking. This will separate the pins by exactly 0.0005. The viscometer is now ready for taking measurements. It is recommended that this gap setting procedure be performed every time the spindle is removed and reattached to the viscometer. It must be performed for each new temperature due to expansion.
  12. With the motor switch OFF, remove the sample cup. Use a disposable syringe to transfer 0.5 (>100 cP)or 1.0 ml (<100 cP) of the sample to the center of the sample cup (if in doubt start with 0.5 ml). Do not reuse the syringe. Make certain that the sample is bubble-free and only slightly spread at the center of the cup. Replace and secure the sample cup, taking care to avoid hitting the spindle with the cup.
  13. Set motor to desired speed using the rotating knob on the left side of the viscometer. Turn motor on and set display to %, a motor speed (RPM) yielding a % reading as close to 100 as possible without exceeding that limit will give the most accurate readings. Also, it should be noted that for maximum accuracy readings below 10 should be avoided.
  14. Allow 2-3 minutes for fluid to reach the desired temperature and for the digital reading to stabilize. Press the CPS pad on the front of the viscometer and record the digital reading along with the temperature. If the reading is unstable or above 100, and you used 0.5 ml of sample, you can add another 0.5 ml and retry the measurement.
  15. Repeat 4-6 times at the same temperature and different RPM values.
  16. When testing is complete, turn off the motor, detach sample cup and cone/spindle and clean cup and cone with tissues and acetone (or another solvent). Avoid turning off the power, if you plan to test another sample (see # 3 above).

RVDV-II+ with Spindle #1 to #7, Operational Procedure:

  1. Gently pour approximately 500 mL of sample into a 600 mL beaker, avoid creating air bubbles. Place the beaker in a water bath set to desired temperature.
  2. Mount the guard leg on the RVDV-II+ Viscometer. Turn on the viscometer using the rocker switch mounted at the lower left rear of the instrument.
  3. Lower the viscometer into the fluid using the knob on the vertical stand. Check the leveling bubble at the rear of the instrument and adjust the stand as necessary.
  4. Insert and center spindle in the sample until the level of the fluid is at the immersion groove on the shaft of the spindle. With a disc-type spindle, tilt the spindle slightly while immersing to avoid trapping air bubbles on its surface. Attach the spindle to the lower shaft of the viscometer. Lift the shaft slightly, holding it firmly with one hand while screwing the spindle on with the other (note left-hand thread). Avoid putting side thrust on the shaft. Verify the proper spindle immersion depth and that the viscometer is level.
  5. Press the Select Spindle. Use UP and DOWN Arrow key to scroll to proper spindle number. Press the Select Spindle key again to set.
  6. Set motor to desired speed using the SET Speed and Up and Down Arrow keys. An appropriate selection of spindle and speed will result in measurements made between 10-100 on the % torque scale. To measure high viscosity, choose a small spindle and/or a slow speed. If the chosen spindle/speed results in a reading above 100%, then reduce the speed or choose a smaller spindle.
  7. Allow the spindle to rotate at least five revolutions. Record the Viscometer reading and temperature.
  8. Press the MOTOR ON/OFF/ESCAPE key and turn the motor OFF when changing a spindle or changing samples. Remove spindle before cleaning.

Note: Full scale range can be calculated by equation:

Full Scale Viscosity Range [cP] = TK * SMC * 10,000 / RPM

Where: TK = Torque Constant = 1 for RVDV-II+

SMC = Spindle Multiplier Constant from the following table.