PROCEDURE FOR DUNOUY TENSIOMETER

SARU DAWAR

29 JULY 2004

The CSC-DuNuoy Tensiometer uses to measure the surface and interfacial tension.

Refer to the manual for the diagram of the tensiometer to understand the various parts and the notations specified in this procedure.

SETUP:

  • Remove the instrument from the box. The instrument may be bolted to the wooden platform if it was stored in its transport mode. Remove the instrument from the wooden platform and place on a level surface.
  • See the instruction manual for a diagram of the instrument components.
  • Carefully remove the platinum ring from its storage box and insert the tip into the vertical arm ‘P’. Be careful to not exert force onto the lever arms.
  • Loosen the clamping jaws ‘X’ with the help of the knurled knob ‘N’.

CALIBRATION:

  • With knob ‘A’, bring the index ‘I’ and its mirror image exactly in line with the reference line on the mirror.
  • Loosen the dial clamp ‘C’ and rotate the dial until the zero on the vernier is nearly opposite the zero on the dial.
  • Tighten the clamp and, using the fine adjustment screw ‘F’, rotate the dial until the zeros coincide exactly.
  • On the strip of paper place an accurately known mass weighing between 500 and 800 milligrams.
  • Turn knob ‘A’ until the index ‘I’ is exactly opposite the reference line on the mirror. Record the scale reading to 0.10 division.

To determine the surface tension:

dynes/cm

If the recorded dial reading is greater than the calculated value by 0.25 dyne/cm or more, the lever arm must be shortened. If the recorded dial reading is less than the calculated value by 0.25 dyne/cm or more, the lever arms must be lengthened.

CLEANING:

The cleaning procedure recommended after insulating oils have been tested is as follows:

  • Clean the glassware by removing any residual oil with petroleum naptha or benzene followed by several washes with methyl ethyl ketone and water and immersion in a hot cleaning solution of chromic acid; rinse thoroughly with tap water and then distilled water.
  • Clean the platinum ring by rinsing it in petroleum naptha or benzene followed by rinsing in methyl ethyl ketone. Then heat in the oxidizing portion of the gas flame. Flame only that portion of the ring which will be immersed in liquid and do not overheat it, otherwise the ring may become loosened from its stirrup.

OPERATION:

  1. To measure surface tension, attach the clean ring to the lever arm.
  2. Place the liquid whose surface tension is to be measured in a clean container, such as evaporating dish, watch glass, or beaker, at least 4.5 cm in diameter.
  3. Place the container on the sample table. With screw ‘B’ in its uppermost position, raise the entire sample table assembly until the ring is immersed approximately 5mm into the liquid and is approximately centered with respect to the container.
  4. Lower the liquid further by means of screw ‘B’ until the ring is just in the surface of the liquid and the index is approximately on zero.
  5. Increase the torsion of the wire by rotating knob ‘A’ and , at the same time, lower the sample table means of screw ‘B’ to keep the index on zero.
  6. The index is to be kept on zero even though the surface of the liquid is distended.
  7. Continue adjusting the knob and the screw simultaneously until the film breaks.
  8. The scale reading at the breaking point of the film is the force of the pull exerted on the ring, or the apparent surface tension ‘p’ dynes/cm.

STORAGE:

  • Clean the components if contaminated with fluid before storing.
  • Remove the torsion wire from the vertical arm ‘P’ andplace in the wooden holder. Be careful to not bend or break the wire.
  • Adjust knob ‘A’ to center the index on the mirror. Tighten the clamping jaws ‘X’ with the help of the knurled knob ‘N’.
  • Tighten knob ‘C’.
  • Place the instrument on the wooden platform in the box. Place the manual and platinum wire ring storage box into the instrument box next to the instrument.
  • If the instrument storage box will be moved from its storage shelf with the instrument in it, the instrument must be bolted to the wooden platform before placing in the box. The screws and washers are stored in the instrument box.

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