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REDWOOD VISCOMETER NO. 1

AIM

To determine the kinematic viscosity and absolute viscosity of the given thin sample of oil at various temperatures and to study the variations of viscosities with temperature using Redwood viscometer No. 1.

PRINCIPLE

Viscosity is the property of a liquid by virtue of which it offers resistance to its own flow. The viscosity of liquids decreases with increasing temperature. The rate at which the viscosity changes with temperature is known as viscosity index.

APPARATUS

Redwood viscometer No. 1, two thermometers (0–100°C), a narrow-necked flask (50 cc), a stop watch.

DESCRIPTION

Oil Cup

It is a silver plated brass cylinder (90 mm in height and 46.5 mm in diameter). The upper end of the cup is open. The bottom of the cylinder is fitted with an agate jet (with a bore diameter of 1.62 mm and a length of 10 mm). The jet is opened or closed by a valve rod, which is a small silver-plated brass ball fixed to a stout wire. The level to which the cylinder is to be filled with oil is indicated by a pointing wire fixed on the inner side of the cylinder. The lid of the cup is fitted with a thermometer, which indicates the temperature of the oil.

Heating Bath

The oil cup is surrounded by a cylindrical copper bath containing water. It is provided with a tap for water outlet and an electric heating coil is fixed at the bottom. A thermometer fixed in the water jacket indicates the temperature of the water.

Stirrer

Outside the cylinder, there is a stirrer, carrying blades, for stirring the water in the bath for maintaining a uniform desired temperature. The stirrer is provided with a cylinder shield at the top to prevent the water from splashing into the oil cylinder.

Levelling Screws

The entire apparatus rests on the three keys provided at the bottom with levelling screws.

Kohlrausch Flask

It is a specially shaped small-mouth flask for receiving oil from the jet outlet. Its capacity is 50 ml up to the mark in its neck.

PROCEDURE

1. Clean the cup and make sure the cup and the jet are free from dust.
2. Close the cup with the help of the ball valve. Fill the cup with the given oil sample up to the tip of the hook gauge.
3. Insert thermometers in the holders, one in the oil cup and the other in the water jacket, and read the room temperature of the oil.
4. Place the clean standard receiver flask of 50 ml capacity just below the jet. Note down the time required to fill 50 cc of oil into the flask at room temperature.
5. Start the heating process and wait until the temperature of the bath is raised by 12 to 15°C.
6. Cut off the heating process when the temperature of the bath is raised by 12 to 15°C.
7. Stir the water continuously until the temperature of the water and the oil becomes the same.
8. Lift the ball valve and start the stop watch. Simultaneously, note down the time required to collect 50 cc of oil into the flask. Note down the temperature.
9. Repeat the experiment at various temperatures and tabulate the observed readings.
10. Find the kinematic viscosity and the absolute viscosity as explained in the following section using the given formulae.

THEORY

1. Viscosity is the property of a liquid or fluid by virtue of which it offers resistance to its own flow.

   Kinematic viscosity of oil = centistokes (centipoise)

   Here, t is the time taken for collecting 50 cc of oil, A and B are viscometer constants. A = 0.26 and B = 171.5.

2. Absolute viscosity = Kinematic viscosity × density

   Density of oil at different temperatures = T − T_r

   Here, T = Density of oil at tested temperature

   T_r = Density of oil at room temperature

MODEL GRAPHS

1. Temperature Vs Redwood seconds
   
2. Temperature Vs kinematic viscosity
   
3. Temperature Vs absolute viscosity
   

RESULT

It is observed from the graphs that as the temperature increases, the Redwood seconds, kinematic viscosity and absolute viscosity decrease.

VIVA QUESTIONS

1. What are the oils used in Redwood viscometer No. 1 and No. 2?
2. What are the differences between Redwood viscometer No. 1 and No. 2?
3. Define viscosity?
4. What are the units of viscosity?
5. What is viscosity index?
6. What is the necessity of using the water bath?
7. How much oil is collected each time?
8. How to regulate the temperature?
9. What is the effect of temperature on the lubricants?
10. What are the differences between absolute viscosity and kinematic viscosity?