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Test setup to determine the flow limit
A very long triangular suspended arm is created from very light strips of balsa wood. Reinforcement is recommendable to make it a bit stiffer so that it does not
immediately break. Very simple but low-friction joints are created with bent wire. These bear the pendulum arm. A paddle made of thin sheet aluminum is glued
to the bottom end of the pendulum arm. Paddle area: 20 x 20 mm. This very light-weight paddle is dipped in the liquid to be investigated.
A paper with parallel lines for reference is placed under the Petri dish.
 
Test setup with a pendulum arm, enlarged section of the gearing, and the Petri dish with a scale
If the pendulum is pulled back and released, gravity moves the pendulum back into its vertical position. The viscosity of the liquid slows this process. It takes time before
the paddle reaches its vertical position since the return force of the pendulum is very small due to its great length and low weight. The depth that the paddle is dipped influences
the sensitivity of the test setup.
If the liquid to be investigated does not have a flow limit such as salad oil, the pendulum always swings back into the vertical position. If the results are doubted, the pendulum
can be swung in the opposite direction, from left to right. The results will be the same.
If a thick salad dressing is used such as 1,000 Island, the results are different. Use a transparent dressing with fine herb particles floating around.
When you shake the bottle, you are probably thinking that the contents are a liquid. Not so - the herbs do not fall to the bottom. The dressing contains one or more thickeners
suitable for foods that give the dressing a very small flow limit. When designing foods, the appearance of the product must be enticing and stimulate the customer's desire to buy.
  
Fig. 10.18, Fig. 10.19, Fig. 10.20
The herb particles float in the salad dressing. Fruit yogurt has a very low flow limit.
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| Fig. 10.21 |
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Fig. 10.22 |
Organic yogurt stays put. It has a high flow limit.
Whether or not the thickener improves the taste is beside the point. Fruit yogurt has an artificial flow limit. The acid and sugars in the fruit destroys the natural flow limit
of organic yogurt. But back to the experiment.
When we investigate the herb dressing, the pendulum stops before it reaches a vertical position. The paddle more or less freezes at the flow limit of the dressing. It never
reaches the vertical position.
(Make sure that nobody wiggles the test setup).
This experimental test setup that only costs a few dollars can determine the flow limit of a substance with a very high level of precision.
The only source of error in this experiment is the friction of the wire bearing. This is negligible, however. To improve the situation, two pieces of thread could be used to
suspend the pendulum.
? Can simple means be used to increase the measuring range of the apparatus?
Yes, by changing the paddle surface, and by attaching a weight close to the paddle .
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