Level Measurement Based on Buoyancy

Question 1

List at least 4 buoyancy level measurement devices.

Answer 1

1. Mercury float
2. Magnetic float
3. Side mounted
4. Magnetic Reed
5. Magneto strictive
6. Tank gauge
7. Guided wire
8. Displacer

Question 2

Why does a level switch need dead band?

Answer 2

In order to prevent chatter

Question 3

What are the pros of mounting a float directly into the process vessel?

Answer 3

Good if plugging is a problem
Cheaper
Simple

Question 4

What are the cons of mounting a float directly into the process vessel?

Answer 4

Turbulence due to inlet or foam can create inaccurate readings

Have to take the tank fully out of service (shutdown) in order to perform maintenance

Question 5

What are the advantages of mounting a float in an external chamber?

Answer 5

Good for excessive turbulence

Easier to isolate and service

Question 6

What are the disadvantages of mounting a float in an external chamber?

Answer 6

Expensive (more materials like flanges are required)

Variation in density de to temperature can give inaccurate readings. A blow down has to be performed.

Question 7

Give an application for a thin-walled torque tube and explain why it may be required.

Answer 7

When density differences are small (interface) or buoyancy does not produce enough force
(light low density product) a thin walled torque tube will allow a more sensitive instrument.

Question 8

What are the advantages of using a point level float?

Answer 8

Few moving parts
Cheap
Simple
Can measure interface

Question 9

What are the disadvantages of using a point level float?

Answer 9

Limited motion
Can’t read continuous level
Change in density is an issue
Fouling is an issue
Can’t measure diff gap
Turbulence/foam gives inaccurate readings

Question 10

What buoyancy devices can be used to measure diff gap?

Answer 10

Magnetic reed switch
2 float devices