Level Measurement not Based on Buoyancy

Question 1

List the different types of level measurements that are commonly used by industry.

Answer 1

Liquid level, solids level, liquid/liquid interface.

Question 2

Explain the function of level glasses.

Answer 2

Local visual indication of level in a vessel.

Question 3

List five possible problems that may effect the reading of a level glass.

Answer 3

• Capillary action (minimized by design, large diameter for viscous fluid)
• Plugging (eliminated by taking proper reading, i.e. blowdown)
• High level due to boiling/foaming liquids
• High condensation rates within the glass can render it useless
• Density differences between vessel and glass (blowdown to solve this)

Question 4

List and explain two applications where a reflex gauge glass cannot be used.

Answer 4

High pressure steam
Acids/bases

Question 5

Explain the principle behind a color level gauge and give two methods for obtaining a
remote readout for this gauge.

Answer 5

Uses light refraction: Both red and green light is directed into a gauge glass, if water is
present the green is refracted through while the red is not. When vapour is present, red is
observed and green is not.
Liquid level is seen as green and vapour as red. Remote reading may be accomplished by
using mirrors, fibre optics, or cameras.

Question 6

Explain the function of a frost plug, and briefly describe the principle of operation of this
device.

Answer 6

Usually used on cryogenic applications. Frost will form under the correct humidity
conditions on the portion of the gauge that is subjected to liquid level. Above the liquid
level the gauge will be dry.

Question 7

Explain what “ringing or blocking” is as it pertains to ultrasonic level measurement.

Answer 7

Process is too close to sensor, not allowing enough time to calculate the distance.

Question 8

Explain general applications for the three types of level capacitance probes (bare, coated,
sheathed).

Answer 8

Bare: non conductive liquids, dry solids, metal vessels
Coated: conductive/non-conductive/corrosive liquids, metal vessels
Sheathed: increased sensitivity, coated/bare, conductive/non-conductive, non cylindrical
vessel

Question 9

What is dead capacitance as it relates to capacitance level probes.

Answer 9

Capacitor formed by insulator between probe and tank fitting. Always present and never
changes as probe cannot short to tank.

Question 10

Explain how to increase the sensitivity of a coated capacitance level probe that is to be
installed in a metal vessel.

Answer 10

A coated probe would be most sensitive when installed at the center of the vessel. (bare
probe is more sensitive closer to tank wall) Thinner coating will also make C4, and C5
larger.

Question 11

What are the disadvantages of ultrasonic level measurement devices?

Answer 11

Foam
Ringing
Dust/Vapor in the headspace
Turbulence
Affected by temperature and pressure
Affected by filling noise

Question 12

What are the advantages of using ultrasonic level measurement?

Answer 12

Can measure interface (bottom mounted)
Can measure corrosive fluid (externally mounted)

Question 13

What device would be used to measure the level of 2 high vapor pressure liquids?

Answer 13

Guided Wave radar

Question 14

What device helps a load cell work?

Answer 14

Strain gauge