Identify Characteristics of Level Measurement Technologies

Identify characteristics of various level-measurement technologies.

An important tip is to commit to memory the operating principle of each instrument type, and then reason from that basis what each type’s characteristics will be:

Sightglass

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Liquids only
2. Yes, but only if both ports submerged
3. No
4. Simple and inexpensive
5. Manual indication only – no output signal

Float

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Liquids yes, solids only if retracted by automated winch
2. Yes, if float density is set properly
3. No
4. Relatively simple and inexpensive
5. Requires guides and other apparatus to stabilize in a tall vessel; doesn’t work well in turbulent processes

Hydrostatic (DP with direct contact)

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Liquids only
2. Yes, if both ports submerged
3. Yes, density!
4. Fairly simple and reliable
5. DP sensor must withstand process fluid corrosion

Hydrostatic bubbler (dip tube)

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Liquids only
2. Yes, if both ports submerged
3. Yes, density!
4. Fairly simple and reliable
5. DP sensor must withstand process fluid corrosion

Displacer

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Liquids only
2. Yes, if displacer is fully submerged
3. Yes, density!
4. Simple and reliable
5. Susceptible to vibration; displacer may become stuck inside of the cage if the process fouls

Ultrasonic

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Yes
2. No
3. Yes, the speed of sound through the transmission media
4. No contact with process fluid required
5. Requires sharp change in density to function (e.g. layers of foam may interfere with measurement); irregular shapes inside of vessel may cause scattering and/or false echoes

Non-contact radar

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Yes
2. No
3. Yes, the speed of light through the transmission media
4. No contact with process fluid required
5. Requires sharp change in permittivity to function (e.g. certain substances such as wood chips may be difficult to detect); irregular shapes inside of vessel may cause scattering and/or false echoes; permittivity of substance(s) above the measured level will affect its accuracy and may change with the density of that substance

Guided-wave radar

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Yes
2. Yes
3. Yes, the speed of light through the transmission media
4. Waveguide helps ensure a strong echo signal
5. Waveguide must withstand corrosion and other stress from the process fluid; requires significant transitions in permittivity to detect surface and interface levels; irregular shapes inside of vessel may cause scattering and/or false echoes; permittivity of substance(s) above the measured level will affect its accuracy and may change with the density of that substance

Magnetostrictive

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. No
2. Yes, if float density is set properly
3. No
4. Highly accurate measurement of float position (sub-millimeter resolution capability!)
5. Susceptible to fouling of float on waveguide rod

Weight (load cells on vessel)

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Yes
2. No, unless total height of liquid is fixed (e.g. using an overflow pipe)
3. Yes, if level (height) measurement is desired; No, if mass measurement is desired
4. No contact with process fluid required; yields a linear measurement regardless of vessel geometry
5. All mechanical stress must be eliminated from vessel

Capacitive

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Yes
2. Yes, so long as probe is fully submerged
3. Yes, permittivity of the process liquid for the non-conducting style
4. Inexpensive
5. Prone to calibration errors due to stray capacitance; irregular shapes inside of vessel may cause nonlinear response

Nuclear

Find the below mentioned the questions.

1. Can it be used to measure the level of both liquid and solid materials?
2. Can it be used to measure liquid-liquid interfaces?
3. Does its calibration depend on some fluid property such as density?
4. What are the Special advantages?
5. What are the Special disadvantages?

Answers

1. Yes
2. Yes, if density or other radiation attenuating/scattering properties are sufficiently different between the two liquids
3. Yes, density!
4. No contact with process fluid required
5. Expensive to license and operate; calibration errors may result with fouling of vessel walls

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