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:
Can it be used to measure the level of both liquid and solid materials?
Can it be used to measure liquid-liquid interfaces?
Does its calibration depend on some fluid property such as density?
What are the Special advantages?
What are the Special disadvantages?
Answers
Yes
No
Yes, the speed of sound through the transmission media
No contact with process fluid required
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
Can it be used to measure the level of both liquid and solid materials?
Can it be used to measure liquid-liquid interfaces?
Does its calibration depend on some fluid property such as density?
What are the Special advantages?
What are the Special disadvantages?
Answers
Yes
No
Yes, the speed of light through the transmission media
No contact with process fluid required
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
Can it be used to measure the level of both liquid and solid materials?
Can it be used to measure liquid-liquid interfaces?
Does its calibration depend on some fluid property such as density?
What are the Special advantages?
What are the Special disadvantages?
Answers
Yes
Yes
Yes, the speed of light through the transmission media
Waveguide helps ensure a strong echo signal
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