1. UNIVERSITÀ DEGLI STUDI DI SALERNO
Bachelor Degree in Chemical Engineering
Course: Process Instrumentation and Control
(Strumentazione e Controllo dei Processi Chimici)
Measuring devices of the main process variables
Level measurement
Rev. 1.5 – April 10, 2019

2. LEVEL SENSORS CLASSIFICATION
1st CLASSIFICATION
Based on the separation surface:
Gas / liquid,
Vapor / liquid,
2 unmixable liquids
Solid (granular or powder) / air
Fluidized solid / air
2nd CLASSIFICATION
Based on the instrument function:
Continuous Level sensors
They continuously measure the height of a separation surface generated by gravity (for liquid and granular solid) from a reference plane.
Point-level sensors or multi-point sensors
They detect when a separation surface (for liquid and granular solid) is above or below a reference height.
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

3. LEVEL SENSORS CLASSIFICATION
3rd CLASSIFICATION
Depending on whether the sensor is in direct contact or not with the measuring medium:
contact sensors,
non-invasive or non-intrusive sensors
4th CLASSIFICATION
Depending on whether the sensor is immersed or not in the measuring medium:
Immersion probes Surface probes
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

4. COMMON LEVEL SENSORS for LIQUIDS
from Magnani, Ferretti and Rocco
(2007)
Hydrostatic sensor
NOTE:
Stevin’s principle is applied.
The measurement is insensitive to foam formation and bottom deposit
l= p 2 − p 1 ρ l g
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

5. COMMON LEVEL SENSORS for LIQUIDS
Float
sensor
float
COUNTER-
WEIGHT
Visual sensor
Float (or probe) sensor
↓ ↓
Level indicator LI
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

6. CAPACITANCE LEVEL SENSORS
Variable Capacitance: They measure the capacitance variation between the sensor and the wall of the vessel caused by level variation of the liquid or the solids (granular or powder)
They are sensitive to the dielectric constant variation of the vessel content
They are recommended also for liquids at high pressure and temperature (up to 500 bar and 500°C) and for powders and granular solids with a constant and much larger permittivity than vacuum (or air) permittivity
They have a good resistance to mechanical and chemical corrosion
Good precision
The calibration has to be performed in the measuring range
It works as a transducer
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

7. Process Instrumentation and Control - Prof M. Miccio
SONAR LEVEL SENSOR
SONAR (SOund Navigation And Ranging): It detects the transmission (or propagation) time of a sound or an ultrasound
tTR =2l/C
l = Level decrease from transmitter/receiver (T/R)
C = Sound (or ultrasound) velocity
They are recommended for liquid at atmospheric pressure or slightly higher and for granular solids having particle sizes larger than a tenth of the wave length of the transmitted sound.
There is no contact with the vessel content.
The measurement does not depend on the electric and physical properties of the content.
Good precision.
They cannot be used for vacuum or pressurized vessels and for acoustic absorbent vapors.
It works as a transducer
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

8. RADIOISOTOPE-EMISSION LEVEL SENSOR
It is used also for solids
It works as a transducer
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

9. Process Instrumentation and Control - Prof M. Miccio
PROBE LEVEL SENSOR
For solids in high silos (tens of meters)
High precision
It requires programmed maintenance for moving components
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

10. CONDUCTIVE LEVEL SENSORS
Conductive: It detects the liquid level from the measurement of the conductivity betwen two or more specific points thanks to the probes installed into the vessel
three points for measuring three level states:
minimum
medium
maximum
They can be used only for conductive liquid having conductance between 1 S and 10-5 S (R from 1  to 105 )
They are use as point-level for discrete-pointor continue measurement
Robustly built (low maintenance is required)
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

11. VIBRATING LEVEL SENSORS
Vibrating: a vibrating probe (piezoelectric element) operating at 400 Hz for liquids and Hz for powders. The frequency changes if the material brushes the probe (e.g. 20%)
 To detect the point-level positions (max and min) in liquids and solids (powders)
04/11/2019
Process Instrumentation and Control - Prof M. Miccio

12. Process Instrumentation and Control - Prof M. Miccio
FINAL SUMMARY
04/11/2019
Process Instrumentation and Control - Prof M. Miccio