Posts

In industrial plants, accurate level measurement of liquids, solids, and semi-solids is vital for process safety, quality control, and operational continuity. Among the available technologies, capacitive level transmitters stand out due to their high accuracy, robust design, and broad range of applications. These devices are widely used in industries such as food, chemicals, petrochemicals, cement, pharmaceuticals, and energy.

Capacitive Level Transmitters

WORKING PRINCIPLE

Capacitive level transmitters operate on the principle of capacitance change. Essentially, the system functions as follows:

Formula: C = ε · A / d

  • C: Capacitance (Farads)
  • ε: Dielectric constant of the medium
  • A: Electrode surface area
  • d: Distance between electrodes

A probe (electrode) inserted into a tank, together with the tank wall, forms a capacitor. As the tank fills, the dielectric constant changes depending on the medium:

  • Air → low dielectric (ε ≈ 1)
  • Water → high dielectric (ε ≈ 80)

As the fill level increases, the capacitance value changes, and the transmitter converts this variation into a level measurement.

EXAMPLE CALCULATION

For a 2 m probe with an electrode area of 50 cm² and a distance of 5 mm:

  • When filled with air: C_air ≈ 8.85 pF
  • When filled with water: C_water ≈ 708 pF

This large difference enables highly accurate level measurement.

APPLICATIONS

  • Liquids: water, acids, solvents, oils, paints
  • Solids: flour, sugar, cement, plastic granules, coal dust
  • Special media: foamy liquids, sticky substances, high-pressure or high-temperature tanks

ADVANTAGES

  • Wide applicability for liquids and solids
  • Resistant to pressure and temperature variations
  • High accuracy and repeatability
  • Corrosion-resistant coatings for aggressive chemicals
  • Easy calibration and setup

DISADVANTAGES

  • Accuracy depends on dielectric constant
  • Build-up of sticky materials may cause errors
  • Improper grounding in metallic tanks may cause faulty readings

TECHNICAL SELECTION CRITERIA

  • Dielectric constant: materials with ε < 2 require high-sensitivity models
  • Temperature and pressure: ensure compatibility with process conditions
  • Tank structure: choose appropriate probe for plastic, metal, or concrete tanks
  • Explosive environments: use ATEX or IECEx certified models

CALIBRATION AND MAINTENANCE NOTES

  • Calibrate at empty and full tank conditions
  • Regularly clean probes from dust or deposits
  • Replace damaged insulation (Teflon or ceramic)
  • Ensure proper grounding to prevent measurement errors

ENGINEERING NOTES

  • Accuracy: up to ±0.5% FS
  • Output signals: 4–20 mA, HART, Profibus, Modbus
  • Measurement range: 0.2 m – 20 m
  • Process connections: flanged, threaded, or clamped

RECOMMENDED DESIGNS BY APPLICATION

Application Dielectric Constant Recommended Design Notes
Water tank ≈ 80 Standard probe High accuracy
Oil tank 2 – 5 Insulated probe Requires calibration due to low ε
Cement silo 3 – 7 Rod/rope probe Regular cleaning required
Acid tank 50 – 70 PTFE-coated probe High corrosion resistance

CONCLUSION

Capacitive level transmitters provide a reliable, economical, and precise measurement solution. Thanks to their dielectric-based principle, they can be used effectively in both liquids and solids. With proper selection, calibration, and maintenance, they ensure long-lasting and accurate measurements in various industries.

Metal tube flowmeters are robust instruments designed for accurate flow measurement under high pressure, high temperature, and aggressive fluid conditions. Compared to glass tube rotameters, they offer greater durability and are widely used in chemical, petrochemical, power generation, water treatment, food, and pharmaceutical industries.

Metal Tube Flowmeters and Industrial Applications

WORKING PRINCIPLE

They operate on the variable area principle. As the fluid flow increases, the float rises. In metal tube designs, the float position is detected either by magnetic sensors or mechanical indicators.

Fundamental equation:

Q = C · A(h) · √(2ΔP / ρ)

Q: flow rate, C: coefficient, A(h): cross-sectional area depending on float position, ΔP: pressure drop, ρ: fluid density.

STRUCTURAL FEATURES

  • Measuring tube: stainless steel or alloy steel
  • Float: selected with different densities for higher accuracy
  • Indicators: Mechanical (pointer type) or electronic (4-20 mA, HART, Profibus)
  • Operating range: up to 100 bar pressure and 400 °C temperature

ADVANTAGES AND LIMITATIONS

Advantages:

  • Reliable under harsh process conditions
  • Suitable for both liquids and gases
  • Easy integration with automation systems through electronic outputs

Limitations:

  • More expensive than glass tube models
  • Limited direct visual observation, relies on indicators

APPLICATION AREAS

  • Acid/alkali measurement in chemical and petrochemical plants
  • Steam and condensate flow monitoring in power plants
  • Chemical dosing control in water treatment facilities
  • Hygienic process monitoring in food and pharmaceutical industries

STANDARDS AND CALIBRATION

  • ISO 5167: Flow measurement standards
  • ASME MFC: Standards for flow measurement devices
  • ATEX-certified versions: For explosive atmospheres
  • Regular calibration required to maintain accuracy

CONCLUSION

Metal tube flowmeters play a vital role in modern industry due to their durability and reliability. By combining with digital technologies, they can be seamlessly integrated into automation systems, contributing to process optimization and efficiency.