Selecting a Level Switch for Viscous Liquids: FTL31 Supports 1 to 10,000 mPa·s

In chemical, lubrication, filtration, and cleaning systems, viscous liquids often challenge conventional point level switches. Float switches may respond slowly due to sticking, while conductive and capacitive sensors may be affected by conductivity, buildup, or air bubbles. For such applications, a vibronic point level switch offers a more direct selection path.

Why Viscous Media Affect Level Switches

Viscous liquids flow poorly and tend to leave residue on sensor surfaces. If solids are also present, the risk of buildup and coating increases. In these cases, switching stability depends not only on the electronics but also on the sensor design and measuring principle.

FTL31 Applicability: Viscosity and Density

According to Endress+Hauser Technical Information TI01147FEN, the Liquiphant FTL31 is suitable for liquids with a dynamic viscosity of 1 to 10,000 mPa·s. For density, the standard version requires > 0.7 g/cm³, while an optional version supports > 0.5 g/cm³. If solids are present, particle size should be θ < 5 mm.

These parameters form the first selection boundary. If they are exceeded, other measuring principles or models should be considered.

Value of the Tuning Fork Principle in Viscous Applications

FTL31 uses the tuning fork measuring principle with no moving parts. Unlike float switches, it does not rely on mechanical displacement, making it less likely to be blocked by sticky media. Wetted parts are made of 316L stainless steel. Process temperature can reach 150°C, and process pressure ranges from -1 to +40 bar.

Mounting and Process Conditions

For tank or vessel installation, a wall distance of ≥ 10 mm is recommended to reduce the effect of buildup on the switch point. For pipe installation, pipe diameter should be ≥ 50 mm, insertion depth ≥ 10 mm, and flow velocity < 5 m/s. These conditions help minimize turbulence and air bubble interference.

Output and Safety Modes

FTL31 is available in 3-wire DC-PNP and 2-wire AC/DC electronic versions. Users can select MAX safety mode for overfill protection or MIN safety mode for pump dry running protection. The device supports external function testing with a test magnet and provides LED indication.

Selection Recommendation

For viscous liquids, FTL31 can serve as an alternative to float, conductive, capacitive, and optical sensors. However, note that FTL31 is not suitable for hazardous areas, and FTL33 is recommended for hygiene areas. Before final selection, confirm that viscosity, density, solids content, process temperature, and pressure fall within the stated ranges.