Select the Right Liquid Level Sensor

It's important to consider a variety of factors when choosing the type of technology.

By John E. Edwards, P & I Design

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Float displacer. Suitable for point or continuous applications, it measures the change in buoyancy via a torque tube, lever or servo arrangement. The continuous measuring range is set by the displacer length immersed in the tank's external cage, which is preferable for noisy applications, or servo mechanism. The point method uses a float, with the range being limited by the length of the float arm.

Nucleonic. Good for point or continuous duties, this non-contact method, which is independent of fluid density and viscosity, measures the signal strength of a radioactive source beamed across a vessel and has typical ranges of 0.24 m to 3.36 m. Accuracies generally claimed are ±2% of reading. It's the preferred method for monitoring level in flash vessels and reboilers under all temperature and pressure conditions.

Radar. Applicable to point or continuous applications, it measures the travel time of an impulse reflected from the liquid surface. Interference echoes from tank internals, and agitators are suppressed and signals can be characterized to give liquid volume. The sensor doesn't contact the liquid but is exposed to headspace conditions, which don't affect the measurement. Reflectivity requires the liquid dielectric constant, εR, to be at least 1.4 (hydrocarbons are 1.9–4.0, organic solvents are 4.0–10 and conductive liquids are over 10). Adjusting the antenna and signal conditions allows tailoring to the particular process, with guided radar used for low εR and turbulent conditions. The method can handle custody transfer because of its claimed accuracy of ±0.5mm.

Capacitance. For point or continuous service, it suits liquids that can act as dielectrics. Sensitivity increases with the difference in dielectric constants, δεR, between the liquid and the vapor space or between the two liquids. Special designs, involving coated and twin probes, are used when δεR is under 1.0, conductivities exceed 100 μmho, or to overcome probe build-up effects, and when vessel material is non-conducting. Typical accuracies claimed are ±0.25% of span. However, fluid properties affect measurements, so the method isn't suitable for changing conditions. Maximum conditions are 200°C at 100 bar and 400°C at 10 bar.

Ultrasonic. Suitable for point or continuous use, it is based on the time-of-flight principle. A sensor emits and detects ultrasonic pulses that are reflected from the surface of the liquid. The method is non-invasive, with some types being non-contact, and isn't affected by εR, conductivity, density or humidity. Maximum conditions are 150°C at 4 bar.

Load cells. Appropriate for point and continuous applications, such devices, which can be based on strain gauge or piezoelectric technology, measure the weight of the process vessel plus contents. Individual load cell accuracy of 0.03% of full scale is achievable but overall performance depends on correct installation practices to exclude external forces due to associated piping and equipment. For vessels with jackets, agitation and complex piping, it's difficult to obtain an acceptable accuracy. When the container can be totally isolated, as in final dispensing and filling applications, precision weighing can be achieved.

Tuning fork. This method can detect point liquid level but isn't suitable for viscous and fouling applications. Maximum conditions are 280°C at 100 bar.

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