Why Do Ultrasonic Level Meters Become "Inaccurate"? A Deep Dive into Four Common Technical Pain Points

Target Audience: Engineering Technicians, Equipment Selection Managers, System Integrators

Introduction

Ultrasonic level meters have become a mainstream choice for water tank, pool, and oil tank monitoring due to their non-contact measurement, easy installation, and wide media applicability. However, in practical applications, ultrasonic level meters are not a "universal solution"—many users report issues like reading drift, signal loss, and measurement instability. What are the root causes? This article will deeply analyze four common technical pain points in the ultrasonic level meter industry.

Pain Point #1: Blind Zone Limitation – The "Invisible" Near-End Level

Ultrasonic level meters have a physical region directly in front of the probe where measurement is impossible, known as the "blind zone." This exists because the piezoelectric crystal requires time for "ring-down" after transmitting ultrasonic pulses, during which it cannot receive echo signals.

Taking the Siemens LU150 as an example: its blind zone is 250mm. When the level rises within 250mm of the probe, the instrument cannot measure normally, potentially causing reading jumps or complete failure. For shallow tanks (e.g., less than 1 meter in height), the blind zone can occupy over a quarter of the measurable range.

Industry Data: Standard ultrasonic level meters typically have blind zones between 200mm and 500mm. For tanks under 1 meter in height, the usable measurement range may be only 50-80cm.
Pain Point #2: Reading Drift Due to Temperature Effects

The measurement principle of ultrasonic level meters makes them highly sensitive to ambient temperature. They calculate distance by measuring the time from sound wave transmission to reflection: Distance = (Speed of Sound × Time) / 2. The speed of sound in air changes with temperature—a 1°C change alters sound speed by approximately 0.6m/s.

Although most ultrasonic level meters have built-in temperature compensation, compensation often fails in these scenarios:

• Large temperature differences at tank top – Outdoor tanks experience dramatic temperature changes between day and night
• Obvious hot air flow – In industrial settings, rising hot air distorts sound wave paths
• Frequent steam/condensation – In scenarios like hot water tanks or sewage treatment pools, steam causes complex sound speed variations

Pain Point #3: False Echoes and Spurious Signal Interference

Ultrasonic waves reflect off any obstacle in their path. Fixed structures inside tanks—support beams, ladders, agitators, inlet baffles—all create "false echoes." When these false echoes are stronger than the true liquid surface echo, the instrument gets "fooled" and misidentifies fixed structures as the liquid level.
Interference from electrical noise: Frequency converters, high-power motors, and other equipment generate electromagnetic noise that couples into sensor signal lines. A Siemens forum user reported that when a Siemens VSD drive started, ultrasonic level meter readings would go "full scale" within 2-4 minutes, returning to normal only when the VSD stopped.

Pain Point #4: Foam/Steam Absorption and Scattering of Sound Waves

Ultrasound is a mechanical wave that relies on medium vibration to transmit energy. When foam exists on the liquid surface, it absorbs and scatters ultrasonic energy, causing severe attenuation of echo signal strength—the instrument cannot "hear" sufficient echo.

Consequences:

Complete measurement failure

Instrument enters "signal loss" state, displaying fault codes

Readings jump, oscillating between true level and erroneous values
Pain Point #5: Stringent Installation Requirements

Ultrasonic level meter accuracy heavily depends on installation quality. Siemens technical documentation specifies critical requirements:

Noise isolation: Maintain distance from high-voltage lines, contactors, and VFDs

Vertical alignment: Probe must be perpendicular to liquid surface

Obstacle avoidance: Keep distance from fill/discharge ports, agitators, and tank walls

Torque control: Probe must not be overtightened, as this increases "ringing" and causes measurement anomalies
Pain Point Summary