Figure 1: Liquid level sensing principle

This method relies on time-of-flight measurement principles based on sound propagation time. The velocity of sound in air varies by temperature. In dry air at 20°C (68°F), the speed of sound is 343 m/s or 2.91 km.

Since sound waves are used for level sensing, unlike a laser, it is challenging to focus the beam on a tiny target. The width of the beam containing the peak ultrasonic power could be significant.

The beam angle α is the angle (figure 2) at which relative power is more than 50% (or -3dB) of the peak ultrasonic power. During the sensor installation in liquid-level sensing, the beam angle α serves as the reference parameter.

Figure 2: Reference beam angle α.

Example of LVCN210, LVCN318, LVCN414 from OMEGA for ultrasonic level sensing

Figure 3: LVCN210 ultrasonic Level transmitter and controller by OMEGA (Source)

The small tank ultrasonic sensor is offered in different models, each with its own set of components. These typically include a sensor unit, USB Fob, and FKM gasket for configuring, installing, and operating the ultrasonic sensor.

The small tank ultrasonic sensor can be configured through a free PC software program provided by the component supplier. The software from the component supplier must first be installed on the computer. The ultrasonic sensor communicates with computer software through a USB interface called a Fob. Before connecting the sensor to the computer through a USB port, you need to download the required software and configure the necessary cable connections. Using the software, users can click on the Flow, Level, pH, Environmental, and Pressure Sections mentioned in the software and press on Products to configure the sensor for the targeted application. Once the ultrasonic sensor is configured and before installation, isolate the white and green wires from active power, which is essential to prevent any shortening of the configuration circuit.

Steps in configuring ultrasonic level sensor

1. Output Configuration

This helps to set the relays regarding pump/valve operations in high or low alarms. It also sets fail‐safe for the relays and output of the sensor (current, voltage, or frequency).

Modes in sensor output operation

Pump/Valve mode: A pump or valve can be selected to fill or empty the tank automatically.

Empties Tank: This will automatically set relay(s) to empty a tank. The start level will be above 4he Stop level for each relay, as shown in Figure 4 (Auto empty).

Fills Tank: This will automatically set relay(s) to fill a tank. Each relay's start level will be below the Stop level, as shown in Figure 4 (Auto Fill).

Not Applicable: This appears when this function is not available.

Figure 4: Empties tank and fills tank mode (Source)

2. Tank Shape Selection

It defines the tank's shape and dimensional information relative to the sensor’s location on the tank. The software allows the sensor to be configured in volumetric units (Gallons or Liters) or distance (Height of liquid) units (inches, cm, feet, or meters). Users can easily switch between units or change from Distance to Volume using the Tank Shape button in the software.

3. Level Configuration

This feature enables users to input distance values for the operational range of the sensor for their particular application. The Sensor Height and Fill Height values determine the operational range for the voltage or frequency output. Sensor Height is the distance from the bottom of the tank to the bottom of the transducer. Fill Height indicates the distance from the bottom of the tank to the maximum liquid Height, as shown in Figure 5.

Figure 5: Sensor height and fill Height (Source)

4. Writing to the sensor

After setting up the tank shape and level configurations, click "Write to Unit" in the software to transfer this setup into the sensor’s memory. When completed, this configuration will remain inside the sensor's memory. It will only change if the sensor is connected again to modify the configurations and a new configuration is written to the sensor. Even if there’s a power loss, the sensor’s memory will retain the configuration. Once done, the sensor can be mounted for the level sensing operation.

Key factors to be considered in ultrasonic-level sensing

• Obstruction avoidance: The tank wall or other obstructions inside the tank may provide error readings. Consider the shapes and dimensions of the tank to avoid inaccuracies.
• Mounting position or placement: Filtering algorithms prioritize large targets over small targets and noise. This helps tank-level monitoring applications, as the liquid level produces the most extensive acoustic returns. The sensor’s mounting position affects reliability and consistent performance.
• Off-the-side reflections: If the tank has smooth walls inside, the sensor may detect the wall instead of the liquid, resulting in inaccurate readings. Corrugated or non-smooth inside walls can also negatively impact the sensor's performance. Narrow beam pattern and stability filtering let the sensors choose the target that gives the most significant acoustic return, generally the liquid level inside the tank.
• Chemical compatibility: Chemicals in liquids can affect electronics/metals, which might cause corrosion or damage. In such cases, F-option (fluoro silicone sealant) for water or particulate protection and P-option (perylene coating) for corrosion protection can facilitate operation near hazardous chemicals. It is also helpful for measuring fossil fuels.
• Oil level sensing in sealed tank: In a few cases of oil level sensing, condensation or frost on the sensor’s surface can be taken as a target, causing the sensor to report minimum distance or reduce the sensitivity. Therefore, in such cases, a self-cleaning feature, where the face of the transducer is heated gently to remove any moisture/condensation on the sensor’s face, helps reduce the moisture on the sensor.

How do you choose the suitable sensor for ultrasonic liquid-level sensing?

Factors to be considered while choosing a level sensor are:

• Accuracy: Is the difference between the sensor's output value and the target's distance. For example, an ultrasonic sensor with up to 1% accuracy of the set measuring range can detect objects as small as 1mm within a 10 cm range. Accuracy is essential for precise level measurements.
• Measuring distance/ range: The measuring range describes the maximum distance the target is accurately measured, typically from a few mm up to 10 meters. Liquid measurements require a sensor with a range of at least 25% greater than the expected maximum measurement. Material size, shape, and orientation can also impact measurement accuracy, with large, flat water surfaces being more easily detected at a distance than curved or granular objects.
• Resolution: This indicates the smallest incremental distance the sensor can range. For example, the resolution of ultrasonic sensors operating at 58 kHz is one cm.
• Environmental conditions: The deployment environment should be considered when selecting sensors. For example, harsh or corrosive environments need corrosion-resistant sensing.
• Output type: Level sensors offer digital, analog, and pulse output types. Choose sensors with the required output for the application, such as RS232, TTL, I2C, Pulse-Width, Analog Voltage, and Analog Envelope.

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Conclusion

When selecting an ultrasonic sensor for a particular application, it is crucial to verify deployment conditions, like sensing range and required accuracy, the nature of the liquid (Chemical / moist forming), etc. Factors like beam angle, power consumption, and operating frequency also play an essential role in reliable output readings. A sensor should be compatible with the existing equipment and environmental conditions. Additionally, a few other aspects of sensors, such as blind zone correction, temperature compensation, narrow beam width, etc., contribute to precise liquid level measurements.