EXTRASOUND
Externally mounted ultrasonic level transmitter
Measuring the level of liquid inside a tank using an innovative externally mounted ultrasonic sensor is a non-invasive and highly effective method for level monitoring. This approach is particularly advantageous in demanding applications where direct contact with the liquid is undesirable, such as in corrosive, hazardous, ultra- low temperature or sterile environments. In industries with high pressure vessels, it can be used as a portable nonintrusive level sensor for inspection and monitoring purpose.
Principle of Operation
The externally mounted ultrasonic sensor operates on the principle of ultrasonic pulse-echo technology. Here’s how it works:
1. Ultrasonic Pulse Generation:
The sensor emits high-frequency ultrasonic pulses (typically in the range of hundred of kilo hertz up to mega hertz) through the wall of the tank (typically this sensor can be attached the bottom of tank). These pulses travel through the tank wall and into the liquid. Only a small part of emitted sound wave enters to the tank and reach the liquid media.
2. Pulse Reflection:
When the ultrasonic pulse reaches the liquid surface, it is partially reflected back toward the sensor. The remaining energy continues to propagate through the gas above the liquid phase.
3. Echo Detection:
The sensor detects the reflected echo and measures the time delay between the emitted pulse and the received echo.
4. Level Calculation:
Using the speed of sound in the liquid (which is a known value or can be builtin calibrated or can be transferred to the sensor by the user), the sensor calculates the distance to the liquid surface. The liquid level is then determined by subtracting this distance from the total height of the tank.
Key Components
1. Ultrasonic Transducer:
o Generates and receives ultrasonic pulses.
o Typically made of piezoelectric materials that vibrate at high frequencies when an electrical signal is applied.
2. Signal Processing Unit:
o Processes the received echo signals and calculates the time delay.
o Converts the time delay into a level measurement.
3. Mounting Hardware:
o Ensures proper coupling between the sensor and the tank wall.
o May include clamps, adhesives, or magnetic mounts for secure attachment.
4. Display/Output Interface:
o Provides real-time level readings.
o Outputs data in various formats (e.g., 4-20 mA, HART, Modbus, or digital communication protocols).
Advantages of Externally Mounted Ultrasonic Sensors
1. Non-Invasive Measurement:
o No need to penetrate the tank, making it ideal for sterile, hazardous, or pressurized environments.
2. No Contact with Liquid:
o Suitable for corrosive, abrasive, or viscous liquids that could damage internal sensors.
3. Easy Installation and Maintenance:
o Can be installed without shutting down the tank or draining the liquid.
o Minimal maintenance required since there are no moving parts or direct exposure to the liquid.
4. Wide Range of Applications:
o Works with various liquids, including water, chemicals, oils, and slurries.
5. Cost-Effective:
o Eliminates the need for complex tank modifications or internal sensor installations.
Challenges and Considerations
1. Tank Wall Material and Thickness:
o The ultrasonic pulse must travel through the tank wall, so the material and thickness must be compatible with the sensor's frequency and power.
o Thicker walls or highly attenuative materials (e.g., certain plastics or composites) may reduce signal strength.
2. Liquid Properties:
o The speed of sound in the liquid must be known or calibrated for accurate measurements.
o Foam, vapor, or suspended solids can interfere with the ultrasonic signal.
3. Temperature Effects:
o Temperature changes can affect the speed of sound in the liquid, requiring compensation or recalibration.
4. Mounting Surface Preparation:
o The tank wall must be smooth and clean to ensure proper coupling of the ultrasonic signal. It is recommended that apply the transducer in a polished area at the bottom of tank.
o It would be better to mount the transducer on the part of the tank which there is enough space for propagation of sound wave free of reflection from internal obstacles. Please note that beam angle of emitted sound is at least 15 degree.
o Irregularities or coatings on the tank wall can attenuate or distort the signal.
Installation Guidelines
1. Sensor Placement:
o Mount the sensor on a flat, smooth area of the tank wall.
o Ensure the sensor is positioned at a height that allows it to measure the full range of liquid levels.
2. Coupling Medium:
o Use a coupling gel or adhesive to ensure efficient transmission of ultrasonic pulses through the tank wall.
3. Calibration:
o Calibrate the sensor by inputting the tank height and the speed of sound in the liquid or use second transducer as calibration device.
o Perform a dry calibration (empty tank) and a wet calibration (known liquid level) for optimal accuracy.
4. Environmental Considerations:
o Protect the sensor from extreme temperatures, moisture, and physical damage.
o Ensure the mounting location is free from vibrations or external noise that could interfere with measurements.
Applications
1. Chemical Storage Tanks:
o Monitor levels of corrosive or hazardous liquids without direct contact.
o High pressure vessels.
o Cryogenic storage tanks.
2. Water and Wastewater Treatment:
o Measure water levels in tanks, reservoirs, or sumps.
3. Food and Beverage Industry:
o Ensure hygiene by avoiding internal sensors in sterile environments.
4. Oil and Gas Industry:
o Monitor fuel or oil levels in storage tanks.
5. Pharmaceutical Manufacturing:
o Measure levels of sensitive or sterile liquids without contamination risk.
Technical Specifications:
• Measurement Range: 0.5 m to 15 m (depending on sensor model)
• Accuracy: ±0.25% of full scale
• Output Signal: 4-20 mA, HART, Modbus
• Power Supply: 24 VDC
• Operating Temperature: -40°C to 80°C
• Tank Wall Thickness: Up to 50 mm (depending on material)
• Material Compatibility: Stainless steel, carbon steel, plastic, fiberglass