Fmu90
Fmu90
In the field of industrial automation and process control, the demand for precise level and flow measurement is paramount. The FMU90 represents a sophisticated class of ultrasonic transmitter and controller units designed to interface with ultrasonic sensors to provide a comprehensive monitoring solution. Unlike integrated transmitters where the sensor and electronics are housed in a single unit, the FMU90 serves as a remote evaluation unit. This separation allows the electronics to be mounted in a safe, accessible location—such as a control cabinet—while the sensor is positioned in the potentially harsh environment of the measurement point.
As a professional manufacturer of industrial level measurement instruments, Welk provides a range of solutions including radar level meters and ultrasonic level sensors. Understanding the technical nuances of controllers like the FMU90 is essential for engineers tasked with managing water treatment, chemical processing, and industrial wastewater applications. This guide explores the principles, applications, and selection criteria for this technology to assist in optimizing process efficiency.
Understanding Ultrasonic Level Measurement Principles
Before diving into the specific functions of the FMU90, it is necessary to understand the underlying physics of ultrasonic measurement. This technology is based on the "Time of Flight" (ToF) principle.
The Time of Flight Principle
An ultrasonic sensor, connected to the FMU90, emits a series of ultrasonic pulses toward the surface of the medium (liquid or solid). These pulses are reflected by the surface and received back by the sensor. The controller measures the time interval ($t$) between the emission and the reception of the pulse. Given the speed of sound ($v$) in the medium (usually air), the distance ($D$) from the sensor to the surface can be calculated using the formula:
$$D = \frac{v \cdot t}{2}$$
Since the total distance ($L$) from the sensor to the bottom of the tank is known through calibration, the level ($H$) of the material is determined by:
$$H = L – D$$
Factors Affecting Sound Velocity
The accuracy of ultrasonic measurement is highly dependent on the speed of sound. In air, the speed of sound is approximately 343 meters per second at 20°C, but it changes by about 0.17% per degree Celsius. To maintain accuracy, the FMU90 utilizes temperature compensation, often via a temperature sensor integrated into the ultrasonic transducer or an external Pt100 sensor. This ensures that fluctuations in ambient temperature do not result in measurement errors.
Key Functions and Capabilities of the FMU90 Controller
The FMU90 is not merely a display unit; it is a multi-functional controller capable of complex signal processing and logic execution. It is typically available in versions that support either one or two sensor inputs, allowing for multi-point measurement or differential level control.
Signal Processing and Echo Analysis
One of the primary advantages of a dedicated controller is its ability to perform advanced echo analysis. In industrial tanks, internal structures like agitators, ladders, or inlet pipes can create "false echoes." The FMU90 features sophisticated algorithms to map out these fixed reflections, ensuring the device tracks only the true level of the medium. This "mapping" or "suppression" is critical in narrow vessels or tanks with complex internals.
Multi-Channel Versatility
With dual-channel versions, the FMU90 can perform simultaneous measurements in two different tanks. Alternatively, it can calculate the difference between two levels (useful for monitoring screen head loss in wastewater plants) or the sum of two levels. This versatility reduces the need for multiple transmitters, lowering the total cost of ownership.
Data Logging and Communication
Modern process environments require seamless data integration. The FMU90 typically offers various output options, including 4-20mA HART, PROFIBUS DP, or Modbus RS485. It often includes integrated data logging capabilities to record minimum and maximum values, as well as totalized flow volumes for environmental reporting.
Open Channel Flow Measurement and Flume Integration
Beyond simple level measurement, the FMU90 is widely utilized for open channel flow measurement in water treatment and irrigation systems. By measuring the head (level) of water upstream of a primary device—such as a weir or a flume—the controller can calculate the flow rate using pre-programmed hydraulic formulas.
Support for Primary Devices
The FMU90 typically includes a library of standard flow curves for common structures, including:
* Khafare Flumes
* Parshall Flumes
* V-Notch Weirs
* Rectangular Weirs
For custom channels, users can input a manual linearization table (up to 32 points) to define the relationship between level and flow. This makes it an ideal solution for monitoring plant influent and effluent where gravity-fed channels are the norm. For more information on various sensor options for these applications, you can Review product options and application support.
Pump Control and Industrial Automation Logic
In many applications, the measurement of level is directly linked to the control of pumps. The FMU90 features integrated relay outputs that can be configured for various control strategies, eliminating the need for an external PLC for basic pump management.
Lead-Lag and Alternation
To ensure even wear on multiple pumps, the FMU90 can implement pump alternation. In a two-pump system, the controller will switch which pump starts first for each cycle. If the level continues to rise despite the first pump running, the controller can activate the second "lag" pump to assist.
Specialized Control Features
* Pump Rake Control: Specifically for wastewater screens, the controller can activate a cleaning rake based on the level differential across the screen.
* Stormwater Overflow Monitoring: Detecting and logging overflow events in combined sewer systems.
* Backwater Detection: Identifying when downstream conditions are impeding flow and adjusting calculations or triggering alarms accordingly.
Technical Selection Criteria for Ultrasonic Systems
When selecting an ultrasonic system involving an FMU90 controller and its associated sensors, several technical parameters must be evaluated to ensure reliable performance.
Selection Table: Application Evaluation
| Parameter | Consideration | Recommendation |
| :— | :— | :— |
| Measurement Range | Maximum distance from sensor to bottom. | Select a sensor with a range 20% greater than the tank height. |
| Medium Type | Liquid, slurry, or solid. | Liquids are ideal; solids require sensors with higher power and dust handling. |
| Process Temperature | Impact on sound speed and sensor material. | Ensure sensor is rated for the maximum process temperature (e.g., -40°C to +80°C). |
| Chemical Compatibility | Corrosive vapors or liquids. | Choose PVDF or PTFE coated sensors for aggressive chemicals. |
| Mounting Environment | Indoor vs. Outdoor (Sun/Wind). | Use sunshades for outdoor sensors to prevent temperature measurement errors. |
Blocking Distance (Dead Zone)
Every ultrasonic sensor has a "blocking distance"—a zone immediately below the sensor face where measurements cannot be taken. This is due to the time required for the sensor membrane to stop vibrating after the pulse is emitted. When designing a system, the sensor must be mounted high enough so that the maximum liquid level never enters this dead zone, which typically ranges from 0.25m to 0.6m depending on the sensor frequency.
Installation Guidelines and Best Practices
Proper installation is the single most important factor in the success of an ultrasonic level measurement system. Even the most advanced controller cannot compensate for a poorly positioned sensor.
1. Avoid the Center: In cylindrical tanks, do not mount the sensor in the exact center, as this can lead to multiple reflections from the tank walls that interfere with the primary signal.
2. Perpendicular Alignment: The sensor face must be perfectly parallel to the liquid surface. A deviation of just a few degrees can cause the reflected pulse to miss the sensor, resulting in a "Loss of Echo" (LOE) error.
3. Clear Path: Ensure there are no obstructions (pipes, brackets, or agitators) within the signal beam. The beam angle is typically around 6° to 11°, meaning the signal spreads as it travels further from the sensor.
4. Inlet Proximity: Do not mount the sensor near the filling inlet. Turbulence and air bubbles created by the inflowing liquid will scatter the ultrasonic signal and cause erratic readings.
5. Nozzle Length: If mounting on a nozzle, ensure the nozzle is short enough that the sensor face extends into the tank, or that the nozzle diameter is large enough to prevent interference with the beam.
Maintenance and Troubleshooting Common Challenges
While ultrasonic systems are non-contact and generally low-maintenance, certain environmental factors can pose challenges.
Foam and Turbulence
Heavy foam on the surface of a liquid acts as an acoustic absorber, deadening the ultrasonic pulse and preventing an echo. In such cases, a stilling well may be required, or a transition to radar technology may be necessary. Welk offers high-frequency radar level meters that are less affected by surface foam.
Steam and Condensation
Heavy steam can change the speed of sound significantly, leading to inaccuracies. Furthermore, condensation on the sensor face can attenuate the signal. Many sensors used with the FMU90 feature a self-cleaning effect through vibration, but in extreme cases, a sensor with a heated face or a drip-off design is preferred.
Dust and Solids
In silo applications, dust generated during filling can scatter the ultrasonic signal. For these environments, lower-frequency sensors (e.g., 10kHz to 20kHz) are used because they have more power to penetrate the dust cloud. However, for extremely dusty conditions, radar level measurement is often the more robust choice.
Frequently Asked Questions (FAQ)
Q: Can the FMU90 be used for solids measurement?
A: Yes, when paired with the appropriate high-power sensor, it can measure solids such as grain, plastic pellets, or ores. However, the angle of repose of the solid must be considered, as it can reflect the signal away from the sensor.
Q: What is the maximum distance the controller can be from the sensor?
A: Typically, the sensor can be located up to 300 meters (approx. 1000 feet) away from the FMU90 controller, provided that shielded, twisted-pair cabling is used to prevent electromagnetic interference.
Q: How does the FMU90 handle power failures?
A: The device stores its configuration in non-volatile memory (EEPROM). Upon restoration of power, the system resumes measurement and control functions immediately without requiring reconfiguration.
Q: Is it possible to simulate levels for testing?
A: Yes, the FMU90 includes a simulation mode that allows technicians to manually override the level reading to test relay outputs, pump logic, and 4-20mA signal transmission to the control room.
For engineers looking to implement reliable level and flow solutions, the FMU90 offers a robust platform for data evaluation and process control. By adhering to strict installation standards and understanding the environmental limitations of ultrasonic technology, users can achieve high-precision measurement across a wide range of industrial applications. For further technical specifications and to explore our full range of measurement instrumentation, visit our Main Page.
