FMU42 Ultrasonic Level Gauge

Today we will introduce an ultrasonic level gauge FMU42 that can be used for level and flow measurement. Below is its display diagram.

Its working principle is that the ultrasonic sensor emits high-frequency pulse sound waves, which reflect when encountering an object. The sensor can obtain the distance based on the time difference between the emitted and received reflected waves, and convert it into a current between 4-20mA for output. It is worth noting that the instrument cannot be in contact with it when measuring the level. The sensor emits ultrasonic pulse signals towards the surface of the liquid. The ultrasonic pulse signal is reflected on the surface of the medium, and the reflected signal is received by the sensor. The device measures the time difference t between sending and receiving pulse signals. Based on the time difference t (and acoustic velocity c), the device calculates the distance between the sensor diaphragm and the surface of the medium, D: D=c ⋅ t/2, and calculates the liquid level L through the distance D. By using the linearization function, the volume V or mass M can be calculated from the liquid level L. The user inputs a known blank distance (E), and the calculation formula for the liquid level (L) is as follows: L=E - D. The built-in temperature sensor (NTC) compensates for the sound velocity changes caused by temperature changes.

• SD safety distance
• BD blind zone distance
• E empty standard distance
• L liquid level
• D sensor diaphragm to medium surface distance
• F range (full standard distance)

The following is a schematic diagram of its measurement system:

• PLC (programmable logic controller)
• Commubox FXA195
• computer, installed with debugging software (such as FieldCare)
• Commubox FXA291, with ToF adapter FXA291
• equipment, such as Prosonic
• Field Xpert
• VIATOR Bluetooth modem, with connecting cable
• connectors: Commubox or Field Xpert
• transmitter power supply unit (built-in communication resistor)

The following is a schematic diagram of installation conditions:

• distance from the tank wall: ¹⁄₆ 2 of the container diameter, installation of protective cover; Avoid direct exposure of instruments to sunlight and rain
• It is prohibited to install the sensor in the center of the tank.
• Avoid measuring in the feeding area.
• It is prohibited to install limit switches or temperature sensors within the beam angle range.
• Internal devices with symmetrical structures, such as heating coils, baffles, etc., will interfere with the measurement.
• Installation precautions for sensors perpendicular to the surface of the medium:
  • Only one device should be installed on the same tank.
  • Install the measuring device on the upstream side, with the installation height as high as possible above the highest liquid level Hmax,
  • The installation of the short tube insertion end adopts an angled inclined socket.
  • The installation position of the measuring equipment must be high enough to ensure that the material will not enter the blind spot distance even when it is at the highest level.

The following figure is an example of installation.

• A uses a universal flange for installation.
• B uses an installation bracket, which is generally used in non explosion proof areas.

Complete the following steps to fix the instrument

• Loosen the fixing screws.
• Rotate the casing to the desired position, with a maximum rotation angle of 350 °.
• Tighten the fixing screws to a maximum torque of 0.5 Nm (0.36 lbf ft).
• Tighten the fixing screws; Use metal specific adhesive.

The above is its basic introduction