PRACTICAL ULTRASONIC LEVEL MEASUREMENT

These devices base their level measurement on a mechanical phenomenon called Piezoelectric. This means that we have a crystal that, when subjected to a voltage, begins to vibrate, emitting pulses. These pulses are emitted from the level sensor, which in turn becomes a receiver, and the time it takes for them to travel that distance is equivalent to the level, that is, the measurement of the separation between the sensor and the surface to be measured.

Within ultrasound measurements, we must be clear about whether we will be measuring level, which means from a zero level upwards, or space, which is from the surface of the sensor to the surface of the material to be measured.

ULTRASONIC SENSOR HEADS

Figure No. 1 shows the piezoelectric material that emits the pulses and the receiver that detects the return.

Each device comes with tables that define the characteristics of the surface to be measured, the medium of propagation of the mechanical wave (sound wave), characteristics of the tank in which it is inserted (shape, material, etc.), and sometimes the altitude of the process relative to sea level. They also have internal temperature sensors, which allow for calculations to correct for changes in the speed of sound (which will remain constant for a long time = 343.2 m/s at 20°C).

These instruments, regardless of their robustness, have some drawbacks; they are affected in their measurement by obstacles in their mapping area, therefore some considerations must be taken into account for their assembly.

Figure No. 2 shows us what we should and should NOT do to achieve a good measurement.

In the 99% case, this equipment must be installed perpendicular to the surface to be measured. If this is not possible, care must be taken to ensure that the surface being measured has the largest possible area. It is important to remember that each device has a specific angle of attack, and the surface closest to the sensor face will be measured. If this is not the surface required, it will lead to measurement errors, resulting in what are known as... FALSE ECHOES.

Now that we have the basic knowledge to define a good installation, we will now look at working with an installed level sensor that is experiencing operational problems.

Each device has a basic and advanced configuration, which we assume is already installed and operational. We should also be aware that each manufacturer provides a manual outlining the most common problems and solutions.

We will create a list of the most common failures and provide the answer with the highest probability of a good result.

-Level Indication Failure, this means that what the display indicates is not the actual level.

Simplicity can be the solution to every problem. What does the equipment see? Is it loose? Are there any exposed wires? Is the equipment clean? Is it secure? Is the measuring surface free of intermediate interference?

Resp: Verify that the sensor membrane is free of impurities and adhered material, although some equipment is able to compensate for this weakness, it is always important to help ensure that it is clear.

Resp: Verify with your fingertips that the pulses are present on the sensor membrane; they should emit pulses periodically.

Answer: Check that there are no cables without their protection or loss of insulation; remember that in some cases we need a high voltage (approximately 500 Volts) to create the pulses in the Piezoelectric.

Resp: Check that the sensor diaphragm is not torn or cut, as this distorts the pulses.

Resp: Verify that the measurement space is not obstructed by any interference obstacles; some equipment allows us to suppress obstacles in its configuration to avoid disturbing the measurement.

Resp: Verify that the supply voltage is as defined by the manufacturer, Phase-Neutral-Ground

Figure No. 3 SENSOR WITH ATTACHED MATERIAL.

These are clear examples that for a given problem, we can see several solutions. We can begin to look at some other solutions for different types of brands that will be specific.

If there is any point that has not been taken into consideration or a specific issue, please send it to:

Email: hcarrasco@instrumentacionycontrol.net  and we'll see.

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