Custom Non-Invasive and Standard Ultrasonic Liquid Level Sensors

Pressure Metrics uses ultrasonic level sensors to design continuous liquid level pressure sensitive film for tanks and bottles. These sensors mount to the top or bottom of a vessel. It is by measuring the time it takes for a transmitted wave to bounce off. the fluid surface and back, they can continuously and reliably measure the height of liquid in a tank. These sensors are versatile and can be used in a wide range of applications.

Some ultrasonic liquid level sensor applications include:

  • Oil tanks
  • Water Reservoir/Water Treatment Systems
  • Chemical treatment tanks
  • Pharmaceutical companies
  • Food manufacturing plants


How Continuous Ultrasonic Liquid Level Sensors Work

A permanent level sensor utilizes piezoelectric transducers to spread a burst of ultrasonic energy that moves at the speed of sound. The ultrasonic wave is reflected on the air/liquid interface and is sent back to the sensor. The time needed for the wave to return is calculated by a signal managing circuit and utilized to analyze the distance, which is then transformed into a functional electronic output signal.

The example in Figure 1 shows sound waves transmitted through a single material (air). In this case, to determine the gap between the sensor and the object planned for exposure, the formula utilized is:

  • D = c*t/2
  • Or:
  • D is the elevation of the liquid to be calculated
  • c is the speed of sound in the medium through which the sound travels
  • t is the exact time it takes for the sound waves to come back to the sensor

How Liquid Level Sensors Work

In applications where the ultrasonic wave travels through several different media before being reflected off an air boundary, the speed of sound through and the thickness of each material must be considered. Also, the speed of sound is temperature dependent and therefore temperature correction factors must be applied.

In a continuous ultrasonic level sensor. almost 100% of the acoustic wave energy is reflected from the air/liquid. The  interface due to the large difference in acoustic impedance between air and water. Acoustic resistance is a land of a material calculated in units called Rayls. It is considered by multiplying the speed of sound.In a material by the thickness of that material. Under typical conditions, air is hundreds of times less dense than liquids. Resulting in a large impedance mismatch between the two media and nearly 100% acoustic reflection.

The transmitted ultrasonic wave will spread out in a conical shape as it travels away from the transducer and will reflect off any solid or liquid surface in its path. Therefore, in many cases, the ultrasonic level sensor must be mounted far enough away from the tank walls to avoid interference.


Non-invasive liquid level sensor solutions: “downward” or “upward” sensors

Most ultrasonic liquid level sensors on the market mount on top of a tank, as shown in Figure 1, and are sometimes referes to as “top down” sensors. The reason this is most often the prefers configuration is that the only medium sound waves will pass through before reflecting is air, a medium for which the speed of sound.

Another configuration is to mount the transducer at the bottom of the tank, as shown in Figure 2, so that the sound waves pass through the liquid and then reflect off the liquid/air interface. This is sometimes calls an “ascending” sensor.

The main disadvantage of this method of detection is that sound travels at different speeds through different liquids. This means that to make an accurate liquid level measurement, the sensor must program to “know” what liquid is in the tank.

This limitation can overcome in a variety of ways, including calibration, and adding additional transducers to use for fluid identification, but this almost always results in the need for a custom design for a specific application. Therefore, you will see very few “off-the-shelf” ascending sensors on the market.

Non-invasive liquid level sensors

Why bother designing a custom “bottom-up” solution when a “top-down” solution is easier and more readily available? There are three main reasons to consider a custom ultrasonic liquid level sensor:

  • A rising level sensor can be places outside the wall of a bottle or tank. resulting in a completely non-invasive continuous liquid level sensor. This is an exceptionally crucial issue in a lot of pharmaceuticals. The medical liquid level detection functions where impotence is a need. Top-down sensors can’t be made non-invasive for the same reason. the technology works so well in the first place. sound traveling through the air will bounce off any solid. The liquid surface you place in front of it. It is including the walls of the room. ship. To achieve airborne ultrasound. the sound waves must be generated through the air. requiring the sensor to be in contact with the air inside the tank. This resulting in an inherently invasive downward sensor.

Standard Ultrasonic Liquid Level Sensors

  • Upward continuous ultrasonic liquid level sensors. It can be temporarily mounts to the outer wall of a vessel using dry coupling means. This means the system designs with a permanent sensor. It is that temporarily interfaces. It is with a single-use disposable bottle or container. which is key in many medical and pharmaceutical. The liquid level sensing applications. The best tank materials for this type of rising sensor are plastic or glass.
  • Since upward sensors are design to send the sound wave through a liquid instead of air, a higher frequency ultrasonic wave can uses without significant ultrasonic attenuation. Most downlink sensors operate at 40 kHz while uplink sensors can go up to 1 MHz. This has many advantages, including a smaller “dead zone” in front of the transducer, higher resolution, and a narrower ultrasonic beam to help avoid interference from obstacles and tank walls.

Despite all these significant advantages of bottom-up type liquid level sensors, few are on the market due to the large amount of customization that must go into each specific application. Fortunately, this is where Pressure Metrics shines.  most sensors we sell are custom designs in conjunction with our customers to solve a specific problem. Additionally, we have significant experience with non-invasive sensors, including bubble sensors, non-invasive pressure indicating film, flow meters and occlusion sensors.


Main components to think when choosing a permanent ultrasonic level sensor:

  • The type of liquid measured. Should it be sterile? Is it an aggressive chemical? Is it highly flammable? If the answer is yes to any of these questions. you may need a sensor made of chemical. resistant material or a non-invasive sensor.
  • What environment will the sensor exposes to? Extreme heat/cold? Be sure that the sensor you select may resist the operating and storage conditions.
  • Do you need an explosion-proof or intrinsically safe rated sensor?
  • What type of output can you support?
  • What input voltage can you supply?
  • How tall is your tank? Can you afford to have small fluid levels that detect? If the tank is very small, you may need a sensor. that will give you a very small dead space range (distance in front of the sensor that it cannot detect).

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