Four considerations when choosing a sensing solution for liquid-level measurement


Let’s say you need to measure the level of a liquid in a tank: perhaps it’s for an automotive gas tank, wiper-washer fluid tank or engine coolant tank. Possible solutions include traditional mechanical or electronic sensing devices.

If you have already decided to use electronic sensing, you still have other choices to make. Electronic capacitive sensing works well for many liquid-level sensing applications, but the unique conditions of automotive applications may require a different approach. To determine the best solution for your application, you need to do some information gathering. You will need to answer four key questions:

  1. What are the properties of the liquid you’re monitoring?
  2. Will you measure the liquid from inside or outside the tank?
  3. What are the conditions of the tank?
  4. What are the accuracy requirements of your system?

Question 1: What are the properties of the liquid you’re monitoring?

Is the liquid viscous, corrosive, homogenous? Corrosive liquids such as urea and alcohol make it increasingly difficult for mechanical devices to measure liquid levels since they typically need to come in contact with the liquid itself. On the other hand, capacitive- and ultrasonic-sensing methods do not come in contact with the liquid since both of these methods allow you to monitor the level of the liquid from the outside of the tank. With capacitive sensing, pairs of conductive plates running down the side of the tank will monitor the level. Ultrasonic sensing involves mounting a piezo sensor on the bottom of the tank. Both methods are noninvasive and not negatively impacted by corrosive liquids. However, liquids that are viscous in nature (power-steering fluid, brake fluid, engine oil) leave a film on the inner wall of the tank when the level goes down; this can be problematic for capacitive sensing.

Question 2: Will you measure the liquid from inside or outside the tank?

One method for measuring fluid level from inside the tank is a resistivity technique. However, this becomes problematic with corrosive liquids. Due to reliability, cost and limitations of internal level-sensing techniques, external electronic-sensing devices such as ultrasonic and capacitive are gaining in popularity for automotive applications. You have fewer choices if the tank is metallic; since capacitive sensing relies on an electric field to make its measurement, it’s not an option here.

Question 3: What are the conditions of the tank?

More than likely, your tank is made of high-density polyethylene (HDPE) or steel. Mechanical devices such as a moving “float arm” can internally monitor liquid levels in tanks of all different materials. However, they have mechanical limitations when the shape of the tank is irregular (something other than a cylinder or rectangular in nature) or if the tank is very tall and thin because they limit the float arm’s capability.

Question 4: What are the accuracy requirements of your system?

While some mechanical devices can achieve accuracies in the single percentiles, capacitive- and ultrasonic-sensing solutions can generally achieve millimeters of accuracy. Capacitive sensing achieves this by having a pair of electrodes below the minimum surface of the liquid and another pair that runs the full height of the tank. If system conditions vary greatly, you may need another pair of electrodes above the liquid level as well. In the case of ultrasonic sensing, a single piezo transducer is sufficient to achieve better-than-millimeter accuracy by applying a time of flight (TOF) measurement technique.

Ultrasonic TOF level measurement works by using a single piezoelectric transducer to create a pulse from the bottom of a tank, as shown in Figure 1. The pulse travels through the tank wall and into the fluid in the tank. It continues to propagate through the fluid until it reaches the fluid surface that represents a large change in acoustic impedance. Due to the large change in acoustic impedance at the fluid surface (fluid to air interface), an echo is created, and the sound wave propagates back toward the piezo transducer. Measuring how long it takes for the echo to return to the transducer is referred to as TOF.

 Figure 1: Ultrasonic time of flight level measurement

The TDC1000 ultrasonic analog front end (AFE) makes TOF measurements simple by exciting the transducer and receiving the echo. In turn, the TDC1000 creates a start and stop pulse, both of which are easily monitored by a microcontroller, which acts like a stopwatch in order to measure TOF and achieve 1mm height accuracy. This process is illustrated in Figure 2.

  

Figure 2: Automotive liquid-level and fluid-identification measurement system block diagram

I hope these four key considerations help you decide which sensing solution for automotive liquid-level measurement is best for your application and give you some understanding how ultrasonic sensing delivers the benefits of both millimeter accuracy and noninvasive sensing. Do you plan to use ultrasonic sensing or a different option for your next project? Leave a note in the comments section below.

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