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Measuring liquid level from the top of the tank (metal tank)

Jose Ramon hernandez
Prodigy 30 points

Hi All,

When measuring liquid level in a tank with non-intrussive ultrasonic sensors almost all documents say the transducer should be placed in the bottom of the tank. In some case, they slightly comment the transducer can be placed in the top and I would like to ask futher details about this.

Since the wave is reflected when there is a media boundary with enough difference of impedance (sound speed) I think there would be no problem if the signal is sent from the top. When the tank is full of liquid the TOF would be shorter than when it is empty.

Why is it recommended to place the transducer on the bottom?

I list my application requirements below so anyone can advise:

- Metal tank (stainless steel).

- Different types of liquids. If the sensor is on the bottom I would need calibration for different liquids, right? But if I place the sensor on the top I think I would not need it. 

- Tank heigth: 1-1,5m. (3-5 feet)

- I am mainly interested in detecting when the tank is empty. Detecting the level is a plus.

Regards, Ramon.

  • 0
    TI__Mastermind 21685 points
    Ramon,

    I am not sure if you already looked at the TI app notes on this topic
    www.ti.com/.../snaa266
    www.ti.com/.../snaa270

    Air (or gas) is very absorptive. When transducer is placed at the top, you need lot more excitation energy to get any echo. That means a high voltage boost circuit and extra cost. Also, you might need some housing to prevent condensation or liquid coming in contact with the transducer. It may be cheaper to mount it on the bottom but depends on the other application factors as well.

    When measuring from bottom, you have to use the speed of sound for the given liquid. See link below for a good summary on this below
    www.ondacorp.com/.../Liquids.pdf
    For high accuracy of level measured, you would need to measure the liquid temperature and compensate. Easier in that case to measure from top especially when you are dealing with different liquids.

    Detecting empty from top on 1-1.5m tank would definitely need very high excitation.

    Take a look at these postings as well

    e2e.ti.com/.../498792

    e2e.ti.com/.../467713

    Thanks,
    vishy
  • 0 in reply to Vishy Viswanathan
    Prodigy 30 points

    Hi Vishy,

    Thanks very much for your detailed answer.

    Well, I guess if I do not want to add a voltage booster I will have to place the sensor at the bottom. Since I might only need to detect when the tank becomes empty (or almost empty), maybe placing the sensor on one side (horizontally) close to the bottom is another choice, but I am wondering how the sensor deals with a curved suface, any idea?

    Sth I did not understand from your answer when talking about the sensor on the top is that you mention I would have to deal with "condensation or liquid coming in contact". These precautions are for in-tank sensors, right? or also for non-intrussive sensors?

    Regards, Ramon.

  • 0 in reply to Vishy Viswanathan
    Prodigy 200 points
    How do you get a lot more excitation energy? What I mean is, is there a name for the high voltage boost circuit other then the booster board that comes with the GASEVM? I'm looking to boost but would rather not buy another EVM board just for the booster board and ideally would like to boost from 5V to 50 or 100V instead of 30V. Thanks!