PGA460: level sensing with transducer sensing in water?

Hello A,

Thank you for the question,

TOF measurements through water are possible, but require additional tuning to implement. Frequency selection is very important to the accuracy of the system, and the ability for the waves to transit through a medium. 300kHz may work in some liquid coupled applications, but generally 1MHz is more common for level sensing. Being that the PGA460 is incapable of using such high frequency, I recommend conducting some preliminary tests to evaluate if the receive echo is measurable with 300kHz. 

Most water level sensing implementations have the transducer mounted on the outside of the tank, requiring the sound waves to transmit through additional attenuating mediums. In this case, a solid material (tank wall) , then into the water, reflect off the surface, and finally pass once again through the solid medium to be sensed by the transducer. 

The principle can work, and the PGA460 has been used successfully in similar applications so long as the system is well tuned. 

You bring up another good point in mentioning the empty tank, this can cause issues for the sensing system. If the ring down of the transducer is longer than the time for the reflected echo signal to return to the transducer, the distance will not be properly measured. This means it will not be possible to realize the full sensing range of the tank.

Finally, the methods to help the implementation succeed.

1. Avoid sensing through metal if possible. Metal will substantially attenuate the burst echo, making sensing difficult without significant modification to burst energy, or receiver gain.

2. More burst energy. The transformer drive of the PGA460 will help in ensuring the receive echo is detectable. While transformer drive is not always required, it often makes the solution implementation easier to realize.

3. Proper mounting of the transducer. The impedance mismatch between the transducer and solid medium will become substantial if the transducer is not properly mounted.   This Application Note provides additional information on selecting and mounting transducers: snaa266a  .

4. Receiver gain will be required for the TOF measurement. For the testing, it is suitable to set the gain very high around the expected response time. It may be necessary to set the gain as high as possible to verify the echo will be detectable.

5. Tune the system. Assuming the echo is detectable, it will be necessary to tune the register settings and the hardware components( adjusting threshold values, optimal burst count, adjust Rdamp to reduce ring down time, manipulate gain map"

Here is another resource for liquid level sensing: SNAA220A. Note, the document is promoting the TDC1000 device, but the principles can be applied to the PGA460.

I am always happy to look at the PGA460 configuration file, and make recommendations for improving the system. 

Please let me know if you have any questions,

Thank you,

Jacob