PGA460: measuring speed of sound in gas (small distance, 1cm) with 2 trancducer

HI Bastiaan,

If you intend to use 2 transducers, then either the PGA460 or the TDC1000 should be able to do what you are looking for. For short distances the main limitation is the ringdown time of a transducer when using a single element as transmitter and receiver, which is not a factor when using two transducers.

It then comes down to how much functionality you need out of the device. The TDC1000 is a fairly simple ultrasonic AFE that does not actually measure the time of flight. It only excites the transducer and receives the return signal, outputting digital START and STOP signals so that another device can measure the time between them. Something like the TDC7200 can be used to measure the time of flight at great accuracy, or the same microcontroller used to configure the TDC1000 could be used to reduce component count while sacrificing some accuracy. The TDC1000 on its own also has limited driving capability which can make gas measurements challenging even at short distances. The TDC1000-GASEVM employs a voltage boosting circuit to raise the excitation voltage to 30V for gas applications.

I've notified my colleague who can speak more to the PGA460.

Regards,

Hi Bastiaan,

As my colleague mentioned, if you plan to use two transducers, then both the PGA460 and the TDC1000 will work at very short distances.

The PGA460 is a more complex device than the TDC1000. It will directly output the time of flight, and it includes a temperature sensor that would be useful in your speed of sound calculations. It also has a digital signal processing block with programmable bandpass filtering and digital gain. The PGA460 has more signal conditioning options than the TDC1000. A PGA460 evaluation module can be found here if you wish to experiment with the various signal conditioning options.

For your PGA460 configuration, a high-frequency bistatic transducer pair is best suited for your 1cm range requirement. The high frequency will ensure you have enough resolution for your speed of sound calculations. If your gas chamber is very pure you can use open-top transducers, which are more sensitive than their closed-top counterparts. If there is any risk of debris you should use closed top transducers. With the open-top transducers you can use a full-bridge driver, as you won’t need the high voltage capability of a transformer for such short distance measurements. If you use the closed-top transducers you’ll need to use a transformer to drive them. More detailed information about recommended configurations can be found in section 3 of the PGA460 Ultrasonic Module Hardware and Software Optimization Guide.

You can find a list of air-coupled ultrasonic transducers and their specifications here.

Regards,