Are ultrasonic Tx/Rx pairs really just the same sensor?

I want to retool the classic HC SR04 rangefinder to be able to do ToF gas flow sensing. I need to be able to do ToF on a column of air in both directions, so it benefits me to be able to use the transceiver as both transmit and receive.

TxR>[=====]>TxR

Otherwise, I either need a larger diameter tube with a pair of Tx/Rx on either side, or two tubes, neither is optimal.

Is there any reason why I can't just use a mux and alternately use one module as Tx and other as Rx, then switch roles?

Here's a datasheet of what I think is a representative transducer.

How could I even go about telling if the two components are different? They are likely piezoelectric, so resistance won't tell me much.

• Looking at the data sheet, I think the module does both Tx and Rx. It has a Tx transducer, and a Rx transducer. I'm not sure I understand what you're looking for. – SteveSh Mar 24 '20 at 19:59
• @SteveSh He wants to modify the module to be able to swap TX and RX at will and on-the-fly in his setup. – DKNguyen Mar 24 '20 at 20:00

Sometimes they are, sometimes they aren't. If transducers are sold as TX/RX pairs they are probably optimized differently and aren't identical.

Transducers do exist that can serve both roles so you can use two side-by-side to eliminate a deadzone, or multiplex just one for both transmission and reception. Or in your case, multiplex swap the role of transmission and reception between the two.

• That is kinda what I figured. The ones I am looking at do have Tx/Rx pairs, which is why I asked. However the packaging is identical save for a T or R stamped on the back. Not even a model number anywhere. If it were separate model numbers then obviously. I guess I figured how different could they really be? Tuning freq is the same, and in both cases, it helps to reduce inertia. Unless maybe one is capacitive while another is deformation based. I ordered a bunch of extras so I am gonna start tearing them apart. – DeusXMachina Mar 25 '20 at 14:57

I also wonder the same way. I've test this module by send them squrewave at 40kHx and monitor another one on osciloscope, both of them work and tx and rx but difference amplitude readed by tx to rx got higher amplitude. By the way, my setup is really poor I just hold it by hand just for see if it can work with both tx and rx mode.

Based on this experiment, my presumption is.

1. Tx use low-voltage, high-current transducer to produce high wace energy with lowvoltage
2. Rx use high-voltage, low-current transducer to pick up signal and convert to higher magnetude of voltage and reciever have high impedance there for we don't need much current.

Note: higher rx voltage reduce signal per noise ratio.

For farther experiment I think I need to fix thing permanently and test many configuration (tx-tx,tx-rx,rx-tx,rx-rx) also test electrical characteristic.