Narrowing the beam of an ultrasonic sensor

Question

I need to measure the distance to a solid or liquid in a round tank about 2 feet wide and 5 feet deep using a sensor that can output PWM and run at 3.3 or 5 volts.

After testing, I've found that the HC-SR04 has too wide a beam to measure accurately at all distances within the tank, from what I assume is echos off the walls. I've found a MaxBotics sensor that works but is too expensive to use when making multiple sensing units.

Do you guys know of any cheap sensors that may be able to fulfill these requirements or a way to narrow the beam of the HC-SR04? It seems like an "exponential horn" on the sensor is one way to narrow the beam, but I can't find any resources on the dimensions of such a horn.

Answer 1

Since your specs are just hand-waving arguments, I can suggest a really small Satelite parabolic dish with an offset sensor.

But for giggles, I would try a long heat-shrunk tubing over the sensor to attenuate the stray angular signals.

But this path loss will also attenuate the sensing range so refocusing all the energy makes more sense. (no pun intended) Focal path dispersion will occur from irregular shapes and result in weaker noisy edges.

Spec: effectual angle: less than 15°; distance: 2cm~500cm

It might be tried to measure the acoustic resonant frequency of the remaining air in the tank using a low-level sweep generator to an acoustically flat external speaker air plenum. (like a bass reflex effect)Then calibrate.

Another possible solution is plastic coated electrodes for capacitance or SRF frequency measurements.

Yet another is using UHF to measure the return loss in a splitter to a diode detector and FET buffered mV reading. Sweep the RF to detect the wavelngth for either a null detection or a peak half wave. Then calibrate for accuracy. The antenna could be a short rubber ducky, that does not need to be even unity gain as antenna efficiency drops with length.

But if you are trying to make a $1 sensor, you got your work cut out for you.