Underwater distance sensor recommendations

Projects General Guidance
1

Hi all,

I've been vaguely sketching out ideas of building a small autonomous submersible (something I could let loose in a backyard pool, for example), and so far my biggest question is what to use to detect obstacles or boundaries. I know sonar is commonly used underwater; does anyone know of a breakout-board-sized sonar transmitter/receiver? It wouldn't have to be very long range at all (less than a metre). So far, all I've found are commercial fish finders, which are a little bigger and more expensive than I had in mind. I'm not against the idea of just having pressure sensors, so the little guy just bumps into stuff and then backs up, but if there's a remote sensing option I'd prefer it.

Suggestions would be much appreciated!

2

A meter long stiff wire attached to a microswitch would work just the same.

Paul

3

You could use an gyro to detect changes in rate. An unintentional change would indicate an obstacle.

4

If you can't find a cheap fish finder to hack, or if bump sensors won't work, you could try the US$100 Maxbotix MB7078, an ultrasonic distance sensor intended for air but reportedly usable as an underwater distance sensor in swimming pools and similar size tanks.

The generally-accepted position of many active posters on this forum is that inexpensive ultrasonic sensors intended for air categorically will not work under water. Plausible science-based reasons are given for that position and the associated claim that ultrasonic transducers intended for underwater ranging are expensive and "require sophisticated, high frequency and high voltage (typically 200V) electronics to operate."

However, that flies in the face of "actual" experiences reported in links in post #3 of the following thread describing success using intended-for-air ultrasonic sensors under water. Help! I can't use an underwater ultrasonic sensor. - Sensors - Arduino Forum

The following link from Maxbotix, a reputable manufacturer of high-end ultrasonic sensors intended for air, describes the challenges of using them for ranging underwater, and it also has some links (at the bottom of the article) to users who have successfully used their intended-for-air sensors under water, even though the "kluged" waterproofing in one of the examples looks "incredibly clumsily built" as one forum member put it (of course, "looks" are irrelevant; what is important is "function," and function...including underwater ranging...has been demonstrated).

My personal experience is that the common and cheap JSN-SR04T "waterproof" sensor does not work under water. Hacking a cheap fish finder or using bump sensors is probably the best way to go. If you do try the Maxbotix, please report back!

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Added the following, for completeness:

@wvmarle reports that the sensor mentioned in the following link can measure the depth of liquid in a container when its head is held in the water just below the surface, so it could do what the OP wants (detect imminent collision of a model sub with the side of a pool). It has a limited range... 2 meters, max...but its blind distance is only 50mm - much better than the Maxbotix minimum reported distance of 20 cm in air (and thus presumably about 90 cm in water). And it is cheaper than the Max, too.

https://forum.arduino.cc/index.php?topic=528653.15

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5

These are all some great starting points, perfect. Thanks guys! I'll report back with progress soon :slight_smile:

6

I wonder if one of the microwave radar sensors like this would work underwater?

7

radio frequencies including microwave does not work well underwater it is absorbed by water. Hence the use of sonar for subs.

8

wolframore:
radio frequencies including microwave does not work well underwater it is absorbed by water. Hence the use of sonar for subs.

Yes but I wondered if it has enough range to prevent bumping into things. No need to be waterproof as it will work through some/most materials.

9

Ok try it and report back to us.

10

updated post #3....

11

Maxbotix explains some of the difficult challenges encountered in underwater sonar distance ranging in this tutorial.

A number of engineers and scientists are working on new techniques for the manufacture of cheap but reliable underwater transducers, with one serious problem being the need to match the acoustic impedance of the transducer with that of water, but they are not there yet.

The current bottom line for hobbyists is that if you can't afford commercial underwater sonar rangers, or fit them into your project, you need to think hard about other means of accomplishing your goals.