Under water distance ? - Page 1
Forum section: electronics / projects

author: B J
author_rank: Contributor
timestamp: July 24, 2022, 11:59:13 pm
content: The normal fish finders and depth finders, that I have found, use systems that have a minimum distance of a few feet. I would like to measure distance from about five feet down to maybe six inches. Looking for some sort of transducer ( maybe a piezo crystal ) that will work at around one mega HZ. Thinking about using one for transmitter and two running in parallel as receivers. So far the transducers I have found that can operate at this frequency, are very big bucks.. Do you have a suggestion of where to look for a source to work in this application? ---- Any other ideas ?

author: rooppoorali
author_rank: Regular Contributor
timestamp: July 25, 2022, 04:44:05 am
content: Does this one fulfill your requirement?https://www.amazon.com/Waterproof-Ultrasonic-JSN-SR04T-Integrated-Transducer/dp/B07FQCNXPP

author: CaptDon
author_rank: Super Contributor
timestamp: July 25, 2022, 01:31:08 pm
content: The transducer you posted "Is waterproof, but won't work under water" right in the spec sheet!! O.P., are you trying to measure in water?? Those transducers operating in the Mhz region are normally for ultrasounding in live flesh and as such are highly over-priced.

author: Marco
author_rank: Super Contributor
timestamp: July 25, 2022, 01:56:35 pm
content: Better processing can do it with cheaper sensors.

author: eugene
author_rank: Frequent Contributor
timestamp: July 25, 2022, 01:58:58 pm
content: If I were in your position I would abandon my search for a fully waterproof transducer and find one that is designed to 'see through' a plastic window. Sensors like that are commonly used to measure depth/level in tanks by looking up through a water tight window in the bottom. I would then build the sensor(s) into a water tight enclosure that could be put under water.Example:https://www.digikey.com/en/products/detail/unictron-technologies-corporation/H2KLPY11000600/9921487EDIT: Looking more closely at the datasheet for the transducer above, it could probably be made waterproof with careful application of some RTV. I don't even mean potting the entire thing, just put a little silicone or RTV around the entry point for the wires and possibly a bead around the joint between the aluminum enclosure and the mounting pad. I wouldn't worry at all about damping the signals; silicone rubber has almost no damping properties at 1MHz. (We want to avoid both damping and dampening, which are not the same thing even though a lot people seem to think so.)

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 25, 2022, 02:38:02 pm
content: Quote from: eugene on July 25, 2022, 01:58:58 pm(We're not all native English speakers, and thedictionariestell us that transitive form of "to dampen" can mean either "to make moist or wet" or "to make less intense". "It dampens our spirits" does not always mean "it waters our drinks". You must admit that that can fool a non-native speaker into thinking damping and dampening can be used interchangeably.)

author: tszaboo
author_rank: Super Contributor
timestamp: July 25, 2022, 02:42:59 pm
content: Do you want just one of these or thousands?

author: B J
author_rank: Contributor
timestamp: July 25, 2022, 05:07:51 pm
content: This is for a one time gadget I'm am working on for an idea. There may be three used, one for for transmitting and two for receiving. ---- rooppoorali, I was looking at the JSN-SRO4T a few days ago, I do think that it has possibilities for my use. I used one in another project a couple years ago and it does its job about every 12 seconds, year round. Doing great, however it is in air. eugene, I was thinking the same thing about using the silicone for a seal. As far as shooting through a window, worth looking at.

author: B J
author_rank: Contributor
timestamp: July 25, 2022, 05:18:24 pm
content: Also wondering about the AO2 YYUW I need more info. than what I've gotten so far. The JSN SR04T minimum may not make it. More head scratching time needed.

author: Marco
author_rank: Super Contributor
timestamp: July 25, 2022, 06:14:24 pm
content: The US1000-21A is availablerelativelycheaplyfrom China, it has been used fordistance sensing underwater for a robot, unfortunately they don't really describe the electronics they used.

author: Henrik_V
author_rank: Regular Contributor
timestamp: August 01, 2022, 02:44:27 pm
content: Playground: Use the soundcard (tweaked to get the bandwidth) and two transducers (piezo) , one for sensing and one for sending.Try chirp signals and crosscorrelation

author: Marco
author_rank: Super Contributor
timestamp: August 01, 2022, 03:03:16 pm
content: Wouldn't really work when using MHz transducers.You could downmix the receiver, but for the transmitter you need to breadboard an inductive kicker at the very least (which would probably do fine in production too, just use an inductor which resonates with the piezo parallel capacitance around a MHz and give it a kick to a couple 100V).

author: free_electron
author_rank: Super Contributor
timestamp: August 01, 2022, 03:48:53 pm
content: problem is that sound travels much faster underwater than in air.pulse delay measuring may not work : you may need to do phase measuring

author: Marco
author_rank: Super Contributor
timestamp: August 01, 2022, 03:58:01 pm
content: With a MHz 6 inches is 100 cycles. That should be enough time for the direct response to die out.

author: free_electron
author_rank: Super Contributor
timestamp: August 01, 2022, 06:01:07 pm
content: Quote from: Marco on August 01, 2022, 03:58:01 pmi don think you can generate an "ultrasonic" pulse of 1MHz .. ultrasonics are typically in the 40KHz range. on top of that : how does water behave at those frequencies ?

author: nfmax
author_rank: Super Contributor
timestamp: August 01, 2022, 06:43:00 pm
content: Quote from: free_electron on August 01, 2022, 06:01:07 pm1 MHz is no problem in water. With appropriate drive, transducer matching, and Tx/Rx switching you should be able to measure down to 2" or less easy using a single transducer. SoS in fresh water is around 1480m/s, rather faster in salt water. What will really give you trouble is either bubbles or suspended sediment: they generate lots of scattering - like fog with light

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 01, 2022, 07:45:19 pm
content: Quote from: free_electron on August 01, 2022, 06:01:07 pmOf course you can.https://www.olympus-ims.com/en/ultrasonic-transducers/highfrequency/Water behaves just fine.

author: Marco
author_rank: Super Contributor
timestamp: August 02, 2022, 01:04:56 pm
content: Quote from: free_electron on August 01, 2022, 06:01:07 pm~20mm ~1 MHz transducers seem semi standardized for liquid Doppler flow meters and thus cheap from multiple producers. As I said, they have been used for robotics before.I think giving them a 2.5A kick with a 22uH inductor should work well enough for generating the pulse.

author: passedpawn
author_rank: Contributor
timestamp: August 03, 2022, 02:59:39 am
content: I wonder if there are UV LiDAR ranging device? I've used some small and inexpensive IR LiDAR rangers from ST Micro, but IR won't get far in water.

author: twospoons
author_rank: Frequent Contributor
timestamp: August 04, 2022, 12:26:45 am
content: Over that range you could go the other way, use 370Hz (4m wavelength in water) continuous tone and measure thephaseof the reflected sound. The hardware gets much simpler, and you can use a narrow band receiver to improve noise immunity. Beam forming would be much more difficult, but then we don't know your application, so it might not matter.

author: Bud
author_rank: Super Contributor
timestamp: August 04, 2022, 12:36:24 am
content: Quote from: Nominal Animal on July 25, 2022, 02:38:02 pmTry this one"No naked conductor or conductors will be allowed in the building without permission"

author: B J
author_rank: Contributor
timestamp: August 04, 2022, 01:20:30 am
content: Thanks for all the ideas and suggestions. Because of a car accident, I have been very slow in responding to all the good inputs. All the feedback is food for thought, and appreciated. That, I'm OK with. The comment from twospoons has me thinking, what would happen if I built an oscillator, using the echo in the feedback loop. Now it is as simple as measuring the frequency. More head scratching time. ---- Hopefully I will be able to start doing some building and experimenting on some of these ideas before much longer. (Apparently the guy that hit me was on his cell phone and not watching where he was driving. I just couldn't get out of his way.) Thanks again to all who responded with thoughts. I look forward to reading them.0

author: passedpawn
author_rank: Contributor
timestamp: August 04, 2022, 02:14:59 am
content: Quote from: evb149 on August 03, 2022, 04:09:06 amThe shorter wavelengths (UV) are much better for water transmission. Longer wavelengths, e.g. red, have VERY poor transmission through water.I have no idea what you were posting there, but I feel it's based on nothing. Why does the sun look red at sunset? I'm done here, good luck with all you'll need it.

author: twospoons
author_rank: Frequent Contributor
timestamp: August 04, 2022, 04:00:04 am
content: Quote from: B J on August 04, 2022, 01:20:30 amIts not as simple as it seems - you would have to ensure oscillation at the fundamental frequency, rather than one of the harmonics. Or have a way to detect which harmonic your oscillator was running at.Sorry to hear you got smashed by an idiot with a car. That sort of thing really pisses me off. When someone is driving that should be the ONLY thing they are doing - not texting, not doing their make-up, not fiddling with the radio, not scratching their nut-sack.Hope you recover soon.

author: passedpawn
author_rank: Contributor
timestamp: August 04, 2022, 04:49:45 am
content: Quote from: evb149 on August 04, 2022, 04:38:52 amSo, absorption is less at UV? OK. I agree.

author: corgon
author_rank: Contributor
timestamp: June 28, 2025, 06:10:25 pm
content: Bringing this old topic back to life — maybe someone will find it useful.Out of curiosity, I wanted to try measuring water depth using the SR04T sensor.Based on what I saw online, most people didn’t have much luck with it, but I gave it a shot anyway.Turns out, the sensor works just fine underwater — no issues at all.The only thing you need to keep in mind is that the speed of sound in water is different from in air.So I did the math and now the display shows the distance in water on the top line, and the distance in air on the bottom line.The minimum detectable depth is about 80 cm, and the max I managed to test was around 30 meters.This is a super rough prototype, just to check if the concept works.I’ve got the JSN-SR04T-2.0 sensor set to AUTO mode, so it sends data over serial every 100 ms.I process that with a CH32V003 — I average 10 readings and update the value once per second.The measurements is displayed on a Nokia 5110 screen, which has great visibility in direct sunlight.A short video showing how it works:

author: corgon
author_rank: Contributor
timestamp: June 29, 2025, 03:07:50 pm
content: After I made sure everything was working, I tweaked the display and added water temperature measurement using a DS18b20 sensor.Now I just need to put it into a waterproof floating case, and it'll work as a portable sonar and water thermometer for paddleboarders,kayakers, swimmers — basically anyone who wants to know how deep the water is underneath them

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2025, 03:34:13 pm
content: The speed of sound in water is temperature sensitive. Are you using temperature compensation? Even if you are, note that surface temperatures are often much higher than even 15m below the surface, especially in a region with a cool climate, during a heatwave. This limits the accuracy of sonar.

author: corgon
author_rank: Contributor
timestamp: June 29, 2025, 08:01:37 pm
content: Yeah, that’s true.I’m not using any kind of temperature compensation. I do have a calibration constant in the code in case I need to adjust the sensor, though.But when I compared the measurements to a commercial fishfinder, the readings were pretty much the same.For my purposes, it’s accurate enough – I honestly didn’t expect this level of performance from the sensor.By the way, I found a sonar sensor on AliExpress that supposedly sends serial data directly and can measure up to 6 meters deep.I ordered it, and I’ll test it once it arrives. This is the one I got:https://vi.aliexpress.com/item/1005005245293992.html?spm=a2g0o.productlist.main.5.4277737bd9XiO2&algo_pvid=6beb7770-1ffb-4394-8684-d44f8bf9ac92&algo_exp_id=6beb7770-1ffb-4394-8684-d44f8bf9ac92-4&pdp_ext_f=%7B%22order%22%3A%2213%22%2C%22eval%22%3A%221%22%7D&pdp_npi=4%40dis%21EUR%2143.11%2136.64%21%21%2149.26%2141.87%21%40211b819117512271584578569e0028%2112000032344918314%21sea%21SK%21106439098%21X&curPageLogUid=QtTrqSs4ZVoU&utparam-url=scene%3Asearch%7Cquery_from%3A

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 29, 2025, 08:14:36 pm
content: Salinity, too.Speed of sound in pure +4°C water is about 1420 m/s, but about 1460 m/s in seawater (35.2g of salt dissolved in 1kg of water).At +20°C, it is about 1480 m/s and 1520 m/s, respectively. (Velocities listed at three significant digits of precision.)Within these temperature and salinity ranges, you can interpolate linearly.

author: corgon
author_rank: Contributor
timestamp: July 01, 2025, 10:43:30 am
content: Just a quick progress update.Waterproof floating case made of polycarbonate. Wireless charging, magnetically controlled on/off switch and other functions.The case is just buoyant enough so it doesn’t turn into a submarine

author: zapta
author_rank: Super Contributor
timestamp: July 02, 2025, 04:20:22 am
content: Quote from: CaptDon on July 25, 2022, 01:31:08 pmThe requirements for underwater transducers and hydrophones is not just being waterproof but also acoustic impedance matching with the water since air and water have very different properties.https://pmc.ncbi.nlm.nih.gov/articles/PMC7411934/

author: calzap
author_rank: Frequent Contributor
timestamp: July 02, 2025, 05:39:23 am
content: Quote from: B J on July 24, 2022, 11:59:13 pmCould you define what you want in more detail? Are you trying to depth-locate moving objects like fish or more sessile objects like plants or pieces of wood? Are you trying to measure distance to bottom of water? Will the measurements be made from a floating, moving platform like a boat or from a stable platform like a dock? What frequency of measurement do you need? Rapid because objects, like fish, are moving or because the platform is moving, or slower because objects and/or platform are sessile or moving slowly? Do you need digital recording or the ability to create contour lines?

author: Marco
author_rank: Super Contributor
timestamp: July 02, 2025, 07:37:51 am
content: An off the shelf solution I didn't see 3 years ago, though I assume if you open it up it has the same transducer I mentioned then.https://www.dfrobot.com/product-2759.html

author: corgon
author_rank: Contributor
timestamp: July 02, 2025, 10:20:43 am
content: That’s probably the same sensor I linked in my post #28.I’ve got one on order, so we’ll see how it performs when it arrives.From what I’ve found online so far, it doesn’t seem to work too well at depths beyond 1.5 meters.We’ll see – I’ll report back once I’ve tested it.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 02, 2025, 10:43:54 am
content: Quote from: zapta on July 02, 2025, 04:20:22 amYea, big time. I placed a fish-finder transducer in a canal and fed it with bursts of AC at its resonant frequency. I then placed another, identical, transducer in the water connected to a sensitive receiver. The receiver was able to detect the pulses at a range of 160 metres.I repeated the experiment in air; the range was a little under 2 metres.

author: Marco
author_rank: Super Contributor
timestamp: July 02, 2025, 10:52:47 am
content: Quote from: corgon on July 02, 2025, 10:20:43 amDepth range or depth?If it's function deteriorates with the depth it is at I'd assume there is an air pocket getting compressed in the housing.

author: corgon
author_rank: Contributor
timestamp: July 02, 2025, 12:48:59 pm
content: This is the video where the guy tests it. The test starts at about 3 minutes in.youtube.com/watch?v=htRHMCk1jtU

author: Marco
author_rank: Super Contributor
timestamp: July 02, 2025, 01:20:52 pm
content: Quote from: corgon on July 02, 2025, 10:20:43 amOkay, so you meant it doesn't measure depth from the surface too well beyond 1.5 meters.That's the problem with letting someone else do really shitty signal processing for you. Likely the raw transducer could do far better with a better system around it.

author: zapta
author_rank: Super Contributor
timestamp: July 02, 2025, 10:24:19 pm
content: Quote from: SteveThackery on July 02, 2025, 10:43:54 amThe range increases as the frequency goes down but it requires larger transducers to keep the acoustic impedance matching, similar to RF antennas.I wonder if this is a reason why elephant have big ears, for long range communication.

author: JDubU
author_rank: Frequent Contributor
timestamp: July 03, 2025, 01:02:24 am
content: Take a look at these:https://www.jrt-measure.com/products.html?keywords=green&searchType=AllGreen light source (520 nm) works for optical underwater distance measuring.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 03, 2025, 07:05:19 am
content: With ultrasonic transducers in water the impedance matching is not that difficut. Acousticly water is not that different from the piezo material. So one can use relatively small transducers also for long wavelenghts. One needs the size to get a directinal transducer / antenna.Elephants have the large flexible ears not so much for hearing but as heat exchangers for cooling.

author: corgon
author_rank: Contributor
timestamp: September 30, 2025, 02:30:46 pm
content: Just a little follow-up on my sonar projects.I built a version with a separate transducer, and it works with both the SR04T and SR04T-A1.Here’s what it can do:Measure water depthMeasure distance in the airShow depth trends over time with a graph (84-second window)Track how long it’s been running (elapsed time)Display battery level (percentage + voltage)Read temperature from multiple DS18B20 sensorsHas a backlit displayCan automatically shut off after 5 hours, or after 15 minutes of no measurement changeDetects and shows low battery voltageGives a warning if the sonar or temp sensors fail or get disconnected

author: corgon
author_rank: Contributor
timestamp: September 30, 2025, 02:34:37 pm
content: Preview of the GUI