Help with a depth sensor for an ROV

Hi Wingnutzac,

You are not (altogether) barking up the wrong tree. As it turns out, a direct depth can be obtained by use of a pressure gauge, MEASURED OUTSIDE THE ROV.

Gauge pressure can be measured in psig OR inches of water OR millimeters of mercury. One psi = 27.68 inches of water. For example, at 50 psig, your depth would be 115 ft.

Now, about using an auto pressure sensor. The sensor is only a switch. It will not give you proportional output of the pressure it is sensing. All it will do is switch a light on if your oil pressure is too low or too high.

What you need is a pressure sensor similar to the ones built by Freescale Semiconductor. See: http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MPX5999&fsrch=1

The output signal from the sensor (typically 0 - 40mV) may need to be amplified/scaled to accomodate your readout.

Hope this helps.

mikerho AKA Mike Hogan

(I can't log in and have left an email with CR4. They haven't gotten back to me)

I think you meant the sender for the oil pressure gauge fitted to some cars, though I must say too few these days. Look for a seventies/eighties Jag in a breakers for instance. I must say though, that Mr Lucas (Prince of Darkness as he became known not long after entering the automotive lighting market) sadly didn't make his stuff to last long in an underwater environment (or to last long period in that era!). There must also be piezo style sensors available that would probably be a good deal more compact.

This is great, gives me a better direction. When I was thinking about the oil pressure gauge I was thinking of one my dad's old truck had, when I was a kid, that actually measured the PSI of the oil. Your solution is much better, I think the sensors you pointed me towards are cheaper then ready made depth sensors which I can't find for a "hobbyist" price. Thanks!

Wingnutzac, unfortunately I couldn`t recommend the MPX5999 because it is not media compatible. Lo cost sensors like this mostly have an unprotected semicon membrane and are for use ONLY ! in dry air; what you need is a sensor functioning like the above but with a stainless steel membrane. These are often filled with oil or another pressure-transport-media between the sensor plate and the outside membrane isolating the electronic parts from the environment.

You find these parts, par example, at sensotec.com.
Regards Uwe

Yeah, I was actually going to ask the question when I started looking into these and noticed the issue in the manufacture drawings of the sensors. So this may be a dumb question, but do I need to do anything special with the sensor to set it up (other then calibration and programming with my microprocessor)? I found several on the sensortec website that will probably work.

No, you don`t. Calibration is a must, and selecting the sensor for the right media cares for a long lifetime.
Regards Uwe

..... of the sensor, of course ...... ;-)

How about the petrol gauge? sounds strange but see if you can follow me!

Get a large syringe and seal the needle end (remove needle first of course!) The plunger is attached to the float arm of the petrol gauge. Thus the deeper the ROV goes, the smaller the space in the syringe, which moves the float arm down (or up by design) ! The signal will be analogue, so if you put 12v in, you get 0-12v out( ok, not 0v but a lower voltage than 12v). Can you use this idea? (Or you could put the variable resistor directly on the plunger) Sorry, haven't a clue what a ATMEL microprocessor board can do, but if it has an analogue in port, it may work! Simple to calibrate but I doubt it would be of use really deep!

This is my attempt at a diy solution!

I think this idea has merit, definitely a cheap solution. I am going to play with this in my pool and see what kind of results I get. Thanks for the idea!

Typical modern plastic 'throw-away' syringes have terrible friction/stiction though. There has to be something that can be robbed from another source. I keep thinking of the washing machines water level sensor, but that doesn't give a continuous reading (just a couple of switches in the ones I've dismantled) and is too sensitive. Car's oil pressure gauge would be too insensitive with a full scale of 60-100psi (depending how deep you intend to go).

I'm looking for a maximum depth right now of about 150ft, so around 6 ATMs or up to 90 psi. I wonder if the syringe friction issue could be remedied with some heavy grease lubrication...

Rather than measuring the pressure to determine depth, perhaps you might consider measuring the distance with an acoustic solution. You could mount an ultrasonic transducer on the top surface of your ROV. Pinging the transducer and then listening for the return, you can calculate your depth based on the speed of sound through the water and time it takes to make the two way trip. The surface of the water presents an impedance change to the acoustic pulse so you will get a reflection. There are a number of acoustic ranging devices at the hobbyist level that might be adaptable for your situation.

You might even be able to mount a Hummingbird fish finder "upside-down". Those can be purchased for less than $100 USD. There may be a digital output that you can tap into to get the depth signal.

Good luck with your quest!!

Nice out of the box solution. In theory would this only work if the ROV was directly under the transducer though? because when I think about it, if the ROV is down 75 feet and 75 feet to the north of the transducer wouldn't the return from the transducer give the actual distance as opposed to the straight up down distance?

I think he means mount the transducer on the ROV and ping the surface! The reverse of pinging for depth under a boat so to speak!

Now that I read it again I think your right. I like this idea, I think I'll try it along with a pressure sensor and see how they both work.

This is an interesting project, but there are certain things you must understand before choosing your sensor, or transducer:

• Media compatibility: Water, many cheap sensors are for non corrosive gas
• Depth: in of water vs psi is your working range
• Depth resolution: See also depth as many sensors have a % over full scale specs
• Sensor type: Gauge, differential, Absolute? You need absolute. Car sensor probably gauge.
• Output: Amplified or not, depends on your electronics ability and budget. Some even have a digital output (pulse-Frequency).

Companies like Digikey, Mouser, Allied Electronics carry many types and companies but don't specify media in their filter, go on the manufacturer web sites for easier searches.

If you don't mind the reduced accuracy and re-calibration, and have the room for it, you could use a cheap sensor made for air and use a water hammer arrester. Some will be a rubber diaphragm, some a plunger like the syringe idea. See Water Hammer arrester at mcmaster.com and google. Some may require more tinkering than others.

Have fun!

Can you help me understand a little more about the differences between Gauge and Absolute sensors? Also in of water, I have seen this while looking at sensors but not really sure how to use it or what it means in relation to pressure?

There are 3 main types of sensors:

Absolute pressure is relative to vacuum, each side of the diaphragm: media/vacuum. This is good for you since the ROV's inside pressure will probably be influenced by the depth you are at. Better to have a reference to vacuum. It will also be useful to zero your depth before a dive and remove the atmospheric pressure error. Another post makes reference to this.

Gauge pressure is relative to the atmosphere, each side of the diaphragm: media/atmosphere. This is the type you see the most. Your tire pressure is a gauge pressure.

Differential is media relative to media, each side of the diaphragm: media/media. This is useful for example in measuring the pressure drop in a pipe to deduct the flow rate.

As for inches of water, this is a unit of pressure originating to the fundamentals barometry, using columns of liquid, water or mercury, to measure pressures. Any unit conversion site will tell you the conversion to psi. I use EngNet.

Basically, if you go to a depth of 120 inches, the weight of the water over you will be the pressure of 120 inches of water, or 4.33psi. This a compressive force of 4.33 pounds per in² on the body you put at this depth under water.

Hope this helps.

DigiKey lists 1600 or so pressure sensors

Here's one that's about $24 and has a roughly 0-5V output

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=480-1927-ND

480-1927-ND

you might need to use some kind of diaphragm, possibly with oil on the inside, to prevent water from getting inside.

Good luck

Dick

I think this one will work. Any reason you recommend the gauge over the absolute sensor?

Hi winnutzac,

Pressure & depth have a v. simple relationship.

An increase in depth of 10m gives 1 bar extra pressure (exact 10.1974m @ 4deg C).

This will vary slightly for different water temperatures (10.2071m @ 15deg C).

You will also have a variation for saltwater or freshwater, but if your are diving in a specific location this should be fairly constant (exceptions are tidal rivers/inlets). Find out the specific gravity (sg) of the water you are diving so you can adjust the calculatiuon accordingly. When diving to very great depths the sg of the water/ocean can change slightly with depth.

Variations in atmospheric pressure will have an effect (+/-40cm), atmospheric pressure can vary (roughly) from 0.85bar to 1.2 bar and doesn't change v. quickly on calm days. If this is tolerable you can ignore. If you need higer accuracy you can compensate for this.

Thus if you can measure the outside water pressure you can calculate your depth pretty reliably.

I hope the above helps you on your way.