Underwater speed?

[Ramius]

Hi Nick!
Here is a link to the radio controller: http://www.conrad-uk.com/ce/en/produ...ains&insert=8X It has two throttles, one for the port motor and one for the starboard motor and a ton of expansion modules! I originally bought it from Ships n' Things. Range and depth are a funtion of the both transmit and receiver antenna's and how they are configured. You basically are trying to simulate a "dipole" for receive and you can use a yagi for transmit. Part of the other factors are where you place all your electrical cables and how you have prevented EMI (Electrical-Mechanical) and RFI (radio frequency) interference. For example motors produce small sparks which are basically a radio signal. So you have to use bypass capacitors. How you ground the motor cases and how you run power all make a difference. The technique I use is called "single point" power and ground where all the power wires connect to one single point and well as the ground wires. In addition power and ground wires have been put in an electric drill and twisted together. There are a lot of technical reasons for doing this. If I strike out with a pressure sensor then the propeller idea will be next!

Best to All, Ed

[NickMu]

Hi Ed,

Thank you for your detailed reply.
I had an application that needed to read the thrust of a jet of water and before our order of force sensors came in we used a paddle wheel to determine the speed of the jet.
The results were within 5% of the real sensors readings.
Depending on your precision and accuracy requirements any of the suggested methods should work but with some penalties.
IMHO your biggest problem will be the changes in water density due to various reasons (salt, temperature…..)

Thank you again and good luck.

Regards,

Nick

[Ramius]

Hi Nick!
Thank you and I would not mind a paddle wheel type except they all seem to be really big! I do not believe water density will be much of a problem since I will only be operating in fresh water and for a sensor I am using an MS5541C's which are temperature compensated. What I really do not know is the actual formula for velocity. Water density values seem to be all over the map. Do you know of or have a simple formula I can program into a PIC (PIC Basic)?

Antenna work is almost a "Black Art". For a transmitter your main concern is what is called SWR (standing Wave Radio). Basically you are sending power up a coax and when the power hits the antenna if you have the correct antenna and correct impedence (AC resistance) then none or a very minor amount of power will be reflected back to the transmitter or used to "heat" the coax. On a receiver you have the same problem in reverse where you want all the received signal you can get! A dipole (two wires 180 degrees from each other) has an impedence of 300 ohm. When the two wire are at 90 degrees you are looking at about 50 ohms. It even becomes more complicated with the lenght of the antenna wires! That is full wave lenght, half wave lenght, 5/8 wave lenght, 1/4 wave lenght. It can boggle the mind!

Best, Ed

[Demon]

I was thinking of these. Just need 2 fittings to connect hoses in/out.

http://koolance.com/ins-fm16-coolant-flow-meter

You should be able to make your own controller with PIC.

Robert

[Demon]

Formula for velocity:

Follow sub in a boat at max speed, look at knots on gps, note reading from flow meter.

Half velocity = 50% knots, etc.

Or is that too simple?

Robert

[NickMu]

Hi Ed,

I knew it is not an easy task to get such system going especially when in my application the transmitter will be above and bellow water at times. I will try some other avenues first and if no success I might bother you with some more questions.

I will be away from my office for a few more days so I do not have access to my notes.
For our test we just calculated the speed of the jet and the volume of the water passing through in a time period. With the water density as a constant we were able to calculate the thrust.
In your case I’m quite sure you will need at least your differential pressure, the surface of the sensor (maybe) and the water viscosity. I will look into getting you a formula combining all the factors from which you can extract the speed as the only unknown.

What will be an acceptable size for the paddle wheel? The one I used was a little over 1” diameter and it is used on Sea-Doo PWC for speed indicator. It gives 4 pulses per revolution and it is a completely sealed unit wit three wires interface (power in and signal out). If you are interested I can send you one as soon as I get access to it. Just PM me your address and you will have a possible back up solution.

For the differential pressure method how hard will be for you to take a couple of readings at half and full speed to see if your numbers will make some sense. From that it will only be a matter of coding.

Regards,

Nick

[NickMu]

What you are dealing with is the case of dynamic pressure in fluids. One of your sensors will read the static pressure and the other one will read the total pressure. The difference will be your dynamic pressure. Using the formula: P = q*V² you can extract your speed.
P= dynamic pressure
q= density of water
V= speed
So you have your readings for dynamic pressure and you have your water density (I’m not sure if you have to compensate for temperature again since your pressure readings are already compensated) and you can calculate your speed.
I had the chance to do some Google search and I found this:

http://www.engineeringtoolbox.com/dy...re-d_1037.html

HTH.

Regards,

Nick

[NickMu]

One mistake on my formula:
The left side should be multiplied by 2 or the right side should be divided by 2.

Regards,

Nick

[AvionicsMaster1]

Just sayin', that's what I said. Even used the same link. Compensation for density in same post. Number 18 if you'd care to peruse.