How to convert water pressure to mph?

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Mike Rappold

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Dec 5, 2003
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Does anyone know the formulas behind converting water pressure(psi) to miles per hour? This is basically the math behind the ADS speedometer that looks like it uses a tire pressure guage.

I am looking at trying to find an electronic pressure sensor that I can connect to a rudder water pickup. The sensors all read in psi so I need to know how to convert it to mph.

Any help would be appreciated.

Mike Rappold
 

TimD

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Jan 1, 2002
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Mike,

Great idea - I too would like to know how it is done...

Tim.
 

Climate Models

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Nov 23, 2003
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140
Not quite sure how to do it, but I would imagine that it would need to have a way to be temperature calibrated.

The difference form 40-degree water to 80-degree water could mean several MPH.

Just a thought.

The math to figure out X amount of water through a certain sized tube at XX MPH is not that hard.

Pick a tube size and I will do the calculation for you.

Peter R.

www.climatemodels.com
 

Preston_Hall

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Nov 4, 2003
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I don't think you can assume the velocity of the water in the cooling line is the same as the velocity of the boat.

It seems there are two unknowns: velocity, pressure

If,

v=velocity

A=cross sectional area

p=pressure

Volume flow rate = Q=Av

Mass flow rate = M=pQ

Then p=M/Av

Also, if the cooling water velocity is zero the pressure gage would still have a reading. Actually there would be a higher reading as there is no pressure relief (cooling jacket outlet). I would think the pressure/speedometer would need its own water pick-up for accurate readings. And to get an idea of the pressure ranges you need to deal you could acquire one of the ADS meters and couple it with a pressure gauge and pressurize with any fluid on the bench.

Just my thinking.
 

Climate Models

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Nov 23, 2003
Messages
140
Prestons correct. My method would not use the water-cooling pick up, but would use a specific sized tube both in length and diameter. (I should have read the original post closer)

The tube would be like a venturi on old aircraft and would read water flowing through it as pressure, or vacuum in the case of a venturi.

Just another thought.

The calibrated airspeed indicator in my full sized airplane works on the principal of air pressure going through a measured tube (pitot tube) and pressing on a coil, expanding it and in turn moving a needle. You could do the same thing digitally with an electronic pressure sensor in the boat.

Peter R.

www.climatemodels.com
 
B

Bill Zuber IMPBA

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Hey Mike where are you loctated now see you are still into boating. I am down in Huntsville if you get a chance give me a call you can find my number on the www.IMPBA.net site under district five.
 

Jerry T

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Nov 5, 2002
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1,529
Here is the chart.

At...

30 mph the pressure is 13.02 psi

45 mph 29.3 psi

60 mph 52.1 psi

70 mph 70.9 psi

80 mph 92.6 psi

90 mph 117.2 psi

100 mph 144.7 psi

118 mph 201.5 psi

Think dragging a pick up tube in the water causes much drag? :( :(
 

Jerry T

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Nov 5, 2002
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Also water pressure is water press no matter the size of the tube,(measuring volume is a different story) the bigger the tube the more drag= slower speed.
 

Preston_Hall

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Nov 4, 2003
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At 118 mph with a 1/8" tube you get 2.5lbs drag! I think we figured before of 20lbs of force required to get to this speed. Now that's drag!
 

Mike Rappold

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Dec 5, 2003
Messages
193
Thanks to all that replied. I agree with the seperate water pickup other than the cooling line. My thought was to use a dual water pickup in the rudder - similar to something that might be used for a twin hydro. One pickup to supply cooling water to the engine and the other to the pressure sensor.

For data collection purposes, I am looking at using the one from Eagle Tree Systems. They said that if I can let them know what type of sensor being used, they can configure the expansion port on it to read it. The data could then be downloaded and analyized in a spreadsheet. Hence the question about the formula to convert water pressure to mph.

I was looking at this way instead of an air type of pitot tube due to the water. If a boat flips and water gets in the air pressure sensor used in the pitot tube, I do not know if it will hold up. Plus there is a little more difficulty in mounting a pitot tube to a boat. The water pressure sensor seems pretty "clean" to me.

Jerry T - how did you get the data on the chart you posted? Formula?

Thanks in advance.

Mike Rappold
 

Chris Attebery

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Jan 13, 2003
Messages
173
Guys,

Here's a suggestion. When I flew high power rockets I had an accelerometer that was able to convert linear acceleration into MPH and altitude. It's a fairly simple integration to do.

Why not use one of those data recorders with the 2 axis accelerometers to extrapolate the speed? Or.. anyone out there interested in building a black box with a 2 axis accelerometer, tach and possibly servo inputs? I can convert the code from my homebuilt flight computers, but I need someone that can layout/etch boards. We can probably get the price down to less than $100 per unit.This would be a long term project for me, as my time is very limited right now.

Chris
 

Jerry T

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Nov 5, 2002
Messages
1,529
Mike

The chart came from the listbot a while back, can't remember who posted it, I left the same meesage there maybe someone will remember. I tried an ADS speedo years ago when they first came out. You do need a dual pick up rudder, the drag from the extra pick up will kill the speed of the boat.
 

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