I'm testing my speedometer. Does anyone know how how many PSI = MPH

[rs2k]

It takes 55 PSI to make the speedo read 36.5 MPH. Is this correct?

 

[dingbat]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

Pivot type speedometers are notoriously inaccurate no matter that you do.

You can probably get a cheap GPS speed sensor for the cost of a new speedometer

 

[rs2k]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

Why is it that they are fairly accurate in aircraft, but they can't make them accurate in boats?

 

[109jb]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

What you are looking for is the dynamic pressure which is:

q=0.5*rho*v*v

q=dynamic pressure
rho=density of the fluid, in this case water. This is 1.94032 slug/ft^3
v=velocity in ft/sec (55 mph=80.7 ft/sec)

so, the dynamic pressure is 0.5*1.94032*80.7*80.7=6318 lb/ft^2

6318/144=44 psi

The density given is for pure water and is a bit different depending on how much stuff is in it.

 

[Silvertip]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

A marine pitot is about $6.00 and the head unit is about $30. Try and buy the hardware just to mount an aircraft pitot and head unit for that money. There also isn't a lot of trash in the sky.

 

[109jb]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

A marine pitot is about $6.00 and the head unit is about $30. Try and buy the hardware just to mount an aircraft pitot and head unit for that money. There also isn't a lot of trash in the sky.

I have an airplane that I built myself and my pitot head cost me less than 50 cents. It is simply a piece of aluminum tube 3/8" diameter and about 6" long. Airspeed indicators also are not very accurate either. There are installation errors due to pitot placement, and instrument errors in the unit itself, and also errors due to changes in air density. The latter is why they have what are called "true airspeed indicators". These have a rotating face which is used to set the pressure altitude and the temperature to compensate for changes in air density. Without this, they can be significantly off at altitude. My boat speedo is only off by about 2 mph (GPS verified) throughout the entire range from 0 to 48 mph in fresh water. 4 percent ain't too bad in my opinion.

 

[mlukich]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

What you are looking for is the dynamic pressure which is:

q=0.5*rho*v*v

q=dynamic pressure
rho=density of the fluid, in this case water. This is 1.94032 slug/ft^3
v=velocity in ft/sec (55 mph=80.7 ft/sec)

so, the dynamic pressure is 0.5*1.94032*80.7*80.7=6318 lb/ft^2

6318/144=44 psi

The density given is for pure water and is a bit different depending on how much stuff is in it.

As Frank Barone (Everybody Loves Raymond) is fond of saying... "HOLY CRAP"...

 

[1970 Chieftain V]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

Marine pitot tubes are also straight pitot tubes. They measure total pressure. And then the gauge I believe is calibrated to show a speed. This can be off if the static pressure is different from what the gauge was calibrated to. Most aircraft use a combined probe called a pitot-static tube or Prandtl probe to measure airspeed. This is more accurate because it takes into account (and measures) both static pressure and dynamic pressure.

So in your case the 55psi you measured is total pressure. To find the dynamic pressure you would need to subtract what ever the static pressure of the water at your location. You can then use 109jb's dynamic pressure equation to work backwards to check the accuracy of your gauge.

 

[QC]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

Pivot type speedometers are notoriously inaccurate no matter that you do.

My last Alpha and my current Bravo, with integral pitots, have both been within 2% anywhere above 15 MPH . . .

Also, all need to remember that GPS only gives you speed over the bottom, not speed over the water . . .

 

[109jb]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

My last Alpha and my current Bravo, with integral pitots, have both been within 2% anywhere above 15 MPH . . .

Also, all need to remember that GPS only gives you speed over the bottom, not speed over the water . . .

Yes. When checking with GPS you need to make runs in multiple directions and average. 2 opposite directions will generally be close enough, but 3 or 4 directions would be better. Mine is within 4% using a 2 direction averaged GPS run on the same location in a river.

 

[erikgreen]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

I pulled out my speedo and pitot, and replaced it with a multifunction display from Lowrance, the LMF-400, for about $200. Now I can flip the display to GPS speedo or fuel flow as needed.

I don't think I'll ever use a pitot speedo again.

Erik

 

[109jb]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

Marine pitot tubes are also straight pitot tubes. They measure total pressure. And then the gauge I believe is calibrated to show a speed. This can be off if the static pressure is different from what the gauge was calibrated to. Most aircraft use a combined probe called a pitot-static tube or Prandtl probe to measure airspeed. This is more accurate because it takes into account (and measures) both static pressure and dynamic pressure.

So in your case the 55psi you measured is total pressure. To find the dynamic pressure you would need to subtract what ever the static pressure of the water at your location. You can then use 109jb's dynamic pressure equation to work backwards to check the accuracy of your gauge.

Everything you said is true. However, the static pressure of water at about the same depth (1 foot or so) isn't going to change appreciably and is very small in comparison to the dynamic pressure, ecpesially at higher speeds. On my boat, the pitot is only about a foot below the surface when not on plane. With fresh water at about 8 lb/gallon and 231 cu inches to a gallon that is a static pressure of about 0.2 psi. It can pretty much be ignored. Therefore, simply applying a gauge pressure equal to the calculated dynamic pressure should yield the calculated speed on the instrument at least at higher speeds. As a test, I went out to the barn and performed a test (I was curious). I first applied 44 psi to the pitot tube line on my boat and the speedo read 56 mph. I then reduced the gauge pressure to 40 psi and the speedo read 52 mph. I was using a shop air gauge which isn't the most accurate, but the results are very close to what the calculation of dynamic pressure said it should be. The calculations say the speedo should have read 55 and 52 respectively.

BTW. I see you live in Lancaster. Do you by chance work for Lockheed? I used to work at LMTAS in Ft. Worth.

 

[dingbat]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

My last Alpha and my current Bravo, with integral pitots, have both been within 2% anywhere above 15 MPH . . .

.

What kind of speedo are you running? It's very hard to get that kind of accuray using a paddle wheel and limiting the range from 0-10 mph.

FWIW: The accuracy of my speedometer anywhere above 15 mph is less than 0.6%

The real question is why does it matter? Has anybody gotten a speeding ticket on the water for doing 30.6 mph in a 30 mph zone?

 

[1970 Chieftain V]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

... On my boat, the pitot is only about a foot below the surface when not on plane. With fresh water at about 8 lb/gallon and 231 cu inches to a gallon that is a static pressure of about 0.2 psi. It can pretty much be ignored. Therefore, simply applying a gauge pressure equal to the calculated dynamic pressure should yield the calculated speed on the instrument at least at higher speeds.

Either way his speedo is wrong or his pressure measurement is wrong. At 55 psig his speedo should be reading approximately 60 mph.

BTW. I see you live in Lancaster. Do you by chance work for Lockheed? I used to work at LMTAS in Ft. Worth.

Yes I work out here at LM Aero.

 

[109jb]

Re: I'm testing my speedometer. Does anyone know how how many PSI = MPH

Agreed. The original poster's speedo is whacked and needs to be replaced.