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Waxing Philosophic about Bernoulli's Principle

ronzet

Jedi Knight
Offline
LBC fans everywhere,

The car, Frame off, total rebuild 1957 MGA...

The screw-up (?), oil in the oil pressure tube all the way up to the gauge...

The query: Am I correct in assuming that the oil pressure gauge is controlled by Bernoulli's principle and as such reads the pressure of the compressed AIR in the feed tube from the engine block as the oil moves up the tube and compresses the air within.

In other words, there should be no oil actually traveling up the tube into the gauge. Correct???? /bcforum/images/%%GRAEMLIN_URL%%/rolleyes.gif

While I am at it... /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

Anyone know where I can purchase the 'hold down' brackets for the Tach and the Speed-o-meter.... Mine have mysteriously disappeared... At least I have been unable to find them in my 'stack-o-stuff'....
 

piman

Darth Vader
Offline
Hello Ron,

yes in theory, but if there is the slightest leak oil will eventually travel up the tube to that point.
Unless the bourdon tube itself leaks then I would suspect (I have never checked) that there is no oil in it?

Alec
 
OP
ronzet

ronzet

Jedi Knight
Offline
Piman,

The genesis of my query came from turning the engine over with the tube connected to the block and not connected to the gauge only to find oil dripping from the instrument END of the tube.

I am trying to decide of the effort to remove the tube and cleaning it out is warranted???

Thanks for the response... /bcforum/images/%%GRAEMLIN_URL%%/cheers.gif
 

Steve_S

Yoda
Offline
I wouldn't worry about it. In fact, the gauge will be more precise with oil in the tube rather than air, since oil is thicker. Just make sure it doesn't leak or you may ruin your new trousers!
 
OP
ronzet

ronzet

Jedi Knight
Offline
Steve,

Good morning...

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Just make sure it doesn't leak or you may ruin your new trousers! [/QUOTE]

Or, more importantly... my brand new carpet and interior when it gets installed.... /bcforum/images/%%GRAEMLIN_URL%%/wink.gif

Thanks for the advice... I am ready to start this puppy since I have been in a concerted effort to get it on the road this summer. I am currently trying to find out why there is no spark. Everything else is ready to go.. Sans interior that is.. /bcforum/images/%%GRAEMLIN_URL%%/cheers.gif
 

David_DuBois

Jedi Warrior
Offline
There was originally a small, leather washer where the simi ridgid tubing screws onto the back of the gauge. If this has been lost, you will wind up with a very small oil leak at that spot. If you don't have the leather washer, go to NAPA and get a small 'O' ring to replace it with.
Cheers,
 

vping

Yoda
Bronze
Country flag
Offline
Useful info and great tips. Can these be applied to all out LBCs?
 

jlaird

Great Pumpkin
Country flag
Offline
Of course, O ring is even better, it lasts your life time.
 
OP
ronzet

ronzet

Jedi Knight
Offline
Gentlemen,

As usual, a wealth of knowledge... On to NAPA... /bcforum/images/%%GRAEMLIN_URL%%/cheers.gif
 

PC

Obi Wan
Country flag
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Just to be clear, the operation of a pressure gauge is not an application of Bernoulli’s principle.

Pressure gauges simply work as springs that deflect in response to applied force. In the case of a fluid pressure gauge the area over which the force is applied is fixed so the deflection is directly proportional to pressure.

The area of applied force is often a coiled tube which unwinds as the force is applied (as piman points out, this is called a Bourdon Tube) and rotates the needle. Low pressure gauges will use diaphragms instead.

The gauge will respond to any pressure, whether liquid or gas. Steve_S correctly points out that the readings will be more stable with liquid (as it is incompressible).

Last time I changed out an oil pressure line I routed the tube to the gauge, but before connecting it I aimed the tube into a can and cranked the motor until oil spewed out. That way I minimized the air in the line so I could get the best readings.


PC.
 

DrEntropy

Great Pumpkin
Platinum
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Oddly, this is one of those: "Don't pet the sweaty things" issues IMO. I've amped over it and "bled" the line, then alternately just rigged 'em up dry... and never saw the diff'rence in the end.

<shrug> Bernoulli-schmernoulli. Banana dana ding-dong...

/bcforum/images/%%GRAEMLIN_URL%%/devilgrin.gif
 
OP
ronzet

ronzet

Jedi Knight
Offline
PC,

What was I thinking?!!?? Now I feel like a real putz. /bcforum/images/%%GRAEMLIN_URL%%/blush.gif

I stand (or sit as the case may be) corrected. You are correct and I am, in this case, not.

As you obviously know, Bernoulli's principle deals with the differences of pressure in a compressable and non-compressable fluid in motion in a closed path or in a streamline.

As you also probably know, this principle is used to calculate airfoil lift, airspeed, et al.

Thanks for correcting me. I guess I should avoid posting when I have had too many adult beverages... /bcforum/images/%%GRAEMLIN_URL%%/cheers.gif
 

PC

Obi Wan
Country flag
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Just be sure to consume those beverages with a steady inviscid adiabatic flow in a uniform gravitational field. /bcforum/images/%%GRAEMLIN_URL%%/thirsty.gif

/bcforum/images/%%GRAEMLIN_URL%%/cheers.gif

PC.
 

DrEntropy

Great Pumpkin
Platinum
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There's th' rub... finding that uniform gravitational field. Around here I swear somebody keeps turnin' it UP.
 
OP
ronzet

ronzet

Jedi Knight
Offline
/bcforum/images/%%GRAEMLIN_URL%%/lol.gif

Gentlemen,

I love laughing when I start my day.... /bcforum/images/%%GRAEMLIN_URL%%/grin.gif

Thanks for the notes... I must tell you the good news...

I started my A last night!! A major accomplishment. After three years of tearing apart and putting back together.. the little guy rewarded me by starting up... not running yet but it kicked over and ran for a few moments!!! /bcforum/images/%%GRAEMLIN_URL%%/banana.gif

Now where did I put that adult beverage... /bcforum/images/%%GRAEMLIN_URL%%/cheers.gif
 

jlaird

Great Pumpkin
Country flag
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Excellent, a good moment.
 
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