Does the length of oil pressure tube/hose, affect the reading of the gauge?.

[daverclasper]

I replaced this on the Vitesse a few years ago, as was looking old and brittle. The original was maybe around a metre in length? and a fairly straightish run from the block to gauge. The only one I found available at a reasonable price was from Moss, I think it was 1.8 M long and is hence circularly, curled around a few times before going through the bulkhead to the gauge.

I never made a note of when I replaced it and never linked it ("doh") though may have been around the same time I noticed  my oil pressure dropped from around 65 at 2000rpm, to 50 at running temp and idle, about 25, to 18.

I assume?, the longer the hose, the less pressure?, though no idea if it makes a difference in this arrangement? 

This was a good few thousand miles ago and still ok, and hasn't got worse, so not too bothered.

Intrigued if it could be related ?, please.

Thanks 

Edited December 8, 2023 by daverclasper

[Nigel Clark]

Simple answer is NO!

Nigel

[Pete Lewis]

the different results from old to new gauge is really just down to  gauge calibration 

which one was right ???    in the end they give an indication not a worry gauge 

needle response may vary  did you bleed the end of the pipe to loose most of the air

it wont change the reading some air might damp the needle movement a little 

but dont loose sleep over this 

Pete

 

[johny]

Think of a closed system, like a pipe and pressure gauge, as a balloon Dave. With a balloon you can stretch it out as long as you like and to any shape you like but the pressure will be the same in all parts of the whole thing. However an open system with a flow through it like a very, very long hose can have a high pressure at the entrance and yet still only trickle out at the far end because of the pressure drop along the length...

[Pete Lewis]

Ha   sound like my personal plumbing restriction 

Pete

[johny]

yes fraid theres no solution for pressure drop except a bigger bore hose😲 

[dougbgt6]

You're noy looking  for exact readings from the oil gauge, just because it says it's 70 doesn't mean it is.  What you're looking for is consistency, is it the same as usual? However If you've messed with the pipes then usual may turn out to be something different. You'll just have to wait and see!

Doug

[Iain T]

Pascal's Law is 

A change in pressure at any point in an enclosed incompressible fluid at rest is transmitted equally and undiminished to all points in all directions throughout the fluid, and acts at right angles to the enclosing walls.

Pressure = Force/Area

You haven't changed the force so the pressure is the same. 

Iain 

Edited December 9, 2023 by Iain T

[daverclasper]

Thanks guys. That's telt me then! 🙂

I guess it would have just been a tiny bonus point, if it may have made a difference🙂   

Edited December 9, 2023 by daverclasper

[Cookie]

my gauge actually read slightly higher when I changed to an aeroquip type braided hose from a plastic one, it also shows more individual fluctuations in pressure at tickover. I think that's because the braided hose is less restricted. Before I changed the hose the gauge showed abut 65psi when hot and running and 30psi hot at tickover, now it shows about 80psi running and just below 50 at tickover.   TBH I treat the gauge more as an indication of any problems than an accurate measure, if it started reading less then I'd know I had an issue starting.

[dougbgt6]

8 hours ago, Cookie said:

I changed to an aeroquip type braided hose from a plastic one,

Yeh, I did that too, relying on a bit of brittle plastic pipe ain't a good idea!

Doug

[Peter Truman]

The thin plastic pipe will swell slowing the pressure to vary unlike the braided SS covered nylon pipe which will be more constrained and build up a higher pressure.

I must upgrade to the nylon braided pipe on the daughters Spit on my Vitesse I’ve always used copper pipe with coils at each end to reduce fatigue

Edited December 11, 2023 by Peter Truman

[Mjit]

On 09/12/2023 at 11:58, Iain T said:

Pascal's Law is 

A change in pressure at any point in an enclosed incompressible fluid at rest is transmitted equally and undiminished to all points in all directions throughout the fluid, and acts at right angles to the enclosing walls.

Pressure = Force/Area

You haven't changed the force so the pressure is the same. 

Iain 

Ah, but that "incompressible fluid" part's important - and certainly on my car there's a good amount of air in the plastic and air is compressible.  If there's more air in the new pipe you'd get a lower reading.

[Iain T]

31 minutes ago, Mjit said:

Ah, but that "incompressible fluid" part's important - and certainly on my car there's a good amount of air in the plastic and air is compressible.  If there's more air in the new pipe you'd get a lower reading.

Very true but I live in a perfect world😂😂😂.

If I remember my physics from 50 years ago Boyles Law explains the relationship of gas and volume. 

Perhaps that's why I'm compressible I'm full of gas! 

Iain 

Edited December 11, 2023 by Iain T

[daverclasper]

2 hours ago, Mjit said:

If there's more air in the new pipe you'd get a lower reading.

I'm pretty sure, I noticed there was more air when I replaced the tube, than previously.

Also, I understand the gauge may well be inaccurate, as to actual pressure. It was because it maybe dropped in a short space of time, that I wondered if was because of, as per thread title.

Thanks again for replies

Edited December 11, 2023 by daverclasper

[GrahamB]

6 hours ago, Mjit said:

If there's more air in the new pipe you'd get a lower reading.

On starting the engine, oil will travel along the pipe compressing the air until the pressure of the air equals the pressure in the oil at which point the oil can no longer advance along the pipe.  The gauge will then record the air pressure but that is equal to the oil pressure.  A narrow bore tube will damp down any variations as the flow of the oil up the tube, or down the tube as the pressure reduces, will be restricted but the effect is likely to be small.

[Pete Lewis]

if you worry about air lock in the tube disconnect ,at the gauge hold over a jam jar and crank till oil arrives 

then just re reconnect ............the miracle of simple  bleeding 

wont make any real difference but you will be happier 

Pete

[Mjit]

1 hour ago, Pete Lewis said:

if you worry about air lock in the tube disconnect ,at the gauge hold over a jam jar and crank till oil arrives 

then just re reconnect ............the miracle of simple  bleeding 

wont make any real difference but you will be happier 

Pete

I seem to remember trying that...only for half the oil to then drain back overnight!

As others have said so long as it's in a consistent place don't worry.