Smiths oil gauge - silly question I expect

[Andrew S]

With a Smiths oil gauge and the clear white pipe feed, do you need to get the air out of the piping? I am thinking that air gets compressed and looking at the pipe, the oil is in segments so surely can't be accurate?
Do you need to bleed the pipe by loosening at the gauge end and if so, the air is expelled, what stops the oil running back down the pipe?
I am guessing the reading would be lower than pure oil all the way
Thanks
Andrew

[davidf]

Leaving air in the pipe will simply make the gauge slower to respond to pressure changes. As there is no flow out of the gauge end, at a steady state, the pressure will be the same all the way along the pipe, regardless of whether it contains oil or air. However, if the pressure increases, then the air will have to compress, which means oil must flow into the pipe from the engine end. Normally, this is a very restricted port, so this will be (relatively) slow. Myself, I carefully loosened the connection at the gauge end while the engine was running, and surrounded it with an absorbent cloth. When the air was out, and the first trace of oil appeared, I tightened the connection.

[Matt Cook]

Pressure is pressure regardless of the fluid type (liquid or gas). The gas (air) is compressible but when there is no flow the pressure will be constant along the pipe to the gauge.

[Andrew S]

Thanks guys
David, just out of interest, did the oil run back down after the bleed? I do now appreciate it shouldn’t affect the pressure reading
cheers
Andrew

[Ritchie6]

I found this topic of interest as I've been toying with the idea of moving my oil pressure gauge up into the dash panel and replacing the clock which no longer works. My oil pressure gauge was fitted by the previous owner in the mid 1990's but for some reason he chose to mount it on the parcel shelf underneath the glove box meaning it cannot easily be seen from the driving seat. I had concerns that if I disconnected it I might have problems getting it to work again. Plus I'm not even sure how long the pipe is and whether I'd need to replace it in order to reach the dash. I assume they are a standard bore on these things-it's a Smiths gauge. If it needed replacement what is access like at the engine end?

Richard

[dasadrew]

Try this thread from post 18 onwards!

https://socforum.com/forum/forum/sta...pressure-gauge

[Lingen]

Andrew.
The oil could only run back to the engine if air could get in at the gauge end. Otherwise, a vacuum would be caused, and as we know, Nature abhors one of them. Also, the bore of the capillary tube is too small to allow air to travel back up it. Once bled, the pipe should stay full.
Mike.

[Ritchie6]

Try this thread from post 18 onwards!

https://socforum.com/forum/forum/sta...pressure-gauge

Thanks for the link Drew. The fact that 022Dave had an issue with his oil pressure gauge pipe coming adrift and emptying the sump while driving along is the thing that rather puts me off tinkering with mine. It does work perfectly but I'd still prefer it to be mounted in the dash which is where I'd definitely put it if I ever had cause to fit a new one. I think for now I'll leave well alone.

Richard

[Andrew S]

Thank you all, very interesting and helpful, as always
Andrew

[mike@thenook]

I got the air out of mine Andrew by not tightening the joint too tight at the receiving end resulting over a long period of time with the dash slowly going a very dark brown around the OPG!

[Andrew S]

Didnt think of that Mike, great idea as I have a spare dash

[Carboy0]

I think the only reason that we are discussing this because the pipe is translucent and you can see the air/oil bubbles. If all the pipes were black (like mine) or metal, none of us would ever obsess about it ;-).