Oil pressure gauge

[Brian-H]

Brian you are using waffle to prove a point that doesn't exist

Well I've added some strawberries to my waffle for you to savour



I don't know whether the oil pressure gauge on the Daimler V8 uses a Bourdon tube or not, but either way the effect of air would be the same on the pressure reading. I just googled "air bubble" in bourdon tube (with the quotes) and came across a post on a Vintage Scuba diving forum https://vintagescuba.proboards.com/post/2499

Quote
I forwarded this to the owner of the dive store where I work. He has been in business since the 1960's. Here is his response:
"Wow.
I don't have a definitive answer, so I shall offer some fact and opinion. I suppose I could also forward this to an engineer or two as well.

Fact: 95% of the oil filled gauges I have ever received as new stock did not have an air bubble.
Fact: those that did were returned to the manufacturer and replaced at no charge.

Opinion: If the gauge is pressure sensitive by nature (i.e. depth gauge) then it would read somewhat
lower in pressure (shallower in depth) due to the compression of the bubble of gas.
In other words, the pressure that would have deflected the bourdon tube that moves the gear that
moves the needle would be dampened with the loss of volume of fluid caused by compression of a
air-filled cavity. The more gas, the lower the deflection, the more inaccurate it becomes. I'm
sure we could test this in our chamber. I have some 70's dacor jobs just begging to have their
bright copper workings exposed once again to the eyes of man.

Fact: I have been told by more that one factory rep that a bubble in a non-pressure sensitive instrument (ie compass) will have absolutely no bearing (pun) on the accuracy of the device.

Opinion: I agree. The bubble will cause it to implode at some ungodly depth, but barring a ticket
to Cayman trench, I don't expect a mere mortal to ever see it happen. However if we're talking major
leakage then compression of the case will result in the "needle" getting pushed onto it's bearing (jewel)and "frozen" in place from unnatural contact with the case."

Brian's post indicates that though there are delays and dampening the final pressure with or without air is the same, regarding bleeding the gauge, air could be introduced into the gauge line during an oil change and in some cases after the engine switch off depending ln the position of the gauge line, never seen any advice regarding bleeding the gauge after oil changes.
I still think the change is possibly caused by an inadvertent adjustment of the calibration screw.

In my explanation, I did not say "though there are delays and dampening the final pressure with or without air is the same" because what I was saying is that air absorbs some of the pressure hence the gauge will move less (it may be waffle to you but that is what I said). This would be particularly true if the air gets into the Bourdon tube.

Hence the possible requirement for bleeding the gauge.

Note that I am not saying that this is the absolute reason for the change as noted by Kevin in his original post. But what I am saying, is that there is a scientific reason for not wanting air bubbles in a mechanical oil gauge, or in the pipe up to the gauge, and if the bubbles move up into the Bourdon tube (or any similar type of pressure gauge to read oil pressure) then I''d expect the reading to be affected.

That'll be £2 for the strawberries, the waffle is free.

[Ian Slade]

Brian, my apologies, in error I entered you instead of Simon when refering to his post with the link.
Reference your post on a divers gauge, divers use a differential pressure gauge in which case air in the tube does matter the gauge on a car is direct reading and air is of no consequence on the stable pressure reading.

[Brian-H]

Well I still want £2 for the strawberries LOL

But in that description (found by googling "air bubble" in bourdon tube ) he does say
"If the gauge is pressure sensitive by nature (i.e. depth gauge) then it would read somewhat lower in pressure (shallower in depth) due to the compression of the bubble of gas. In other words, the pressure that would have deflected the bourdon tube that moves the gear that moves the needle would be dampened with the loss of volume of fluid caused by compression of a air-filled cavity. The more gas, the lower the deflection, the more inaccurate it becomes."

So it matters not how the pressure arrives at the Bourdon tube, the fact is that air inside a Bourdon tube filled with fluid, will cause the tube to deflect less.

[Ian Slade]

I didn't eat them , the bourdon gauge you quote is differential, i.e the reading is at atmospheric pressure on the surface, as the depth increases the pressure in the tube increases but because of the air bubble the gauge will always be 1 atmosphere out throughout the dive, this will impact on the decompression time which is calculated on depth and time submerged, this can or may increase the chance of the "bends".

[Chris_R]

I'm not understanding any of this but how would you explain an oil pressure gauge connected to the engine by a pipe and the needle fluctuates up and down? Whereas at the same time the electrically fed dash gauge maintains a steady reading.

[Brian-H]

I didn't eat them

You should have had them, they were very nice

I think that we're missing something else here
1. what sort of mechanical oil pressure gauge is used in the Daimler V8 ?
2. when you switch off the engine, surely the oil drains out anyway ? (and thus when you start the engine next time, there's air in the pipe)

If the gauge does use a Bourdon tube, then, either air gets into the section of Bourdon tube anyway (thus IMO the gauge is unlikely to be constructed that way), or, there's a diaphragm at the end of the section of Bourdon tube and something has happened there ?

[Brian-H]

I'm not understanding any of this but how would you explain an oil pressure gauge connected to the engine by a pipe and the needle fluctuates up and down? Whereas at the same time the electrically fed dash gauge maintains a steady reading.

It's easier to dampen out the needle movement in an electrical gauge.

[Chris_R]

OK Brian, that explains a steady electrical gauge, but what would cause a fluctuating reading on the gauge connected by the tube directly to the engine? The needle is bouncing by around 10psi between upper and lower readings quite rapidly.

[Brian-H]

Without having ever seen the mechanical oil pressure gauge used in the Daimler V8, the gauge probably does use a standard Bourdon tube which, as with an air pressure gauge (which I have had apart in the past), is sealed at the "top" end of the Bourdon tube and is open at the fixture end.

The gauge relies on engine oil travelling up the connecting pipe and into the Bourdon tube. However, when the engine has been off for a while, some (if not all) of the oil drains out of the Bourdon tube, down the connecting pipe, and back into the engine. So you're always going to get a mixture of air and engine oil all the way up, including in the Bourdon tube itself.

Based on that logic alone, air bubbles cannot matter . I don't think that the brakes analogy is a good analogy for several reasons, which I'm not going to go into (tldr reasons). I also think that comparison with a diving gauge is not right (again tldr stops me elaborating).

Regarding the issue originally mentioned by Kevin I've changed my mind about air being the cause - there's some other reason, it can't be due to air as there will always be air mixed in with the engine oil, all the way up. The only thing I can think of is that maybe some "dirt" or sludge has worked all the way up into the Bourdon tube ?

OK Brian, that explains a steady electrical gauge, but what would cause a fluctuating reading on the gauge connected by the tube directly to the engine? The needle is bouncing by around 10psi between upper and lower readings quite rapidly.

This video that I just found won't explain why, but it gives the answers to how mechanical gauges can be dampened.



So if the gauge is bouncing around then maybe some dampening mechanism has come adrift or altered somehow ? Or maybe there's excessive vibration in the pipe between engine and gauge, or maybe there's an excessive amount of air in the pipe. Or if it's a new gauge maybe it's not right for that application.

[Ian Slade]

Or the relief vave is bouncing due to a failing pump, the V8 saloon is electrical, when originally sold or instaalled by manufacturers they were sold or fitted as a pair, sensor and gauge or calibrated, the SP Bourdon gauge type is just a union screwed into the oil filter housing and then a pipe to the gauge, no diaphragms or anything else, therefore any fluctuations would emanate from that area.