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MG MGB Technical - oil pressure gauge

Forgive me, this is probably a dumb question. I thought I would add another oil pressure gauge to the vehicle. The original one combined with the water temperature gauge is working fine. It would be interesting to have the two oil gauges, so I have a 't' piece to add the new one. If I connect and tighten all the unions, will I have an 'air column' that will not disperse? Do I have to bleed the system so to speak? Mike
J.M. Doust

Mike,
Pressure is pressure, regardless of the fluid. You don't have to bleed the system.
There is probably some air in the Bourdon tube of the gauge already.
David
David Overington

Thanks David, I thought as much. Funny you get older and you start to question, what would have been very simply executed when you were younger! Age brings about a certain hesitancy! Mike
J.M. Doust

Mike, Thanks for asking this question. I am at the point of reinstalling my gauge and had exactly the same moment of doubt. However, whilst as David says "pressure is pressure", air is much more compressible than fluids so won't air in the tube produce a different dial reading than if there is no air? Am I missing something?
Help anyone?
Richard Coombs

Richard

You're correct in thinking that air is compressible were as oil isn't, however this doesn't matter because as the oil pressure builds up the air will be compressed and take up less space (Boyle's law), the pressure will however remain the same as more oil will be forced into the capillary tube to take up the space.

Bob
R.A Davis

Quite right Bob, Which then leads us all into the problem of the bleeding brakes, where no air can be tolerated! But then again, here we are not measuring, but using fluid to transmit a force. Mike
J.M. Doust

Mike. A direct reading pressure gauge (one which does not attach to an electric sending unit) seldom fails. But, adding a second gauge to the system increases the possibility of some form of failure significantly, mostly at the connection point(s). Plus, with so many of the newly manufactured test instruments (some ever "quality brands"), made in China, I have found that many do not hold up as well as my older test equipment. Do you really want to add a potential problem for very little practical gain?

Les
Les Bengtson

It sounds a bit lame but I realised my mistake in elementary physics as soon as I got to bed. Lost some sleep so I'll take that as the punishment.
Richard Coombs

With brakes there is only limited movement of the pedal to displace fluid, the more of that taken up by compressing air, the less there is available to push the pistons out. With an engine there is unlimited oil displacement available - within the capacity of the sump - so the air will always be compressed to the same pressure as the oil.
PaulH Solihull

This thread was discussed between 17/10/2011 and 19/10/2011

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