Should oil in pressure line for gauge go all the way to the gauge?

In general, true. However I recall hearing for the first time,
from my first semester thermodynamics professor (Dr.
Izawa), that some liquids can be relatively compressible,
namely both ethane and carbon dioxide.


"Mike Hunter" <Mikehunt2@lycos,com> wrote in message
...
> Basic physics will solve your dilemma. Gases compress, liquids do not.
>
> "hls" > wrote in message
> ...
> >
> > "Tegger" > wrote in message
> >>
> >>
> >> Well, I never took physics, so this is all somewhat Greek-ish to me.
> >>
> >> If the air in the line were to expand due to heat, wouldn't it simply
> >> push
> >> back on the column of oil being pushed up from the engine?
> >>
> >> I can understand that pressure would rise with heat-induced gas
expansion
> >> in a sealed system, but the oil pressure gauge isn't a sealed system,
is
> >> it?
> >>
> >>
> >> --
> >> Tegger
> >
> > And the answer is yes. The unusual pressure due to the air would push
> > backwards
> > against the oil pressure. Here is where it becomes dicey.. The amount
of
> > air in
> > the line is small. So even if it expands, its expansion is limited by
its
> > volume,
> > or weight, as a part of the system.
> >
> > The differences are usually much smaller than the case I related to you.

Heron McKeister wrote:
> In general, true. However I recall hearing for the first time,
> from my first semester thermodynamics professor (Dr.
> Izawa), that some liquids can be relatively compressible,

Liquid helium is compressable, too, IIRC. You need to get down to around
4 K (about -255 C or -450 F) to get liquid helium.

Jeff


> "Mike Hunter" <Mikehunt2@lycos,com> wrote in message
> ...
>> Basic physics will solve your dilemma. Gases compress, liquids do not.
>>
>> "hls" > wrote in message
>> ...
>>> "Tegger" > wrote in message
>>>>
>>>> Well, I never took physics, so this is all somewhat Greek-ish to me.
>>>>
>>>> If the air in the line were to expand due to heat, wouldn't it simply
>>>> push
>>>> back on the column of oil being pushed up from the engine?
>>>>
>>>> I can understand that pressure would rise with heat-induced gas
> expansion
>>>> in a sealed system, but the oil pressure gauge isn't a sealed system,
> is
>>>> it?
>>>>
>>>>
>>>> --
>>>> Tegger
>>> And the answer is yes. The unusual pressure due to the air would push
>>> backwards
>>> against the oil pressure. Here is where it becomes dicey.. The amount
> of
>>> air in
>>> the line is small. So even if it expands, its expansion is limited by
> its
>>> volume,
>>> or weight, as a part of the system.
>>>
>>> The differences are usually much smaller than the case I related to you.
>
>

Yes, liquid helium exhibits some other very non-intuitve
behaviors.as well. Despite how it may have come across
though, I hadn't actually meant the examples I mentioned
as being a comprehensive listing ;^)


"dr_jeff" > wrote in message
...
> Heron McKeister wrote:
> > In general, true. However I recall hearing for the first time,
> > from my first semester thermodynamics professor (Dr.
> > Izawa), that some liquids can be relatively compressible,
>
> Liquid helium is compressable, too, IIRC. You need to get down to around
> 4 K (about -255 C or -450 F) to get liquid helium.
>
> Jeff
>
>
> > "Mike Hunter" <Mikehunt2@lycos,com> wrote in message
> > ...
> >> Basic physics will solve your dilemma. Gases compress, liquids do
not.
> >>
> >> "hls" > wrote in message
> >> ...
> >>> "Tegger" > wrote in message
> >>>>
> >>>> Well, I never took physics, so this is all somewhat Greek-ish to me.
> >>>>
> >>>> If the air in the line were to expand due to heat, wouldn't it simply
> >>>> push
> >>>> back on the column of oil being pushed up from the engine?
> >>>>
> >>>> I can understand that pressure would rise with heat-induced gas
> > expansion
> >>>> in a sealed system, but the oil pressure gauge isn't a sealed system,
> > is
> >>>> it?
> >>>>
> >>>>
> >>>> --
> >>>> Tegger
> >>> And the answer is yes. The unusual pressure due to the air would push
> >>> backwards
> >>> against the oil pressure. Here is where it becomes dicey.. The amount
> > of
> >>> air in
> >>> the line is small. So even if it expands, its expansion is limited by
> > its
> >>> volume,
> >>> or weight, as a part of the system.
> >>>
> >>> The differences are usually much smaller than the case I related to
you.

"dr_jeff" > wrote in message
>
> Liquid helium is compressable, too, IIRC. You need to get down to around
> 4 K (about -255 C or -450 F) to get liquid helium.
>
> Jeff

It is a matter of degree. Liquids have very very low compressabilities.
You would not be able to measure the compressability of motor oil
on a simple automotive oil gauge.

Put it under several hundred or several thousands of atmospheres, then
you could measure it with a very sensitive instrument.

It is not even a factor in this general discussion.

"hls" > wrote in message
...

http://www.engineersedge.com/lubrica...ulic_fluid.htm.

Here is a link with some approximate information.

How much helium do YOU think will be in the line to his oil gauge, dr_jeff?


"dr_jeff" > wrote in message
...
> Heron McKeister wrote:
>> In general, true. However I recall hearing for the first time,
>> from my first semester thermodynamics professor (Dr.
>> Izawa), that some liquids can be relatively compressible,
>
> Liquid helium is compressable, too, IIRC. You need to get down to around 4
> K (about -255 C or -450 F) to get liquid helium.
>
> Jeff
>
>
>> "Mike Hunter" <Mikehunt2@lycos,com> wrote in message
>> ...
>>> Basic physics will solve your dilemma. Gases compress, liquids do not.
>>>
>>> "hls" > wrote in message
>>> ...
>>>> "Tegger" > wrote in message
>>>>>
>>>>> Well, I never took physics, so this is all somewhat Greek-ish to me.
>>>>>
>>>>> If the air in the line were to expand due to heat, wouldn't it simply
>>>>> push
>>>>> back on the column of oil being pushed up from the engine?
>>>>>
>>>>> I can understand that pressure would rise with heat-induced gas
>> expansion
>>>>> in a sealed system, but the oil pressure gauge isn't a sealed system,
>> is
>>>>> it?
>>>>>
>>>>>
>>>>> --
>>>>> Tegger
>>>> And the answer is yes. The unusual pressure due to the air would push
>>>> backwards
>>>> against the oil pressure. Here is where it becomes dicey.. The amount
>> of
>>>> air in
>>>> the line is small. So even if it expands, its expansion is limited by
>> its
>>>> volume,
>>>> or weight, as a part of the system.
>>>>
>>>> The differences are usually much smaller than the case I related to
>>>> you.
>>

C. E. White wrote:
>
> "muzician21" > wrote in message
> ...
>> Hooking up an oil pressure gauge, should the oil go all the way to the
>> gauge or should there be an air buffer?
>>
>> Thanks
>
> It shouldn't matter.If there is air in the line, it will be compressed
> so that the "air" pressure is the same as the oil pressure.
>
> I don't really like gauges that require running oil lines to the dash
> -especially the sort sold in most mass market auto parts stores. I
> prefer good quality electical gauges.
>
> Ed


My experience is that electric gauges are awful. Very inaccurate, though
not as completely useless as factory gauges. Even fairly cheap
mechanical gauges are much more accurate.

Go to a high-performance shop and get COPPER capillary tubing if you're
worried about the hard nylon tubing that comes with gauges. Although
I've never had the hard nylon stuff fail either- but you do have to
route it carefully to avoid abrasion or getting it against something too
hot.

hls wrote:
>
> "dr_jeff" > wrote in message
>> Why would having the oil go all the way up to the gauge be a big deal?
>> Would there always be oil in the line?
>
> It wouldnt normally make much difference.

That's the correct answer. It doesn't really make much difference.
Having a little air in the line does help smooth out pulsations in oil
pressure (from the pump, from the crank and camshafts alternately
covering and uncovering various passages.) Air in the line does not
decrease accuracy.

Tegger wrote:
> "hls" > wrote in
> :
>
>> "dr_jeff" > wrote in message
>>> Why would having the oil go all the way up to the gauge be a big
>>> deal? Would there always be oil in the line?
>> It wouldnt normally make much difference. If the line, partially
>> filled with air,
>> experienced heating or cooling, I guess the expansion or contraction
>> of the air could be more of a factor than it would be with a tube
>> filled with oil alone.
>>
>
>
> Air is compressible. Oil is not. Air in the line will result in a falsely
> low gauge reading.

Ummm... no.

The air compresses to the exact same pressure as the oil in the engine
and the gauge reads correctly.

"Steve" > wrote in message
...
> C. E. White wrote:
>>
>> "muzician21" > wrote in message
>> ...
>>> Hooking up an oil pressure gauge, should the oil go all the way to the
>>> gauge or should there be an air buffer?
>>>
>>> Thanks
>>
>> It shouldn't matter.If there is air in the line, it will be compressed so
>> that the "air" pressure is the same as the oil pressure.
>>
>> I don't really like gauges that require running oil lines to the
>> dash -especially the sort sold in most mass market auto parts stores. I
>> prefer good quality electical gauges.
>>
>> Ed
>
>
> My experience is that electric gauges are awful. Very inaccurate, though
> not as completely useless as factory gauges. Even fairly cheap mechanical
> gauges are much more accurate.
>
> Go to a high-performance shop and get COPPER capillary tubing if you're
> worried about the hard nylon tubing that comes with gauges. Although I've
> never had the hard nylon stuff fail either- but you do have to route it
> carefully to avoid abrasion or getting it against something too hot.

Double ditto...I have seen MANY times that people route these nylon lines
too close to a heat source and they eventually melt or decompose. If you
do it properly, nylon is a good material, but this is no place to get
sloppy.