Randall Young writes:
>Dave :
>
>OK, answer me this. What happens when a saturated vapor comes in contact
>with a (relatively) cool surface ? Answer : it condenses, giving up it's
>latent heat, and thereby heating the surface. This process will continue
>until the surface is no longer cooler than the vapor.
Fine. The vapor condenses. Then what? It stays a liquid. But liquid
is still a fliud and it will exert the same pressure on it's containment
vessel (cap tube and bourdon tube) as the gas in the bulb. And at
temperatures all the way down to the freezing point of the fliud.
Your original assesment that the gauge actually reads pressure is indeed
correct. But a thermosyphon action is not required for it to work.
>Heat pipes, as sometimes used in
>high efficiency heat exchangers, also use the phase change to transfer
heat,
>without any mechanical circulation. So do old fashioned steam radiators.
In order for a thermosyphon to work there must be fluid flow from the heat
source to the heat sink and back again. The capillary tube used in the
temp
gauge is much too small to allow sufficient fliud flow for significant heat
transfer. I have a broken temperature gauge (its from an MG but don't hold
that against me) and the ID is very small, indeed!
>I suppose the real way to answer the question is for someone to
disassemble
>a working Tgauge and check how hot the Bourdon tube is. Any volunteers ?
><g>
Actually, I would suggest that one measure the temperature of the capillary
tube along it's length and determine how much heat is transferred down it's
length. I'll bet that there is more heat transfer due to conduction in the
steel
tube than due to fluid movement. Measuring the temperature of such a small
tube would be difficult but there are tiny thermocouples that are well
suited
to the task.
>Randall
Randall, (and Fred),
Consider this example:
If you have a half full bottle of pop (soda for us easterner's) (and since
Fred
doesn't drink I won't use a beer example) in the fridge. There is a gas
pressure due to the CO2 disolved in the H2O. If you squeeze the plastic
bottle both above and below the liquid level you will feel the same amount
of
resistance in either case. This is because in a fluid filled system the
pressure
is equal at all points.
If you set the bottle on the counter all afternoon and repeat the test the
pressure will be greater but still equal throughout the system. It doesn't
have
to be gaseous, it just has to be a fluid.
I hope this is clear enough. Its the best I can do in layman's terms at
the
moment.
Whilst I am on the soap box... All this talk about bleeding the air out of
the
line to the oil pressure gauge is, well, in a word, poppycock! For the
reasons
stated above air in the system will not cause errors in the gauge reading.]
Consider a five foot oil pressure gauge line full of air with the engine
off (at
one atmosphere). Start the engine and build up oil pressure to 60 PSI
which
is four atmospheres. Since this is four atmospheres above the one we had
before we have five atmospheres of (absolute) pressure.
The air in the line now occupies 20 % of the volume as before, or one foot.
The remaining four feet of gauge line will fill up with oil. But the
pressure
will be the same thoughout and the gauge will read the correct pressure.
The only down side is that as the pressure changes up and down the air
in the system will expand and contract causing oil to flow into and out of
the
gauge line. If there is a restriction to the flow the gauge will respond
to
changes in oil pressure more slowly than it would if the air was purged
out but in the real world that is probably not an issue.
Enough! (Sorry, too much coffee this morning. Now I must go yell at the
dog and kick the kids) (Just kidding. I don't have a dog or kids)
Dave Massey
St. Louis.
whhi
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