SNIP

This in essense is what a sight gage does = a

fuel

manometer measuring fuel pressure.

 Vance

Bill Freeman wrote:

Vance,

I don't see this at all.  A manometer measures a

pressure

due to the different heights of the two ends of a

column of

liquid, where the pressure difference is

proportional to the

column height and liquid density.  A sight gage

has no

pressure difference.  It just telegraphs the

actual liquid height

to a more convenient location.  A sight gage has

no delta

press, a manometer has a delta pressure.

A sight gage is exactly equivalent to looking at a

transparent

sided fuel tank, just more convenient - plus it

can be remotely

located.

I do agree that it seems incredible what

inaccurate and unreliable

fuel gages have been put in A/C.  It *is* more

difficult than

a car due to the very poor shape of A/C tanks

(usually), which

might excuse some of the inaccuracy, but not the

unreliability.

I trust my Long EZ sight gages totally.  When it

says you have

4 gallons left you have exactly 4 gallons that

will flow to the

carb in level flight, since I calibrated it that

way.  The sumps

do a good job of covering momentary slips and

skids.

Bill
Long EZ builder & pilot
mechanical engineer

You have to look at it as a classic hydraulic problem,
the height in a remote sight gage is supported by the
pressure from the tank. if you put a non imiscible
lighter wt fluid on the top of your sight gage it will
read higher. also if you vent the sight gage to
atmosphere instead of the tank - pressure or lack of
pressure from the tank vent will change that level in
error. You can use a higher density fluid to compress
the scale, but you would need a flexible diaphragm at
the interface. Does this help it look like a
manometer?
Damping can be provided by a restriction, or
electrically if a transducer is used. - Vance

I think it is a sub class of a manometer in that the pressure in 
the top of the tank is always the same pressure as in the top of the U-tube
there fore it will always read the proper fuel level.

Usually, in the classic manometer the top of the U tube goes to 
an independent reference pressure. The intention of course is to
determine the pressure of the unknown source compared to the
reference pressure.

IMHO it is stretching things to call it a manometer Vance :-)

Paul Lamar
 
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