Should an oil pressure gauge read absolute pressure or should it read
what is called "gauge pressure" which involves a vent to the outside
air?
A normal bourdon gauge, in an unpressurised cockpit, will read gauge
pressure unless the casing of the instrument is sealed. An electronic transducer which is UNvented will read absolute pressure.
The error is quite small but significant: at 20,000ft the pressure is
500mb which is worth about 7psi. So, if you have an unvented
transducer, as you climb from SL to 20,000ft the pressure reading will
be 7psi lower than the actual oil pressure. This isn't much - oil
pressure varies more than this in normal operation.
I ask this because I am fitting a backup oil pressure gauge. The
transducer is vented but this introduces a well known failure mode
which is moisture ingress through the vent which corrodes the
transducer electronics... that vent hole should really be sealed,
after the air inside has been dried.
On Jan 21, 2:55.am, Peter <nos...@nospam9876.com> wrote:
Should an oil pressure gauge read absolute pressure or should it read
what is called "gauge pressure" which involves a vent to the outside
air?
A normal bourdon gauge, in an unpressurised cockpit, will read gauge
pressure unless the casing of the instrument is sealed. An electronic
transducer which is UNvented will read absolute pressure.
The error is quite small but significant: at 20,000ft the pressure is
500mb which is worth about 7psi. So, if you have an unvented
transducer, as you climb from SL to 20,000ft the pressure reading will
be 7psi lower than the actual oil pressure. This isn't much - oil
pressure varies more than this in normal operation.
I ask this because I am fitting a backup oil pressure gauge. The
transducer is vented but this introduces a well known failure mode
which is moisture ingress through the vent which corrodes the
transducer electronics... that vent hole should really be sealed,
after the air inside has been dried.
Gauge pressure is what drives the oil around the engine.
It is what
should be measured. Two solutions spring to mind: If _condensing_
water vapor is the cause of your electronic problem then maybe you
could attach a tube filled with silica gel to gradually get rid of the >humidity in the gauge? The other way of fixing it is to fill the
transducer with silicon oil with a plug and small hole to vent the
gauge. Unless you do a lot of inverted flight the oil should not be
lost while it provides protection from water.
I am not a qualified aircraft mechanic so any of the above
modifications must be evaluated by you and applied at your own risk.
WingFlaps <Moreflaps@gmail.com> wrote.
Gauge pressure is what drives the oil around the engine.
Well, yes, to the extent that if the atmospheric pressure (and thus
the crankcase air pressure) was 1000psi the gear pump would not be
able to push the oil around :)
But the oil pump is a gear pump - a constant volume device which is
going to deliver oil at a constant volume. There is a spring loaded
bypass valve which regulates the oil pressure after the oil pump and
the spring on that enforces a constant absolute oil pressure.
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