On Jan 21, 2:55=A0am, Peter  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.

Cheers


 with oil

to a silca  f you want to keep the electronics
--- SBBSecho 2.12-Win32