WingFlaps  wrote

>On Jan 21, 2:55.am, 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. 

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.

>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.

The liquid fill is a clever solution :) I have actually plugged the
hole, having first put the transducer in with a load of silica gel for
a few weeks to dry out the air inside.

The industrial solution to this problem is a nipple on the vent hole,
enabling one to run a tube to somewhere where the air is not wet. Such
gauges are quite expensive though.

>I am not a qualified aircraft mechanic so any of the above
>modifications must be evaluated by you and applied at your own risk.

Sure.
--- SBBSecho 2.12-Win32