Sunday evening, after installing the replacement vacuum gauge (replacement for the one I dropped) on the carburetor balancing fixture, decided to roll the RV-12 out of the hangar and warm up the engine so the carburetors could be balanced.
Prior to mounting the fixture on the piano hinge for the upper cowl, I
placed the balancing fixture inside the airplane and ran the vacuum lines through
the air vent on the side of the fuselage. The reason for doing this for the
first start is … should the needles on the gauges bounce around, the valves in-line
with both gauges can be adjusted as necessary to dampen out the gauges.
The plan was to run the engine until it had warmed up and, if necessary,
dampen the gauges. Next take stock of the readings and then shutdown and move the fixture outside and hang it on the cowl hinge then make the necessary first adjustment. Much to my surprise, after
starting the engine an initial glance at the gauges showed 0 vacuum on both
gauges. What???? That could not possibly be right. Upon taking a closer look at
the gauges, one of them was not quite on 0 and I could tell the gauges were
being subjected to far more vacuum than the gauges could handle. The needles on
the gauges had gone all the way around the face of the gauge and pegging on the
I blame the DOG Aviation procurement department for this oversight … obviously
the wrong type of vacuum gauge was ordered. The gauges that were ordered were
0-30 inches of water … OOPS!!! The gauges should have been 0-30 inches of
mercury, AKA … in Hg. Duh!!
This is the aftermath resulting from using the wrong type of vacuum
gauge. The ones shown here are inches of water and they should have been inches
of mercury or in Hg. There was so much vacuum that it internally deformed the internal
diaphragm in both gauges.
I don’t know how I let that get by me ... but the end result is I have
destroyed both the gauges. Internally, both gauges are severely tweaked and now
read over 5 inches of water without any vacuum being applied. The DOG Aviation
procurement department has two 0-30 in Hg gauges on the way and they should be
here later in the week.