Whatsis?

Okay, a disclaimer: technically, this isn't Plane Pr0n, it's Helicopter Pr0n, but it *is* an exposé of some of the fiddly bits.

Y'see, my collection differs from John's in that my items are either obsolete, non-functional (and uneconomically repairable), and/or of "no further military value"--and (except for the weaponry) they all are souvenirs of odd things that occurred in the course of one of my flights.

In a couple of instances, they were the *cause* of the odd things that occurred, hence, the non-functional aspect.

This thing is still functional.

Yeah--a water gauge. It's from the instrument panel of one of the (several--heh) types and models of rotary-winged beasties I used to hurl myself through the heavens in and it ain't PhotoShopped. If you're suspicious, check out the hi-rez here. Sorry about the quality--my camera-holding paw is still a bit shaky.

The gauge is a tad over two inches (51.2mm) in diameter across the face and exactly two inches (50.8mm) across the body. Note the slippage mark at 3 o'clock--that tells you the operating ranges are externally applied. The shiny stuff on the face of the gauge is Scotch (don't forget--tomorrow's "Wear A Kilt To Work" Day) tape.

Here's the two-part question:

1. What helicopter used a water gauge on its instrument panel and

2. Why?

Heh. Guess away. I won't be around until later tonight, so you'll have plenty of time to research in between ironing the pleats of your kilts...
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Judging from the comments, it's time for a hint.

As we all know, oil and water don't mix.