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Sometimes on analog flight instruments (aka steam gauges) there is a haze or fogginess on the INSIDE of the glass that is NOT condensation. You can often find it on older aircraft with older instruments. But the instruments don't all develop the haze. I've spent some time trying to determine what this is and I found a few explanations, but only one of them addresses haze/fogginess inside the glass. This is one explanation I have found:

  1. On instruments connected to pitot static system, it is the result of small air leaks which allows air to flow into the instrument and behind the glass, overtime it causes glass to get hazy on inside due to particulates in the vacuum air.

I find this to be credible but would like confirmation.

Additionally, I've noticed similar haziness/fogginess on non-pitot static instruments like Manifold Pressure, Tachometer, and others. So, there is likely another explanation for this.

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This can depend on what the actual instrument is and what (if anything) the glass is coated in.

  1. In some cases the instrument may not have a glass panel at all, it may be acrylic or any other type of similar material. You are mot likely seeing sun damage in this case.
  2. It may be mold or fungus. Camera lenses suffer from a similar issue. If the aircraft resides in a humid environment, maybe even trapped in a damp hanger you can get build up. Since instruments get rebuilt sometimes (some more often than others) they are opened up and resealed quite a bit in their life times. This may not always happen in a perfect environment and even a small amount of fungus can quickly damage something.
  3. It could be interior lubricants (oil) that are thrown around the instrument as it spins at high speed.
  4. Some of the older units are not sealed, and for that matter some may even be vented to the atmosphere as is suggested here.
  5. It could be dust drawn in through the vacuum system that the filter failed to draw out.

Vacuum systems are very common for driving gyro instruments. In a vacuum system, a stream of air directed against the rotor vanes turns the rotor at high speed. The action is similar to a water wheel. Air at atmospheric pressure is first drawn through a filter(s).

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A lot of materials give off 'gas' (for want of a better description) and these can settle on the interior surfaces. This process may be accelerated by heat which is generated by the unit itself or by the environment the unit is installed in. Oil-based materials are prone to this including plastics. This is also what creates that new car smell.

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