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:
- 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.