Regarding some vintage gauges.

[marianoberna]

I have always noticed, that on many old (think 30 yrs and more) a/c, some or all of the pressure based gauges (VSI, Altimeter, ASI, or Manifold Pressure), have this peculiar withish/fogged look, whereas other instruments look normal.

Never understood why! If i were to make a guess, i'd say that pressure changes over time volatilize lubricants inside the instrument case and they get deposited everywhere (including the inner side of the glass/plastic cover).

Anyone got any idea why this happens?

PS: i include an image that i found on the internet, very noticeable on the ASI and somewhat noticeable on the VSI but i have seen this on many airplanes personally.

https://c2.staticflickr.com/8/7184/6845522442_6151888d3a_z.jpg

Anyone got an idea why?

[Dont Hang Up]

However interestingly not noticeable on the altimeter?

Could it just be that older instruments get fogged and the pressure driven ones tend to live forever while most of the electrically driven ones will get replaced at some point?

That's just as much a guess as your idea.

[marianoberna]

https://fbcdn-sphotos-e-a.akamaihd.net/hphotos-ak-xfa1/v/t1.0-9/10991112_825976807470273_6151573161704527496_n.jpg?oh=d02c9e 78fb7c83377bf7c06a03546214&oe=55958EDA&__gda__=1430899436_720699cb7eecfe1b0a502ef00904cca0

Here's another example. I asked this question to many people, and so far nothing more than guesses :D

Another explanation i heard was that maybe the pressurization/depresurization cycles on the instruments might be causing microscopic fatigue cracks on the glass/plastic (similar to how cold bent acrilic becomes hazy)

[Miserlou]

Second photo looks like you were wearing a striped top. Reflection.

In both cases looks like light pollution.

[skyhighfallguy]

Its from the instructor tapping on the airspeed or altimeter to get the student to fly to more closer tolerance.