O ring, sealing, Tri Gauge, Qty: 2
Is your Tri-Gauge misting up?
It's probably these guys. Release the three small Philips screws at the rear of the instrument and carefully withdraw the inner assembly three quarters of an inch or so, to reveal these two identical O rings.
(You'll be digging that red sealing stuff out of the depths of those tiny Philips slot heads for a while, by the way.)
Why do instrument service people put red goop over the screws they have just assembled ? I expect it's what they are taught to do...
It reminds me of recently departed British TV Personality Sir David Frost (1939-2013), who much earlier, starting his career as David Frost, TV Comedian, described being very annoyed by the State Run Gas Board's energy bill that would come in the mail, huge areas of which bearing the officious words 'do not write in this space'.
Which prompted him to get a wax candle and painstakingly smear those areas of the bill before mailing it back to them, so the Gas Board couldn't write there either.