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Why are gauge type instruments filled with glycerine ?

Gauge type instruments are filled with glycerine.  Glycerine serves three functions

It acts as a vibration damper.  pressure guage glycerine

Gauge type instruments can be fitted on machines and pipes where there can be high vibration.  These vibrations can cause the intricate components in the gauge to fall apart. Glycerine, being a highly viscous liquid, serves to dampen these vibration.

It acts as coolant

It also prevents the external heat from reaching the instruments and affecting their accuracy.

It acts as a lubricant

Glycerine is a good lubricant.  The gauge instruments have a number of mechanical parts such as the rack and pinion, springs and pointer.  These need lubrication to function properly.