Why do they put glycerin in gauges?
Look at these gauges. See the bubbles in the glass? That's glycerin. What's it doing in there?
These type of gauges are designed for use where shock or pulsation cause their premature failure. Filling them with glycerin dampens the tube and pointer movement. The fill also lubricates and protects the movement and prolongs the life of the gauge.
So, if one of your gauges loses its glycerin fill, don't worry so much. It will still read pressure but the needle will be flickering and eventually? It will fall right off. So have a replacement on hand if you find a dry gauge.