?? Used lots of liquid (glycol) filled gauges in the oil patch. The liquid lubricated the mechanism and helps settle vibration. Liquid filled gauges have a rubber plug on the top with a little nipple sticking up in the center. The top of that nipple is supposed to be cut off after installation. That is only about the top 1/32" to 1/16". This will leave a tiny vent hole so it reads accurately. These are generally a bunch more expensive than dry gauges.