Vacuum gauges monitor element condition, and as the filter element slowly becomes clogged with contaminates the restriction (resistance to flow) increases. At the first indication of decreased performance, note the dial reading, or apply the 'red line' decal provided with most kits. This will assist in knowing when to change the filter at the next interval. This vacuum gauge kit from Parker includes gauge and 2 fittings. 1/8" male NPT x 1/4" (#4). 1/4" swivel x 1/4" (#4) hose barb. 1/4" (#4) hose not included.