A water pressure gauge can be screwed onto a hose bibb or water heater drain valve to measure line pressure and to check for thermal expansion issues. Normal pressure is anything under 80 psi. In closed systems, there are times when pressure can spike far beyond that and necessitate the installation of an expansion tank.
There are several variants of the gauge. One has a red needle that will register the highest pressure reached while it's left in a given spot and that can be useful sometimes.
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