Thats fine but why replace it, when you have the boiler test the inspector should mark Max working pressure on it you could scale it down from that ?

Terry,

Have you connected the gauge directly to a reference or are you connecting it through the siphon? Due to the small bore of the siphon it us not uncommon for them to become blocked with scale and hence give a false reading! The age of a guage shouldn't be detrimental it's the hours in steam that will wear it out! If it's been sat 20 years doing nothing it's not deteriorating if it's been dry stored?

You can get a replacement from various sources but worthy of note its best to get one with a pre marked red line on the dial as if you open up an unmarked one to mark it the supplier usually says your warranty is void! Miniature Pressure Guage Supply Co offer pre marked gauges of various sizes and styles. (No connection but have purchased from them in the past).

Paul.