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Can anybody tell me what is the difference between blow out disc & blow back disc in pressure gauges?

Arch,
The blow out disk is often like a rubber stopper in the top, you often see this in a gauge with a SS case whereas a gauge in plastic (phenolic or fiberglass reinforced plastic) often has a metal back that will blow out. Both cases are opposed to the old fashion gauges where the glass was likely to burst out if the bourdon tube developed a leak.

In 40 years I have never seen a gauge explode but there's always a first time. Most modern gauges have two compartments the front for the scale and pointer separated from the back by a solid wall (solid front). the back has the movement. You will often see something like "Solid Front, Blowout Back"

Regards
Roy