As per ASME B 40.1, pressure gauges case is defined as the housing or container that supports, protects, and surrounds the internals. Its not consider as the wetted part of the pressure gauges. It just protects the wetted part of the gauges.
The choices are cast iron, aluminum, and 316 SS or phenolic. When the gauges are used in the corrosive area such as for offshore application the case is preferred to be 316 SS.
When the weight of the gauges is a big concern then we could use aluminum case. When the only consideration is ruggedness then case iron can be a best choice.