Vaporstat question

Ran into a vaporstat the other day. Does anyone know what the main and diff should be on these things? I know there are two different models, one in psi and one in ounces. Either way, I know that operated at a lower pressure than a pressuretol, just not sure how much lower. thanks

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If this is for a low pressure vapor steam system I believe they ran at 1LB or less - we could be talking about different things as there are not a lot of low pressure vapor steam boilers still set up out there.