from a material such as aluminum or stainless steel that can withstand chemical cleaning; a main unit 92, which is attached to the base plate 91 using, for example, attachment screws, so as to be removable, and that maintains an airtight seal with an inner space 96, where there is an O-ring 95 (seal member) between the main unit 92 and the base 91; a pressure sensor 93 that detects the air pressu