A probe for use with a measurement instrument includes a circuit connected to the probe. The probe comprises a process adaptor for mounting to a process vessel and including a through bore extending from a connector end to a process end. A tubular body is received in the process adaptor at the connector end and has an outer diameter less than an inner diameter of the process adaptor at the connector end to define a cavity therebetween. The cavity is open to the process vessel. A coupling secures the body to the process adaptor. A center conductor is coaxial with the tubular body and the process adaptor. A seal element is disposed between the tubular body and the center conductor. Process pressure in the process vessel increases compressive force between the tubular body and the seal element.