A magnetic flowmeter includes a pipe having a non-conductive liner, afield coil adjacent to the pipe and configured to generate a magnetic field across a fluid flowing through the pipe, a first electrode located in a first tunnel passing through the pipe and into the liner, and a second electrode located in a second tunnel passing through the pipe and into the liner. The electrodes are configured to sense a voltage induced by the magnetic field across the fluid flowing through the pipe. The flowmeter also includes a sealed compartment attached to the pipe and enclosing the field coil, the first electrode, or the second electrode. The flowmeter further includes a vapor sensor within the sealed compartment configured to sense relative humidity in the compartment, and an electronics compartment having transmitter circuitry connected to the field coil, the first electrode, the second electrode, and the vapor sensor.