Sensor and seal for sensor

An electrochemical gas sensor comprising a housing, the housing containing a plurality of electrodes and a liquid electrolyte, the housing having a gas inlet for receiving analyte gas, the sensor defining at least one port through the housing having a metal interface pin therein, the interface pin extending through the port from the exterior of the housing, being electrically connected to at least one of the plurality of electrodes in the interior of the housing, the sensor additionally comprising a seal located around the metal interface pin within the port, and compressed between a wall of the port and the metal interface pin thereby restricting egress of liquid electrolyte from the housing, wherein the seal comprises an annular seal which in its uncompressed form either presents a cross-section which is concave in a radially extending direction of the seal, or whose width is constant over an extended axial direction of the seal, wherein the seal functions as a valve, allowing the controlled egress of fluid/liquid when there is a sufficient pressure build up within the sensor.