A method of installing a probe for monitoring a surface level of a fluid in a vessel inside the vessel from outside the vessel, the method includes providing a vessel side wall with a first opening, providing the vessel side wall with a second opening, passing the second end of the probe, a second device for fastening the probe and a detection unit of the probe through the first opening and inside the vessel, orienting the second end of the probe towards the second opening, attaching the second end of the probe to the side wall of the vessel at the second opening, and fastening the first end of the probe to the side wall of the vessel at the first opening.