Tension meter (82; 282) for measuring a mechanical tension along a longitudinal direction between a first element and a second element deployed in a well containing a fluid having a fluid pressure, the tension meter (82; 282) comprising: - a first part configured to be attached to the first element, - a second part configured to be attached to the second element, - a bar extending longitudinally between the first part and the second part, the bar comprising a first portion mechanically connected with the first part, a second portion mechanically connected with the second part, and a measurement portion extending longitudinally between the first portion and the second portion, and - a hollow member configured for receiving at least the measurement portion, the hollow member and the measurement portion defining a first chamber surrounding the measurement portion, the second portion being outside the first chamber and the bar being free to expand within the hollow member under the mechanical tension to be measured, characterized in that: - the measurement portion includes at least one strain gauge, - the tension meter (82; 282) includes first sealing elements (174, 176, 178, 191) configured for keeping the first chamber at a first chamber pressure, the measurement portion configured to for a compression force due to a difference between the fluid pressure and the first chamber pressure and - the tension meter (82; 282) includes means (182, 204, 206) configured for converting the fluid pressure into a traction force applied on the second portion, wherein the compression force and the traction force compensate. - a first part configured to be attached to the first element, - a second part configured to be attached to the second element, - a bar extending longitudinally between the first part and the second part, the bar comprising a first portion mechanically connected with the first part, a second portion mechanically connected with the second part, and a measurement portion extending longitudinally between the first portion and the second portion, and - a hollow member configured for receiving at least the measurement portion, the hollow member and the measurement portion defining a first chamber surrounding the measurement portion, the second portion being outside the first chamber and the bar being free to expand within the hollow member under the mechanical tension to be measured, characterized in that: - the measurement portion includes at least one strain gauge, - the tension meter (82; 282) includes first sealing elements (174, 176, 178, 191) configured for keeping the first chamber at a first chamber pressure, the measurement portion configured to for a compression force due to a difference between the fluid pressure and the first chamber pressure and - the tension meter (82; 282) includes means (182, 204, 206) configured for converting the fluid pressure into a traction force applied on the second portion, wherein the compression force and the traction force compensate. - the measurement portion includes at least one strain gauge, - the tension meter (82; 282) includes first sealing elements (174, 176, 178, 191) configured for keeping the first chamber at a first chamber pressure, the measurement portion configured to for a compression force due to a difference between the fluid pressure and the first chamber pressure and - the tension meter (82; 282) includes means (182, 204, 206) configured for converting the fluid pressure into a traction force applied on the second portion, wherein the compression force and the traction force compensate.