Tension meter (82) for measuring a mechanical tension (F1) along a longitudinal direction (L) between a first element (84) and a second element (80) deployed in a well containing a fluid (50) having a fluid pressure, the tension meter comprising: - a bar (108) comprising a first portion (152), a second portion (154), and a measurement portion (156), and - a hollow member (110) defining a first chamber (169) surrounding the measurement portion, the bar being free to expand within the hollow member under the mechanical tension (F1 ) to be measured. The measurement portion includes at least one strain gauge (172). The tension meter includes first sealing elements for keeping the first chamber at a first chamber pressure, the measurement portion being subject to a compression force (F2) due to a difference between the fluid pressure and the first chamber pressure. The tension meter includes means for converting the fluid pressure into a traction force (F3) applied on the second portion (154), wherein the compression force (F2) and the traction force (F3) compensate. Related subassembly and method.