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.