Compensation of rate variation in a watch

A method for compensating the rate as a function of the temperature of a water-resistant watch (1), wherein the water-resistant case (2) contains a movement (3) including an oscillator (4), said case (2) containing, on leaving the factory after the initial rate setting, an internal volume V occupied by n moles of a gas of constant R substantially observing the ideal gas law, wherein the pressure coefficient Cp of said movement (3) is determined in the factory by measurement and/or calculation, defining the relatively linear variation of the rate of said movement (3) as a function of the pressure P of said gas, wherein a value of the humidity coefficient Ch of said movement (3) is determined in the factory after measurement and/or calculation, defining the maximum relatively linear variation of the rate of said movement (3) as a function of the humidity H in said movement (3), wherein an optimal value Cto of the thermal coefficient Ct of said oscillator (4) is calculated defining the relatively linear variation of the rate of said oscillation (4) as a function of the temperature T, said optimal value Cto being intended to compensate the pressure and humidity deviations according to the formula:

• Cto=−[Cp*(n*R)/V]−[(Ch*H)/T],
• and wherein,
• either for an after-sales application or when casing up in the factory, said watch (1) is equipped with compensation means (10) arranged to vary, inside said case (2), the pressure P and/or the nature of the gas and its constant R and/or the quantity of gas and its number of moles n and/or the temperature T,
• or for a preparation in the factory, the thermal coefficient of the elastic return means included in said oscillator (4) is modified by modifying an oxide layer thickness and/or applying a coating and/or by local ablation, and/or the number of moles of gas in said watch and/or the nature of the gas in the watch are modified, and/or the internal volume of said case (2) is modified.