or at an output of the attenuator the frequency fp of this output signal which is the resonance frequency of the beam is measured and it is this measurement that will be used for the calculation of the pressure the frequency measurement is preferably carried out by a square wave rising or falling edge counting method in particular the number of periods of the signal at the resonance frequency fp can be counted over a known fixed duration tc a frequency fp can be directly deduced from this if the value of tc is precisely known however the preferred counting method is a double counting method which is explained with reference to fig5 and 6 and which comprise the following operations first of all the signal y t at the resonance frequency coming from the closed loop control is transformed into a square wave signal whose rising and falling edges coincide with the zero crossings of the signal fig5 this operation is conventional and uses for example a circuit known as a u201c schmitt trigger u201d which detects the passages through zero with a switching threshold defined in such a manner as to avoid spurious switching events in the presence of noise under nominal operation this circuit produces a square wave signal at the resonance frequency fp subsequently a signal is established at a relatively low frequency fc for example.