surfaces to close gaps due to surface finish or geometry irregularities the spring 169 provides this extra force and introduces a bias in the sense that some pressure is required to open it that pressure limits the range of regulation when used for backpressure control referring next to fig3 a through 3c valve 160 will be particularly discussed in conjunction with its placement in the cathode flowpath 140 of the system 100 of fig2 although it will be appreciated that the same applies mutatis mutandis to the anode flowpath 130 as well valve 160 protects the cathode side of the stack 120 from air intrusion during times when stack 120 is not operational correct operation of the valve 160 is especially important in situations where there is either no backpressure valve or one that does not seal well thus for example in situations where either a backpressure valve was designed with clearance which allows oxygen to diffuse into the stack 120 or those using an expander system both tend to permit a significant amount of diffusion so it is up to the isolation valve to be the diffusion barrier significantly the localized connection surface 170 made up of the tab regions 167 a of valve plate 168 can be u2014 upon application of load l u2014 flexed against a rigid member that is part of the body 161 placed behind the flexing valve plate 168 such that a moment m is induced in the tab regions 167 a this has the effect of focusing the pneumatic force generated by the opening pressure of the reactant bearing fluid that flows along flow direction f on the tab regions 167 a and apertures 168 b rather than.