I have purchased a fuel cell, and the supply pressure must not exceed 0.55 bar. My question is how can I supply the hydrogen at this pressure given that it is below atmospheric pressure.
The fuelcellstore.com website offers a pressure reducing regulator that supplies the 0.55 bar. It does not say if this is a gauge or atmospheric.
The regulator uses a fitting for silicone tubing and the site suggests using silicone tubing. The tubing they suggest using is not a vacuum rated tubing (usually wire/polyester reinforced silicone) but rather tubing with a max pressure rating of 10 psi. (Again, they fail to specify psig rather than psia).
I feel safe assuming gauge pressures for these (i.e. 0.55 barg = 1.55 bar absolute at sea level) but suggest you continue researching this since Hydrogen can be dangerous.
The rating is for gauge pressure. So as Brandon Morris mentioned; it will be 0.55 bar as read on a gauge and 1.55 bar absolute (read from vacuum). If it was below atmospheric pressure it would not be feasible. They are basically limiting the pressure because the structure of the fuel cell and its membranes can not take a large differential pressure without damage. You will just need to source a regulator that delivers this low of pressure.