[folivier]

Looking at the shape of the meniscus it is probably a glycerin or silicone filled gauge. These are better since they keep contamination (lime scaling) out of the gauge innards. Any gauge with the same fitting, probably 1/4" MNPT would work. But before you buy one pull this one out and make sure the tiny orifice is not scaled over. If it is just clean it off and try it again. Usually when they go bad you'll see some corrosion inside the gauge or the needle will fall off.