I got this gauge with others last year at an estate sale. Have sold all but this one. None have been returned so I assume they have all worked. I do not know how to test it so if it does not function I will refund money. All I can tell you is what is on the back of gauge. Indicator pressure multi purpose general aero products pn 86425-4.It must have been military as it has U.S. property. Please look at all the photos and email with any questions. Thanks for looking.