Just an alternative point of view on bicycle pumps and tire pressure....
I look at the pressure gauge on most pumps like I do body weight scales; they're usually a bit off.
At home and with my own scale, it really doesn't matter how far off it is because all I'm looking for is confirmation that when I stand on the thing I can see if there has been any change since the last time.
The same thing goes with my pumps at home, they all read a bit different but I've figured out what number to look for on each gauge to know that I've gotten my tire pressure where I like it, which I can usually verify by pushing my thumb into the sidewall of my tire and then confirm as I / we ride. Now, to be fair, I pretty much use just one of the three pumps I have laying around and have forgotten how the other two read. So, when I pump up our tires I know that the gauge reads about 10% low and with that in mind I know that I'm shooting for a reading of X every time I pump up a given tire, e.g., Debbie's tires run at something around 120psi, mine a little higher and the tandems even higher still.
Yeah, it would be nice to know exactly what it is and there are some really good gauges out there that do that. However, until you figure out how to get a valve chuck off with a nice crisp "pop" you'll always be losing a few psi from whatever you saw on the gauge before you pulled the chuck off the tire. As someone else noted, every time thereafter you check again with another gauge you'll lose additional psi and, unlike a car tire that has huge volume and low psi, a bicycle tire inflated to 130psi looses a lot of that pressure in an instant.
Bottom Line: Close-enough is usually good enough.