Sunbeamlotus

I had a broken pressure gauge before that displayed 120psi all the time. I bought a cheap one (about £8) from china to replace it. When i installed it the pressure displays 0 PSI when the car is idle. When it moves it goes to about 20. I can only assume the pressure is fine and it's because I bought a cheap version..? Has anyone else had this issue? I don't know if it's worth purchasing the proper sensor that's about £80