Originally Posted by
rans6andrew
It seems that a little water pooled in the pitot pipes can give sufficient pressure due to gravity to cause a significant reading on the ASI.
10 meters of water equals one bar of atmospheric pressure. So 1 (vertical) cm of water equals 1 mb, equals 27 feet of altitude. It doesn't take much for the altimeter and ASI to be off.
Apart from that, you did the right thing by first taking the pipes off the ASI (and I assume ALT and V/S as well) before starting to mess with the pitot/static ports. All pitot/static instruments are extremely sensitive and can be damaged from a person blowing into, or sucking from their pipework. Let alone if you start to use power tools.