Static line testing question

[Cfrisella]

Is it normal to see an increase in airspeed when testing the static system with a slight vacuum? I tested the pitot side and that is leak free and operating fine. When I pulled a vacuum on the static side both my GRT and Vans ASI show a speed increase. My first thought is a calibration task not yet performed or the system needs a positive pressure from the pitot, which it's not getting. Or I just hooked something up wrong. Most likely reason.

[Mark Burns]

The airspeed indicator is really just a differiential pressure guage.
You changed the static pressure, so there was a difference between static and pitot!

Mark

[Cfrisella]

I didn't think it was hooked up wrong. I've surprised myself before though. Thanks!

[paul330]

IAS is a function of dynamic pressure (P). When standing still, the pitot senses static pressure (S). When moving, the pitot senses P+S. Hence the need to subtratc static pressure from pitot pressure to get an accurate reading. The upshot is that any static errors will result in an IAS error.