Help me understand my Turbo Guage

[prizmatik]

This Greddy Turbo Guage came with my car when I purchased it. It is not in PSI, but I have been able to convert it to understand it in PSI. I am currently boosting about 15psi, which is just past the 1.0.
My question is, while driving, the needle adjusts appropriately, but when I am barely on the gas crusing, the needle is below the 0 mark. According, to my calculation, that is negative boost. does this make any sense?
http://img.photobucket.com/albums/v81/Prizmatic/b75474f1.jpg

[blacka4]

yes negative boost is vacuum. At idle, the car will be pulling a vacuum and not producing boost. which is normal.

and that gauge is in Bar. 1 bar = 1 atmospheric pressure = 14.7 PSI. But you said you figured that out already.

[prizmatik]

Thanks for the schoolin. I am curious how that works. How is vacuum created in the intake? Is the turbo not spinning much at that low RPM?

[offroader1006]

The throttle blocks off incoming air, yet the cylinders are still pulling in more, therefore you get a vacuum.

[prizmatik]

I was thinking about this a little more today.
I'm envisioning, the turbo turbine not having enough force from the exhaust to spin it at very low RPM's, so the air in the intake is just coasting by, causing the vacuum. Does that sound logical?

[sean1.8t]

turbo's don't produce much air until there's load on them and they can build pressure.

the engine itself is pulling in more air than the turbo is producing at idle(or any RPM where there is no load on the engine), hence the vacuum.

[prizmatik]

Yeah, that make a lot of sense. I never had a boost guage in any of my A4's previously. Now that I do, I watch the characteristics of the turbo in action, and have learned about it. Thanks for the input to all of you, I really appreciate it.