Does the Size of a Manometer Tube Affect Pressure Measurement Accuracy?

[fishingspree2]

Hello

The teacher said that one way to measure the pressure of a gas enclosed in a room is to attach a manometer to it, like in that picture:
[PLAIN]http://img842.imageshack.us/img842/3994/manometer.jpg

By doing so, aren't we changing the volume the gas has access to, and thus changing the pressure of the gas?

Thank you very much

 

[vlado_skopsko]

Yes, there will be volume increase so decrease in pressure in the system. But there is no difference how big the diameter of the tube will be so more precise measurement you will have if the diameter of the tube is small so the increase of volume is small in respect of the volume of the system so you don't take it into account.