Ink maintained at a negative pressure state is supplied to an ink-jet recording head via an ink supply mechanism constructed as a differential pressure valve having a coil spring and a movable membrane normally contacted elastically with a valve seat by the coil spring....http://www.google.com.au/patents/US7350907?utm_source=gb-gplus-sharePatent US7350907 - Ink-jet recording device and ink supply unit suitable for it
