Abstract: The present invention relates to Ink Level Detection in a Pressure Regulated Pen. A liquid level detector for a liquid containment device, such as an ink-jet pen, provides a signal indicative of a lowering level of liquid in the containment device. In exemplary embodiments, a refillable or replaceable ink reservoir supplies ink to the printhead of the pen under pressure. Electrical or electromagnetic devices are used individually or in combination with a pressure regulator apparatus to sense a back pressure in the containment device after the reservoir has gone dry to trigger a signal indicating a low or out-of-ink condition.