Abstract: Methods and systems for employing a sensor to detect disconnection of a liquid supply tank and/or a low fill level of the liquid supply tank are disclosed herein. The sensor may have an optical sensor to detect liquid, e.g., to detect a liquid level in a removable liquid supply tank and/or liquid at a pump. The optical sensor can include a body made of a resilient material and including first and second portions and an opening located between the first and second portions. The opening can be configured to receive and frictionally engage with a conduit for carrying liquid, with the first portion configured to support a light emitter and the second portion configured to support a light detector.