All pH electrodes (and DO sensors) can be mounted vertically, into a vessel headplate. Electrodes and sensors can also be installed on large vessels, through a side port. For side-entry installations, the probe must be at least 15° above horizontal to insure proper operation. Remember, there is liquid inside all pH electrodes and DO sensors. This slight 15° incline is enough to prevent a bubble from lodging itself at the tip of the sensor, preventing the sensor from operating properly.