Editing Space Shuttle external tank (section)

====Sensors====
[[File:Sts et ecographic.jpg|thumb|300px|The location of ECO sensors in the LH<sub>2</sub> tank]]
There are eight propellant-depletion sensors, four each for fuel and oxidizer. The fuel-depletion sensors are located in the bottom of the fuel tank. The oxidizer sensors are mounted in the orbiter liquid oxygen feed line manifold downstream of the feed line disconnect. During RS-25 thrusting, the orbiter general-purpose computers constantly compute the instantaneous mass of the vehicle due to the usage of the propellants. Normally, main engine cutoff is based on a predetermined velocity; however, if any two of the fuel or oxidizer sensors sense a dry condition, the engines will be shut down.

The locations of the liquid oxygen sensors allow the maximum amount of oxidizer to be consumed in the engines, while allowing sufficient time to shut down the engines before the oxidizer pumps [[cavitation|cavitate]] (run dry). In addition, {{convert|1100|lb|kg|abbr=on}} of liquid hydrogen are loaded over and above that required by the 6:1 oxidizer–fuel engine mixture ratio. This assures that cutoff from the depletion sensors is fuel-rich; oxidizer-rich engine shutdowns can cause burning and severe erosion of engine components, potentially leading to loss of the vehicle and crew.

Unexplained, erroneous readings from fuel depletion sensors have delayed several shuttle launch attempts, most notably [[STS-122]]. On December 18, 2007, a tanking test determined the cause of the errors to be a fault in a wiring connector, rather than a failure of the sensors themselves.<ref>
{{cite web | url = http://afp.google.com/article/ALeqM5hwoRZdYGkG1X3lZOF3x4GduJuKNQ | title = NASA eyes faulty gauge wires as source of shuttle problems | date = December 18, 2007 | publisher = AFP | url-status = dead | archive-url = https://web.archive.org/web/20080218101202/http://afp.google.com/article/ALeqM5hwoRZdYGkG1X3lZOF3x4GduJuKNQ | archive-date = February 18, 2008
}}</ref>

Four [[pressure transducer]]s located at the top of the liquid oxygen and liquid hydrogen tanks monitor the ullage pressures.

The ET also has two electrical umbilicals that carry electrical power from the orbiter to the tank and the two SRBs and provide information from the SRBs and ET to the orbiter.

The ET has external cameras mounted in the brackets attached to the shuttle along with transmitters that can continue to send video data long after the shuttle and the ET have separated.