The three-hour fueling process was completed by around 8:20 a.m. EST. During the process the ECO sensors should read “dry” before fuel is added and “wet” after fuel has been added (to show that the sensors are immersed in fuel).

At various points during the process, NASA engineers will further test the sensors.

According to new rules by NASA at least three of the four fuel gauges must be working properly for a mission to be approved for launch. The ECO sensors are part of a backup system that assure that the three space shuttle main engines (SSMEs) do not run dry (operate without fuel in the external tank) while they operate for about 8.5 minutes from liftoff to Main Engine Cut Off (MECO).

(If listening to the mission's Capcom during a launch, you will hear a “press to MECO” just before the main engines are turned off.)

Engineers at the NASA Marshall Space Flight Center, Huntsville, Alabama, put the ECO sensors through a series of tests to finally find that connector pins (part of a monitoring cable) were giving false readings. The sensors help to monitor fuel flow and mixtures (between the liquid hydrogen and oxygen) from the external tank to the shuttle’s SSMEs via the monitoring cable.
 
The engineers hard-wired the connector pins in place for the STS-122 mission. They will decide what to do for future flights based partially on how well this fix works.

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