Question: Why would Barry need Kara's help in gaining extra speed to return to his own dimension considering that the device he had on was capable of doing that?
Supergirl (2015)
1 question in season 1
Starring: Calista Flockhart, Melissa Benoist, Mehcad Brooks, David Harewood
Continuity mistake: Rick Malverne has handcuffs on when J'onn goes in as his dad. When he figures out it was J'onn, he tells them they have "23 hours and 14 minutes," looks at his watch and the cuffs are gone. (00:21:30 - 00:25:00)
Cat Grant: All four of you standing there doing nothing, you look like the attractive yet non-threatening, racially diverse cast of a CW show.
More trivia for Supergirl
Answer: It was an experimental device and was not meant to propel him into another world, it happened by accident. He need an extra boost of speed to get him back home.
Except that the device did give him the extra boost of speed, which caused him to end up in Kara's world, so he wouldn't really need Kara's help to return home since the device could do it for Barry.