Sean Culmer, a 20-year-old former Oberlin College student, was convicted tonight of aggravated murder for strangling his college girlfriend, a sophomore from Northern Virginia, and was sentenced to life imprisonment, with no eligibility for parole for 20 years.

A Lorain County Common Pleas Court jury deliberated almost eight hours before finding him guilty of killing Grace B. Austin, 20, in her dormitory room last April 24, rejecting Culmer's plea of innocent by reason of insanity.

Culmer, whose home is in New York City, was taken to the Lorain County Correctional Facility and will be taken to a prison in Columbus within five days.

Because he has twice tried to commit suicide since killing Austin, he is being kept in a separate cell, closely guarded. Lorain County sheriff's deputies said he was crying heavily after being jailed tonight.

Although Culmer said he hid a necktie in Austin's college dormitory room and strangled her, his attorneys said he had a "psychotic episode" and was unable to stop himself.

Culmer, a psychology major, and Austin, majoring in English and economics, had been a popular couple on campus, seldom seen apart, involved in many organizations. Austin had been a homecoming princess at Fort Hunt High School in Fairfax County in 1981.

Culmer was unable to accept the fact Austin wanted to break up because he was becoming too possessive, psychiatrists testified during the trial.

Before being sentenced, Culmer told the judge, "I am sorry that all this happened . . . I will always believe that I was not competent at the time that the incident occurred and there is nothing that will change my mind. I loved Grace, that is all."

Culmer's mother, Mrs. Barbara Culmer, fainted when the mandatory sentence was read. His sister, Karen, spun around facing the Austin family seated behind her, and yelled repeatedly, "I hope you're all happy now."

Both were taken by ambulance to a local hospital for treatment.

Grace Austin's mother, Theora Austin, who was present along with her husband, the Rev. Clinton Austin, and their daughter Kimberly, said of the Culmer family later: "No one called to say, 'We're sorry.' They never came to say one word through all our sorrow. Now they say we're vicious."

Clinton Austin commented: "No one wins in situations like this. There is no joy at a moment like this, but I think justice has been done. It is sad when we lose promising black young people. It is even worse when they cause their own self-destruction. There is no excuse for a responsible person to commit a brutal, vicious murder to a defenseless person."