Dana's answer is good. Cannot tell without the sentence. Here are a few more. Maybe one will help. . The leaking valve lead to a build up in oxygen concentration over the experimental period ? The leaking valve accounted for the built up oxygen concentration seen at the end of the experimental period ? As we concluded we recorded the value of the total built up oxygen concentration.

I don't know the rest of the sentence, so I can't help fully, but science writing loves passives. A good excerpt would be "as the oxygen concentration was gradually increased." Example full sentences: 1) The sample was added to the solution as the oxygen concentration was gradually increased. 2) The specimen was placed in an environment in which the oxygen concentration was gradually increased over a span of 3 hours.

"built up" is OK. "Accumulated" might be better. If the process of building up concludes at the time you are talking about, you could use "final". I might be able to give better suggestions with a bit more context.