As Arun hints at it could be that the column is character rather than numeric. If you are sure that all the values are correct you could coerce the values with
d_Temp_Experiment$value <- as.numeric(d_Temp_Experiment$value)
You might have the below sort of business going on.
myvector <- c(0,1,2,3,4,5,"6","7")
mv <- as.numeric(myvector)
mean(myvector)
mean(mv)