Sentence:  Although the tape was run for over 1 hr. , a steady state was not reached , and it was concluded that the reason for this was that the back <WSD>pressure</WSD> of the manometer was built up from the material fed from between the blocks and this was available at a very slow rate . 
Meaning of the word marked with <WSD> tags:  the force applied to a unit area of surface; measured in pascals (SI unit) or in dynes (cgs unit)