Sentence:  Over a temperature range from 25 to 200 * * f and at pressures up to 250 <WSD>atm</WSD> , an overload of 300 psi , applied for a period of one day , results in an uncertainty in the pressure of , at most , one millimeter of mercury . 
Meaning of the word marked with <WSD> tags:  a unit of pressure: the pressure that will support a column of mercury 760 mm high at sea level and 0 degrees centigrade