PROBLEM 9.1
FIND:
Convert to units of gauge pressure in N/m
2
SOLUTION
Absolute pressure reference scale:
1 atm abs = 14.69 psia = 101.325 kPa abs = 101,325 N/m
2
abs
1 atm abs = 760 mm Hg abs = 406 in H
2
O abs
Conversion factors:
14.69 psi = 101.325 kPa = 101,325 N/m
2
14.69 psi = 1 atm = 29.92 in Hg = 406 in H
2
O = 760 mm Hg
1 bar = 14.505 lb/in
2
= 100,000 N/m
2
p(gauge) = p(absolute) – p(reference)
Using p(reference) = 101,325 N/m
2
:
(a) 10.8 psia x 101325 N/m
2
/14.69 psi = 74 442 N/m
2
abs
p = 74,442 – 101,325 N/m
2
= -26,883 N/m
2
= -26.883 kPa
(b) 1.75 bars abs x 100,000 N/m
2
= 175,000 N/m
2
abs
p = 175,000 – 101,325 N/m
2
= 73,675 N/m
2
= 73.675 kPa
(c) 30.36 in H
2
O abs x 101,325 N/m
2
/406 in H
2
O = 7,577 N/m
2
abs
p = 7,577 – 101,325 N/m
2
= -93,748 N/m
2
= - 93.748 kPa
(d) 791 mm Hg abs x 101,325 N/m
2
/760mm Hg = 105,458 N/m
2
abs
p = 105,458 – 101,325 N/m
2
= 4,133 N/m
2
= 4,133 kPa
COMMENT
Note that gauge pressure can be negative or positive relative to the reference pressure.
Absolute pressure, which is measured relative to a perfect vacuum, is always a positive
value.