Question

Convert the following reading of pressure to kPa, assuming that the barometer reads 760mm Hg.

(1) 90cm Hg gauge (2) 40cm Hg vacuum (3) 1.2m H₂O gauge

Answer 1

Given that h = 760mm of Hg for P_atm 

P_atm = ρgh = 13.6 × 10³ × 9.81 × 760/1000 = 101396.16

 N/m² = 101396.16Pa = 101.39KPa                                                              ...(i)                                                       

(a) 90cm Hg gauge 

P_gauge = ρgh = 13.6 × 10³ × 9.81 × 90/100 = 120.07KPa                             ...(ii)

P_abs = P_atm + P_gauge  = 101.39 + 120.07 

Pabs = 221.46KPa .......ANS

 (b) 40cm Hg vacuum

 P_vacc = ρgh = 13.6 × 10³ × 9.81 × 40/100 = 53.366KPa                              ...(iii)

Pabs = Patm – Pvacc 

= 101.39 – 53.366

 Pabs = 48.02KPa .......ANS

 (c)1.2m Water gauge

 P_gauge = ρgh = 1000 × 9.81 × 1.2 = 11.772KPa                                          ...(iv)

P_abs = P_atm + P_gauge

 = 101.39 + 11.772

 P_abs = 113.162KPa .......ANS