Absolute vs Gauge Pressure and Compound Gauge Essentials

  • PSIA vs PSIG and atmospheric reference
    • PSIA = pounds per square inch absolute; PSIG = pounds per square inch gauge.
    • Absolute pressure includes atmospheric pressure; 0 PSIA = perfect vacuum.
    • At sea level, atmospheric pressure ≈ Patm ≈ 14.7 psiP_{atm} \,\approx\, 14.7\ \text{psi} = ≈ 29.92 inHg.\approx\ 29.92\ \text{inHg}.
    • Conversion: P<em>abs=P</em>gauge+P<em>atmP<em>{abs} = P</em>{gauge} + P<em>{atm}; with P</em>atm≈14.7 psiP</em>{atm} \approx 14.7\ \text{psi}.
  • Barometer reference (comparison)
    • 29.92 inHg corresponds to ~14.7 psi; shows how atmospheric pressure translates across units.
  • Pressure gauges: low-side/compound gauge
    • Two scales: outer scale (PSIG) and bottom scale (inches of mercury vacuum).
    • Outer numbers indicate gauge pressure: Pgauge.P_{gauge}.
    • Bottom numbers indicate vacuum: inHg vacuum.\text{inHg vacuum}.
    • Key relation to remember: P<em>abs=P</em>gauge+Patm.P<em>{abs} = P</em>{gauge} + P_{atm}.
  • Reading a live gauge (example concepts)
    • A reading of about Pgauge≈69 psigP_{gauge} \approx 69\ \mathrm{psig} is typical for an R-22 system.
    • If the gauge reads zero inHg vacuum on the bottom scale and the hoses are open to atmosphere, the gauge reads P<em>gauge=0 psigP<em>{gauge} = 0\ \mathrm{psig}, and the absolute pressure is P</em>abs≈Patm≈14.7 psi.P</em>{abs} \approx P_{atm} \approx 14.7\ \mathrm{psi}.
    • When the gauge shows a nonzero gauge pressure, you compute absolute pressure by adding 14.7 psi14.7\ \text{psi}:
    • Example: P<em>gauge=30 psig⇒P</em>abs=30+14.7=44.7 psia.P<em>{gauge} = 30\ \mathrm{psig}\Rightarrow P</em>{abs} = 30 + 14.7 = 44.7\ \mathrm{psia}.
  • Bottom scale details
    • Bottom scale ranges from 0 to 30 inches of mercury vacuum.
    • Perfect vacuum corresponds to 30 inHg vacuum30\ \text{inHg vacuum} (i.e., nearly zero absolute pressure).
    • The gauge’s bottom scale extreme (0 inHg vacuum) would align with ~30 inHg vacuum, reflecting the relationship with absolute pressure.
  • Practical notes
    • Mercury barometers are hazardous; gauges are safer and more practical for everyday work.
    • Zero on the gauge (PSIG) indicates atmospheric pressure, not zero absolute pressure.
  • Quick recap
    • Always convert PSIG to PSIA with P<em>abs=P</em>gauge+P<em>atmP<em>{abs} = P</em>{gauge} + P<em>{atm} where P</em>atm≈14.7 psiP</em>{atm} \approx 14.7\ \text{psi} at sea level.
    • Outer scale = PSIG; bottom scale = inHg vacuum.
    • Perfect vacuum = 30 inHg vacuum30\ \text{inHg vacuum}; atmospheric pressure ≈ 14.7 psi14.7\ \text{psi} or 29.92 inHg.29.92\ \text{inHg}.