A technician is charging an R-410A system at 5,000 feet elevation, where local atmospheric pressure is approximately 12.2 psia (about 2.5 psi below the sea-level standard of 14.7 psia). Using a standard sea-level pressure-temperature chart, what correction should be applied to the gauge reading to determine the correct saturation temperature?
- ASubtract approximately 2.5 psi from the actual gauge reading before referencing the standard P-T chart
- BAdd approximately 2.5 psi to the actual gauge reading before referencing the standard P-T chart
- CMultiply the actual gauge reading by a factor of 1.17 to compensate for elevation
- DNo correction is needed; gauge pressure always reads true absolute pressure regardless of elevation
Show answer & explanationAnswer & explanation
Correct answer: A. Subtract approximately 2.5 psi from the actual gauge reading before referencing the standard P-T chart
A pressure gauge reads relative to local atmospheric pressure (absolute pressure minus local atmosphere). Standard P-T charts assume a sea-level atmosphere of 14.7 psia is added to the gauge reading to get absolute pressure. At altitude, since local atmosphere is only 12.2 psia (2.5 psi less), the actual gauge reading will run about 2.5 psi higher than the standard chart's assumed gauge value for the same true absolute pressure. Subtracting 2.5 psi from the field reading before consulting the chart corrects for this offset and yields the accurate saturation temperature.
Why the other options are wrong
- B. Adding to the reading moves further from, not toward, the correct absolute pressure equivalent.
- C. There is no standard multiplication factor used for altitude correction in P-T chart work.
- D. Gauge pressure is relative to local atmosphere, which varies with elevation, so a correction is needed.
Altitude Correction for P-T Charts
Standard pressure-temperature charts assume sea-level atmospheric pressure; at higher elevations, gauge readings must be corrected (typically subtracted) by the atmospheric pressure deficit before referencing the chart.
- Standard atmosphere = 14.7 psia at sea level
- At altitude, local atmosphere is lower, so gauge readings run higher for the same absolute pressure
- Subtract the atmospheric deficit from the gauge reading before using a standard sea-level P-T chart
Memory trick: High up, air is thin — subtract before you dive in (the chart)