R-Pro › Field cases › Refrigerators & showcases
PT Chart Usage — Pressure↔Temperature Conversion Diagnosis
Likely causes
Refrigerant-specific PT chart — same pressure yields different saturation temperature for each refrigerant
Check: Always confirm refrigerant type before measurement — check nameplate or charging label. Using the wrong PT chart makes superheat/subcooling diagnosis completely wrong. Example: at 1.0 MPa(g), R-22=24°C, R-410A=11°C, R-134a=39°C, R-404A=5°C.
What to do: 1) On digital manifold, select correct refrigerant from menu — auto PT conversion. 2) Carry paper PT chart for 7 refrigerants (R-22/R-134a/R-410A/R-404A/R-32/R-454B/R-744). 3) If refrigerant unknown, stop work — never guess. 4) Verify whether chart uses gauge (g) or absolute (abs) pressure — 1 atm (0.1 MPa) difference.
Glide refrigerants — single pressure has two saturation temperatures (liquid/vapor)
Check: Blends like R-407C, R-454B, R-448A, R-449A have glide. PT chart shows 'Bubble' (liquid saturation) and 'Dew' (vapor saturation) columns. Subcooling calc → use Bubble temp; superheat calc → use Dew temp. Single-column chart yields 5°C+ error.
What to do: 1) For glide refrigerants, always use dual-column PT chart. 2) Digital manifolds auto-separate Bubble/Dew — check Glide indication when selecting refrigerant. 3) Charge in liquid phase only (gas-phase charging causes composition separation). 4) Never top off after a leak — glide refrigerants change composition when leaking; recover full charge and replace with new refrigerant.
Pressure unit confusion — conversion errors between MPa, bar, psi, kPa
Check: Conversion: 1 MPa = 10 bar = 145 psi = 1000 kPa. Errors common when mixing US gauges (psi) with Korean/Japanese PT charts (MPa). Verify with both unit systems — e.g., R-410A normal high-side ≈ 3.0 MPa = 30 bar = 435 psi.
What to do: 1) Set digital manifold unit menu to single unit matching local standard. 2) Use paper charts with both units printed. 3) When unsure, use a converter app (Refrigerant Slider, etc.). 4) Correct any wrong-unit diagnostic notes immediately — prevent confusion for next tech.
PT charts are reference tools to keep on hand, not to memorize. Standard practice: digital manifold + paper chart backup. Glide refrigerants and pressure unit confusion are the two most common error sources.
All 565 cases work offline in R-Pro, with error codes for 62 brands and P-T data for 108 refrigerants.
Open R-Pro →Related cases
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- Abnormal Subcooling — Low / High Diagnosis
- Zero Subcooling — Continuous Bubbles in Liquid Sight Glass
- Refrigerant Type Cannot Be Identified — Missing Nameplate or Painted Over