Question 1

What is the difference between bubble point and dew point on a PT chart?

Accepted Answer

For zeotropic blends like R-410A and R-454B, the refrigerant boils across a range — bubble point is the temperature where the first vapor bubble forms (liquid side), dew point is where the last liquid droplet evaporates (vapor side). Charge as liquid using bubble point. Calculate superheat using dew point at the suction pressure.

Question 2

Which PT values do I use for superheat vs subcool?

Accepted Answer

Superheat — use the DEW point at the suction pressure to find saturation temp, then subtract that from your actual suction line temp. Subcool — use the BUBBLE point at the liquid pressure, then subtract your actual liquid line temp from that.

Question 3

Why does R-410A read higher pressure than R-22 at the same temperature?

Accepted Answer

R-410A operates at roughly 50-60% higher pressure than R-22 at any given temperature. For example, at 95°F outdoor with normal operation: R-22 high side reads ~250 psi while R-410A reads ~395 psi. Always check your gauges are rated for the higher R-410A or R-32 pressures (700+ psi rated).

Question 4

Is R-454B charging the same as R-410A?

Accepted Answer

Very similar pressures — R-454B runs ~5-7% lower than R-410A at the same temp — but R-454B is mildly flammable (A2L) so the charging procedure changes: leak-test before opening, ventilate the work area, and use only A2L-rated recovery and charging gear.

Question 5

What is the safe upper pressure for R-32 in residential work?

Accepted Answer

R-32 saturation pressure at 130°F is roughly 490 psi (per the Hudson Technologies manufacturer chart). Most R-32 systems are designed to a 600-650 psi maximum working pressure. If you see liquid-side pressure climbing toward that ceiling during operation, shut down and find the cause (overcharge, restricted condenser, non-condensables) — do not vent.

Interactive PT Chart

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Interactive PT Chart

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