Why Pressure Tells You a Temperature (But Not the One You Measured)
In a working vapor-compression system, pressure and saturation temperature are linked for a given refrigerant. A P–T chart (pressure–temperature chart) lets you convert a measured pressure into the saturation temperature—the temperature at which that refrigerant would be boiling/condensing at that pressure.
Key skill: keep two temperatures separate in your mind and on your notepad:
- Saturation temperature (Tsat): comes from pressure using a P–T chart for the correct refrigerant.
- Actual line temperature (Tline): measured with a temperature clamp/probe on the tubing.
You compare them to determine:
- Superheat (typically on the suction/evaporator outlet side):
Superheat = Tline − Tsat - Subcooling (typically on the liquid/condensing outlet side):
Subcooling = Tsat − Tline
Notice the subtraction order changes. That is not a math trick; it reflects what you expect physically: suction vapor should be warmer than its saturation temperature (superheated), and liquid line should be cooler than its saturation temperature (subcooled).
The 4-Step Field Method (Use This Every Time)
Step 1) Measure pressure (and know what kind of pressure it is)
- Connect your gauge/manifold or digital probes to the correct service port.
- Read gauge pressure (psig, barg). Most field gauges show gauge pressure, not absolute.
- Identify whether you are on the low side (suction/evaporator pressure) or high side (discharge/liquid/condensing pressure).
Quick check: If your “low side” reading is higher than your “high side,” you are either on the wrong port, your valves are mis-positioned, or the system is off equalized.
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Step 2) Convert pressure to saturation temperature using a P–T chart
- Select the correct refrigerant scale (R-410A, R-134a, R-404A, etc.).
- Convert the measured pressure to Tsat using the chart/app.
- Write it down as
Tsat@measured pressureso you don’t confuse it with a thermometer reading.
Quick check: Many blends have a bubble/dew distinction. For basic superheat/subcooling work, technicians commonly use dew for superheat (evaporating) and bubble for subcooling (condensing). If your tool shows both, pick the one that matches the measurement you’re doing.
Step 3) Measure the actual line temperature at the correct location
- Superheat measurement location: suction line near the evaporator outlet (or at the suction service valve if that’s your standard procedure), with good clamp contact and insulation over the probe if possible.
- Subcooling measurement location: liquid line near the condenser outlet (or liquid service valve), again with solid contact and minimal influence from ambient air.
Quick check: A shiny copper line in hot sun can read falsely high if the probe is not insulated. Always aim for good thermal contact and shield from radiant heat.
Step 4) Compare Tsat to Tline to determine superheat or subcooling
Use the correct formula for the side you are on:
- Superheat (suction):
SH = Tline(suction) − Tsat(evap) - Subcooling (liquid):
SC = Tsat(cond) − Tline(liquid)
Interpretation basics:
- Positive superheat means vapor is warmer than its boiling point at that pressure (expected).
- Positive subcooling means liquid is cooler than its condensing point at that pressure (expected).
- Near-zero or negative values usually indicate a measurement/setup error or an abnormal operating condition that must be verified carefully.
Worked Examples (Realistic Numbers)
These examples focus on the skill: pressure → Tsat, then compare with measured line temperature.
Example 1: Suction superheat on an R-410A comfort cooling system
- Step 1: Measure suction pressure:
118 psig - Step 2: P–T chart (R-410A) gives
Tsat ≈ 40°Fat 118 psig - Step 3: Measure suction line temperature at evaporator outlet:
Tline = 52°F - Step 4:
Superheat = 52 − 40 = 12°F
Meaning: The suction vapor is 12°F above its saturation temperature at that pressure.
Example 2: Liquid subcooling on the same R-410A system
- Step 1: Measure liquid/high-side pressure:
360 psig - Step 2: P–T chart (R-410A) gives
Tsat ≈ 110°Fat 360 psig - Step 3: Measure liquid line temperature leaving condenser:
Tline = 98°F - Step 4:
Subcooling = 110 − 98 = 12°F
Meaning: The liquid is 12°F below its saturation temperature at that pressure (subcooled liquid).
Example 3: R-134a medium-temp system—superheat check
- Step 1: Suction pressure:
22 psig - Step 2: P–T chart (R-134a) gives
Tsat ≈ 26°F - Step 3: Suction line temperature:
Tline = 38°F - Step 4:
Superheat = 38 − 26 = 12°F
Common mistake check: If you accidentally used an R-410A scale at 22 psig, you’d get a completely different Tsat and your calculated superheat would be meaningless. Always confirm refrigerant selection before trusting the number.
Example 4: R-134a subcooling check with a probe placement error
- Step 1: High-side pressure:
118 psig - Step 2: P–T chart (R-134a) gives
Tsat ≈ 95°F - Step 3 (bad measurement): Liquid line temperature measured on a section of line near a hot compressor discharge line:
Tline = 102°F - Step 4:
Subcooling = 95 − 102 = −7°F
What to do: Negative subcooling often means you are not actually measuring the liquid line temperature correctly (or you are not on a true liquid column). Re-measure at the condenser outlet/liquid service valve, ensure the line is actually the liquid line, improve probe contact, and insulate the probe. If after correcting measurement technique you still get near-zero/negative subcooling, then you investigate system conditions.
Example 5: Confusing high side and low side (how it shows up in the math)
Suppose you intended to calculate superheat but accidentally used the high-side pressure to find Tsat.
- Measured suction line temperature:
Tline(suction) = 55°F - Accidentally used high-side pressure:
350 psigon R-410A →Tsat ≈ 108°F - Calculated “superheat”:
55 − 108 = −53°F
Quick diagnosis: A huge negative “superheat” is a red flag. You likely used the wrong pressure (high side instead of suction), the wrong refrigerant scale, or the system is off and equalized. Re-check Step 1 and Step 2.
Mini Checklist: Common Mistakes and Fast Fixes
| Mistake | What you’ll see | Fast fix |
|---|---|---|
| Wrong refrigerant selected on gauge/app | Tsat doesn’t make sense for the operating condition; superheat/subcooling looks extreme | Verify refrigerant from nameplate/work order; set correct scale before converting pressure |
| Mixing saturation temperature with actual temperature | Writing down one temperature and forgetting which it is; inconsistent calculations | Label readings explicitly: Tsat@P and Tline |
| Using the wrong side pressure (high vs low) | Negative or wildly large superheat/subcooling | Confirm hose/probe location; identify suction vs liquid line by temperature and routing |
| Poor temperature measurement technique | Subcooling goes negative near hot components; readings change when you touch the probe | Improve clamp contact, clean pipe, insulate probe, avoid radiant heat sources |
| System not stabilized | Pressure and temperature readings drift quickly; calculations jump around | Allow operating conditions to settle before recording final values |
Practice Drill (Do This on Paper Before You Touch a System)
For each scenario, write down: P, Tsat, Tline, then compute SH or SC.
- Scenario A (superheat): R-410A suction pressure 125 psig; Tsat from chart; suction line temp 54°F.
- Scenario B (subcooling): R-410A liquid pressure 390 psig; Tsat from chart; liquid line temp 101°F.
- Scenario C (superheat): R-134a suction pressure 18 psig; Tsat from chart; suction line temp 35°F.
When you check your work, focus less on the final number and more on whether you used the correct refrigerant scale, correct side pressure, and correct subtraction order.
