What You'll Learn

• How the saturation curve (P/T relationship) works and why each refrigerant has a different one
• How measureQuick uses your refrigerant selection and pressure readings to calculate condensing and evaporating temperatures
• Why selecting the wrong refrigerant in the system profile produces incorrect diagnostics
• What "Calculated" pressure sources mean in measureQuick and when the app uses them
• How non-invasive (temperature-only) testing works and when it applies
• How P/T calculations feed into superheat and subcooling
• What changes with newer refrigerants like R454B and R32

What You'll Need

• Device: iPhone (iOS 15+) or Android phone (Android 10+) with measureQuick installed
• Account: Logged in with an active measureQuick account
• Context: A test in progress, a completed test, or Demo Mode enabled
• Knowledge: Basic understanding of the refrigeration cycle (see Refrigeration Cycle Basics)
• Time: 10 minutes to read

The P/T Relationship

Every refrigerant has a fixed relationship between pressure and temperature at which it changes state (boils or condenses). This is the saturation curve. At a given pressure, the refrigerant boils at one specific temperature. At a given temperature, the refrigerant condenses at one specific pressure. The curve is unique to each refrigerant.

This is what P/T charts describe: the saturation temperature for any measured pressure, and vice versa.

measureQuick has these curves built in for every supported refrigerant. When you measure a pressure, the app looks up the corresponding saturation temperature automatically. You never need to consult a printed P/T chart.

How measureQuick Uses P/T

When you create a system profile in the app, you select the refrigerant type (R410A, R22, R454B, R32, etc.). That selection tells measureQuick which saturation curve to use for every calculation in the test.

From Pressures to Saturation Temperatures

The app takes two pressure readings and converts each one to a saturation temperature:

1. Discharge/liquid line pressure (high side) - The app looks up the saturation temperature on the selected refrigerant's curve. This is the condensing temperature, the temperature at which refrigerant is condensing in the outdoor coil.

2. Suction pressure (low side) - The app looks up the saturation temperature on the same curve. This is the evaporating temperature, the temperature at which refrigerant is boiling in the indoor coil.

measureQuick diagnostic screen showing measured suction pressure and the calculated evaporating temperature derived from it

From Saturation Temperatures to Superheat and Subcooling

Once the app has condensing and evaporating temperatures, it calculates the two values that define charge status:

• Superheat = Suction line temperature (measured by pipe clamp) minus Evaporating temperature (calculated from suction pressure)
• Subcooling = Condensing temperature (calculated from discharge pressure) minus Liquid line temperature (measured by pipe clamp)

The P/T lookup is the bridge between your pressure probes and the diagnostic results you see on screen. Without it, the app has pressures and temperatures but no way to determine whether the system is properly charged.

measureQuick showing superheat and subcooling values with the underlying pressure, saturation temp, and line temp values visible

Why the Refrigerant Selection Matters

Different refrigerants have very different saturation curves. The same pressure reading produces a different saturation temperature depending on which refrigerant you select.

In the third row, the system actually has R410A, but the profile says R22. The app calculates a 72F evaporating temperature instead of 40F. Every downstream calculation (superheat, subcooling, charge evaluation) is now wrong. The diagnostic results are meaningless.

This is a common source of false failures and false passes. If a test result does not match what you observe on site, check the refrigerant selection in the system profile first.

Blended Refrigerants and Temperature Glide

Pure refrigerants like R22 have a single saturation temperature at any given pressure. Blended refrigerants (zeotropic blends like R407C, R410A, R454B, R458A) have a temperature range called "glide" between the dew point (where the last liquid droplet evaporates) and the bubble point (where the first vapor bubble appears).

measureQuick handles this differently from most other apps and digital gauges. Where many tools display the dew point or bubble point temperature under the gauge, measureQuick displays the average coil temperature using the weighted midpoint of the glide. This gives you a more representative picture of the actual coil operating temperature.

For superheat and subcooling calculations, the app uses dew point and bubble point in the background:

• Superheat is calculated from the dew point temperature (not the average coil temperature)
• Subcooling is calculated from the bubble point temperature (not the average coil temperature)

This is why the saturation temperature displayed in measureQuick may differ from your P/T chart, your digital gauges, or another refrigerant app, even though the superheat and subcooling values agree. The displayed saturation temperature represents average coil temperature; the diagnostic calculations use dew point and bubble point. Both are correct for their respective purposes.

For high-glide refrigerants (like R458A, which can have 15F+ of glide), this distinction is significant. For R410A (which has less than 1F of glide), the difference is negligible in practice.

Newer Refrigerants

R454B (Opteon XL41) and R32 are entering the residential market as lower-GWP replacements for R410A. Each has its own saturation curve. R454B operates at slightly lower pressures than R410A. R32 operates at slightly higher pressures.

measureQuick supports both. Selecting the correct one matters just as much as distinguishing R410A from R22. As the transition to A2L refrigerants accelerates, accurate identification becomes more critical, not less.

Tip: If you are working on newer equipment and are unsure which refrigerant it uses, check the data plate on the outdoor unit. The refrigerant type is listed there. Do not assume R410A on post-2024 installations.

"Calculated" Pressure Sources

measureQuick tracks where each measurement comes from. When you look at a test record, the pressure source can be one of several types:

• Physical pressure probe (manifold gauge, wireless pressure transducer) - A direct reading from an instrument connected to the system's service ports.
• Calculated - The app derived a pressure value from temperature measurements and the known refrigerant type. No physical pressure instrument was connected.

When the source is "Calculated," the app worked the P/T relationship in reverse. Instead of looking up a saturation temperature from a measured pressure, it estimated a saturation pressure from a measured temperature. This is how non-invasive testing works.

Outdoor Measurements showing pressure readings with physical probe source icons

Indoor Measurements showing calculated (calculator icon) vs physical (blue probe icon) pressure sources

Non-Invasive Testing

measureQuick can evaluate system performance without connecting manifold gauges. This is non-invasive testing, sometimes called temperature-only diagnostics.

How It Works

The app uses temperature readings from pipe clamps (suction line, liquid line) plus outdoor ambient temperature and the selected refrigerant type. From these inputs, it estimates what the system pressures should be and derives superheat and subcooling accordingly.

The process:

1. Pipe clamp temperatures stream from your probes into the app.
2. The app estimates expected suction and discharge pressures based on the refrigerant type and operating conditions.
3. Those estimated pressures run through the P/T curve to produce saturation temperatures.
4. Superheat and subcooling are calculated from the saturation temperatures and the measured line temperatures.

When to Use It

• Service calls where you do not want to connect gauges - Every manifold connection is a potential leak point. Non-invasive testing avoids that risk entirely.
• Return visits and follow-ups - Quick verification that the system is still performing as expected after a previous repair.
• Screening multiple systems - When you need to assess several units quickly and flag which ones need deeper investigation.

Limitations

Non-invasive testing is less precise than pressure-based measurement. It relies on assumptions about system behavior and ambient conditions. The results are useful for screening and trend analysis, but for a definitive charge evaluation, physical pressure readings provide higher confidence.

If a non-invasive test flags a potential issue, connect gauges on the follow-up visit to confirm.

Tip: Non-invasive testing still requires accurate outdoor temperature. If the app's weather data does not match conditions at the job site (direct sunlight on the probe, sheltered location, microclimate), enter the outdoor temperature manually.

Video Walkthrough

These videos cover the P/T relationship and how measureQuick applies it:

• YouTube (HVAC School): (15,014 views, 1:41). Common charging errors including wrong refrigerant selection, with coverage of P/T relationship implications for system profiling and probe placement

• YouTube: (294,298 views, 30 min). Deep explanation of P/T relationships, saturation curves, and how pressures translate to superheat and subcooling

• YouTube: (31,780 views, 4 min). Shows how measured pressures are converted through the P/T relationship during a live charging session

• YouTube: (3,448 views, 11 min). Demonstrates non-invasive testing methodology: temperature-only diagnostics using the P/T relationship in reverse

• YouTube: (4,695 views, 9 min). Explains how the app calculates diagnostic results from raw measurements, including the P/T lookup step

• YouTube: (18,992 views, 53 min). Advanced diagnostic walkthrough covering system analysis and interpretation of P/T-derived values

Tips & Common Issues

Superheat and subcooling values seem wrong

Check the refrigerant type in the system profile first. A mismatched refrigerant is the most common cause of calculations that do not match what you observe on the gauges. Open the system profile and verify the refrigerant matches the data plate.

"Calculated" source on a test where you connected gauges

If you see "Calculated" as the pressure source even though you had gauges connected, the pressure readings may not have been captured by the app. Verify that your manifold or pressure probes were paired and streaming data during the test. Check the Tools screen to confirm live data was flowing.

Non-invasive results differ from gauge-based results

This is expected. Non-invasive testing estimates pressures from temperatures and operating assumptions. The values will be close but not identical to physical pressure readings. Use non-invasive results for screening. Use gauge-based results for definitive charge evaluation.

Working with a refrigerant you have not used before

If you encounter R454B, R32, or another newer refrigerant for the first time, make sure your version of measureQuick supports it. Update the app if necessary. Select the exact refrigerant from the profile options. Do not substitute a "close enough" refrigerant; the saturation curves are different enough to produce incorrect results.

The app shows a saturation temperature that does not match your P/T chart

There are two common causes. First, confirm you are comparing the same refrigerant and the same pressure units (PSIG vs PSIA). measureQuick uses gauge pressure (PSIG) by default. If your chart uses absolute pressure (PSIA), the readings will differ by approximately 14.7 PSI.

Second, for blended refrigerants, measureQuick displays average coil temperature rather than the dew point or bubble point that most P/T charts and other apps show. The superheat and subcooling values will still agree between apps because they are calculated from dew point and bubble point in all cases. The displayed saturation temperature differs because measureQuick uses the weighted midpoint of the glide to represent actual coil operating temperature. See the "Blended Refrigerants and Temperature Glide" section above for details.

Related Articles

Prerequisites:

• Refrigeration Cycle Basics

Follow-up articles:

• Superheat & Subcooling
• Design Temperature Difference (DTD)
• Charge & Airflow Balance
• A/C Service Workflow
• Non-Invasive Testing

Need Help?

Contact measureQuick support: support@measurequick.com