What You'll Learn

• How measureQuick calculates superheat and subcooling from your probe data
• Which metric to target based on the metering device type in your system profile
• How target superheat is derived from outdoor ambient and return air wet bulb conditions
• How to read the superheat and subcooling detail screens, including target values and acceptable ranges
• What the pass/fail indicators mean for refrigerant charge evaluation
• When and why to override a superheat or subcooling result
• How metering device selection in the system profile controls which metric measureQuick evaluates

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 cooling test in progress with live probe data, or a completed test to review (Demo Mode works for familiarization)
• System profile: Equipment profiled with the correct metering device type (TXV or Piston/Fixed Orifice)
• Knowledge: Familiarity with pressure-temperature relationships (see Pressure-Temperature Relationship)
• Time: 10 minutes to read; 15-20 minutes to walk through with a live or demo test

How measureQuick Calculates These Values

measureQuick does not measure superheat or subcooling directly. It derives them from temperature and pressure readings captured by your connected probes, combined with the refrigerant type specified in the system profile.

Superheat = Suction line temperature - Evaporating temperature

• Suction line temperature comes from your temperature probe on the suction line.
• Evaporating temperature is calculated from the suction pressure reading and the refrigerant's pressure-temperature curve. This is why the app needs both a pressure instrument and a temperature probe on the suction side.

Subcooling = Condensing temperature - Liquid line temperature

• Condensing temperature is calculated from the discharge (liquid/high-side) pressure reading and the refrigerant's P/T curve.
• Liquid line temperature comes from your temperature probe on the liquid line.

If either the pressure or temperature input is missing, the app cannot calculate the value, and the corresponding diagnostic will show as gray (not measured).

Blended Refrigerants: Dew Point and Bubble Point

For blended (zeotropic) refrigerants, measureQuick calculates superheat and subcooling using dew point and bubble point temperatures rather than a single saturation temperature:

• Superheat = Suction line temperature - Dew point temperature (at suction pressure)
• Subcooling = Bubble point temperature (at discharge pressure) - Liquid line temperature

The displayed saturation temperature under the gauge in measureQuick is the average coil temperature (the weighted midpoint of the glide), not the dew point or bubble point. This means the saturation temperature shown on screen will differ from what you see on a P/T chart or another app, but the calculated superheat and subcooling values will agree because all apps use dew point and bubble point for those calculations.

For R410A (less than 1F of glide), this distinction is negligible. For higher-glide refrigerants like R458A, the difference between displayed saturation temperature and dew/bubble point can be 5F or more. Tap the target indicator on the gauge screen to see the dew point, bubble point, and glide values for the selected refrigerant.

Diagnostics screen showing superheat and subcooling values with live probe data connected

Metering Device Determines Which Metric to Target

The metering device type in your system profile controls which measurement measureQuick uses as the primary charge indicator. This is one of the most common sources of incorrect diagnostics: if the metering device is set wrong, the app targets the wrong metric.

Key point: On a TXV system, superheat is still calculated and displayed, but it is not the charge indicator. The TXV actively controls superheat to protect the compressor. A TXV system with correct subcooling but unusual superheat may have a TXV problem, not a charge problem. The reverse applies to piston systems: subcooling is displayed but is not the primary charge diagnostic.

How Target Superheat Is Calculated

For piston/fixed orifice systems, measureQuick does not use a single fixed target. It calculates the target superheat based on three inputs:

1. Outdoor ambient temperature - from your outdoor temperature probe or manual entry
2. Return air wet bulb temperature - from your indoor wet bulb probe or psychrometric calculation
3. AHRI standard conditions - the app applies the manufacturer's charging chart logic

As outdoor ambient increases or indoor wet bulb changes, the target superheat shifts. This is why two technicians testing the same system on different days may see different target values. The target is condition-dependent, not fixed.

The detail screen shows the calculated target alongside the acceptable range so you can see exactly what the app is evaluating against.

Tip: If the target superheat displayed seems unreasonable (below 5F or above 25F), check your outdoor ambient and return air wet bulb readings. A misplaced or disconnected probe will feed bad data into the target calculation.

How Target Subcooling Works

For TXV systems, the target subcooling is simpler: it comes from the manufacturer's specifications. Most systems target 8-14F of subcooling, with 10F being common when the manufacturer does not specify otherwise.

You enter or confirm the target subcooling in the system profile. measureQuick applies an acceptable range around that target (typically +/- 3F, depending on the system) and evaluates your measured subcooling against it.

Unlike superheat targeting, subcooling targets do not shift with operating conditions. The target is a fixed value based on the equipment design.

Reading the Detail Screens

Tap any pass/fail indicator on the Diagnostics screen to open the detail view. For superheat and subcooling, the detail screen shows:

• Current measured value - the live superheat or subcooling calculated from your probes
• Design target - the target value (calculated for superheat, profile-specified for subcooling)
• Acceptable range - the tolerance band around the target
• Trend - how the value has changed as readings stabilize

Subcooling detail screen showing current value, typical target, allowable range, and in-range indicator

The color coding matches the Diagnostics screen:

A red superheat or subcooling flag is the primary indicator of a refrigerant charge problem. From mQ's V12 database of 115,706 quality-filtered cooling tests, 56.0% of piston-metered systems fail the charge evaluation, with superheat outside the acceptable range. The overall charge failure rate across all metering device types is 45.4%. Charge failure is the single most common diagnostic finding across all subsystems.

Interpreting Results

Superheat Too High (Piston Systems)

High superheat typically indicates undercharge. The evaporator is starved for refrigerant, so the suction gas superheats beyond the target. Other possible causes: restricted metering device, low airflow across the evaporator, or a partially blocked liquid line.

Superheat Too Low (Piston Systems)

Low superheat typically indicates overcharge. Excess refrigerant floods the evaporator, leaving liquid in the suction line. This risks compressor damage from liquid slugging. Other possible causes: high airflow, dirty condenser restricting heat rejection.

Subcooling Too High (TXV Systems)

High subcooling typically indicates overcharge. Excess refrigerant backs up in the condenser, subcooling the liquid beyond the target. Other possible causes: restricted liquid line, faulty TXV not opening fully.

Subcooling Too Low (TXV Systems)

Low subcooling typically indicates undercharge. Insufficient refrigerant means the condenser cannot fully condense and subcool the liquid. Other possible causes: dirty condenser, restriction upstream of the sensing point.

In all cases, verify that the system has stabilized (10-15 minutes of runtime) before drawing conclusions. Unstable readings during the first few minutes of operation do not reflect actual charge condition. See System Stabilization.

Video Walkthrough

• YouTube (HVAC School): (15,014 views, 1:41). Common charging errors including incorrect superheat/subcooling targeting, wrong metering device selection, and probe placement issues that affect charge evaluation

• YouTube: (899,460 views, 8 min). High-level explanation of superheat and subcooling concepts with visual diagrams

• YouTube: (248,586 views, 11 min). Visual walkthrough of subcooling-based charging with 3D system animation

• YouTube: (31,780 views, 4 min). Shows the subcooling targeting workflow in measureQuick with live probes

• YouTube: (20,805 views, 17 min). Full charge workflow from start to finish, including interpreting superheat and subcooling results

• YouTube: (19,067 views, 10 min). Demonstrates how abnormal superheat readings help diagnose a suction line restriction

• YouTube: (18,992 views, 53 min). Deep diagnostic walkthrough including superheat/subcooling interpretation in context

Tips & Common Issues

Superheat and subcooling both show gray (not measured)

The app needs both a pressure reading and a temperature reading on each side to calculate these values. Verify that your suction pressure probe, liquid/discharge pressure probe, suction line temperature probe, and liquid line temperature probe are all connected and transmitting. Check the probe connection indicators on the measurement screen.

Target superheat seems too high or too low

Target superheat is condition-dependent. If the outdoor ambient or return air wet bulb probe is misplaced, disconnected, or reading incorrectly, the calculated target will be wrong. Verify your ambient and wet bulb readings before questioning the target.

Wrong metering device in the profile

If the system profile says TXV but the system actually has a piston (or vice versa), measureQuick will target the wrong metric. The most common symptom: the charge diagnostic fails even though the system is clearly operating well. Open the system profile, correct the metering device type, and the diagnostics will recalculate.

Subcooling target does not match manufacturer specs

The default subcooling target may not match your specific equipment. Check the manufacturer's installation manual for the specified subcooling target and update it in the system profile. A 2-3F difference between the default and actual target can swing a result from pass to fail.

FAQ: Why is my subcooling target not 10?

Subcooling targets are not universal. They range from 6 to 18 depending on the manufacturer. The default of 10 is an assumption the app uses when no manufacturer value is entered. Always check the data plate on the outdoor unit for the manufacturer's specified target.

This is one of the most common confusion points observed across live training events. Students assume the target is always 10, then wonder why the app flags a system as failing when the charge is correct. The AI profiler sometimes misses the subcooling target when reading data plates. If the data plate says 8, or 15, or any value other than the default, enter that manually in the system profile. The diagnostic pass/fail depends on it.

Readings fluctuate and will not settle

Allow the system at least 10-15 minutes of continuous runtime before evaluating superheat or subcooling. Variable-speed and multi-stage systems may take longer. If readings remain unstable after 20 minutes, the instability itself may indicate a problem: intermittent TXV, restriction, or airflow issue.

Should I override a failing charge result?

Only if you have a specific technical reason. Valid reasons: the system was just charged and has not reached steady state, or you are aware of a condition the app cannot account for (non-standard refrigerant blend, extreme ambient conditions outside the app's calculation range). If the system has been running for 15+ minutes and the reading is still outside the acceptable range, that is likely a real finding. Document it.

Calculated pressure sources and superheat/subcooling

measureQuick can derive subcooling and superheat even when the pressure source is "Calculated" rather than a live pressure instrument. In these cases, the app uses the refrigerant type and known system conditions to estimate the saturation pressures. Tests with live pressure instruments produce more reliable superheat/subcooling values. If you see a charge failure on a test with calculated pressures, confirm with a physical manifold or wireless pressure probe before making service decisions.

Think total system performance, not just superheat and subcooling

As Brian Feenie explains in his HVAC Talk discussion: "I frame things as like total system performance, not subcooling, superheat, temp split, and charge." Superheat and subcooling are critical diagnostic inputs, but they are part of a larger picture. A system can have correct superheat and subcooling but still underperform due to airflow restrictions, duct leakage, or electrical issues. measureQuick's Vitals score (which requires 9+ physical probes) evaluates the system holistically rather than relying on any single measurement.

Reference Material

Gauge Screen Exercise

Gauge Screen Exercise - Page 1

Gauge Screen Exercise - Page 2

Related Articles

Prerequisites (complete these first):

• Pressure-Temperature Relationship

Follow-up articles (next steps after this one):

• Design Temperature Difference (DTD)
• Charge & Airflow Balance
• Recognizing Overcharge and Undercharge
• Metering Device Selection
• Target Zones

Related in the same domain:

• Refrigeration Cycle Basics
• A/C Service Workflow
• Quick Charge Mode
• System Stabilization
• Understanding Diagnostic Screens

Need Help?

If you get stuck or this article does not answer your question:

• Check the Related Articles section above
• Contact measureQuick support: support@measurequick.com