What You'll Learn

• How measureQuick evaluates refrigerant charge based on metering device type
• What the pass/fail indicator means and how it is calculated
• What the field data shows: 56.0% of piston systems fail charge evaluation, 45.4% overall
• How technician overrides work and what the 12.8% override rate means
• Common charge fault patterns and what they indicate
• How DTD (Condenser Temperature Over Ambient) corroborates charge diagnosis
• Why charge evaluation requires both pressure and temperature probes
• When a "Fail" result may not require immediate action

What You'll Need

• Device: iPhone (iOS 15+) or Android phone (Android 10+) with measureQuick installed
• Account: measureQuick account with active subscription
• Probes: Bluetooth-compatible pressure transducers (manifold or wireless) AND temperature clamps on suction and liquid lines at minimum
• System requirements: Running system with at least 7-8 minutes of stabilization time
• Familiarity: Superheat and subcooling concepts (see E3), overcharge/undercharge recognition (see E7), metering device types (see D5)
• Time: 15 minutes to read; ongoing reference during field diagnostics

How measureQuick Evaluates Charge

measureQuick does not use a single formula for all systems. The charge evaluation method depends on the metering device type you select during system profiling.

Piston (Fixed Orifice) Systems

For systems with a fixed metering device (piston or capillary tube), measureQuick evaluates charge using superheat. The app compares your measured superheat to a target superheat calculated from the system's operating conditions - primarily outdoor ambient temperature and return wet-bulb temperature.

The target superheat method is the industry standard for piston systems because the fixed orifice does not regulate refrigerant flow. Superheat responds directly to charge level: add refrigerant, superheat drops; remove refrigerant, superheat rises.

measureQuick flags the charge subsystem as Fail when measured superheat deviates more than approximately +/- 5F from the calculated target.

TXV (Thermostatic Expansion Valve) Systems

For systems with a TXV, measureQuick evaluates charge using subcooling. The TXV actively regulates superheat, so superheat is not a reliable indicator of charge level on these systems. Subcooling responds to the amount of liquid refrigerant in the condenser, which correlates with total system charge.

measureQuick flags the charge subsystem as Fail when measured subcooling deviates more than approximately +/- 3F from the manufacturer's specified target (typically 8-12F subcooling for most residential systems).

The tighter tolerance on TXV systems (+/- 3F vs +/- 5F) reflects the fact that subcooling is a more precise indicator. Small changes in charge produce measurable changes in subcooling.

measureQuick diagnostic screen showing refrigerant charge evaluation with superheat or subcooling highlighted, target range visible, and pass/fail indicator

What the Data Shows

measureQuick's V12 database contains 115,706 quality-filtered cooling tests. The charge evaluation results across this dataset:

More than half of piston-metered systems tested in measureQuick fail the charge evaluation. This is the single most common refrigerant-side fault in the field.

The 45.4% overall rate includes both piston and TXV systems. TXV systems fail at a lower rate because the expansion valve compensates for minor charge variations, and because TXV systems are more common in newer, higher-efficiency equipment that tends to be better maintained.

The override rate of 12.8% means that roughly one in eight charge evaluations is changed by the technician after the app makes its determination. Overrides go both directions - some technicians override a "Fail" to "Pass" (borderline readings, known conditions), and some override a "Pass" to "Fail" (other evidence suggests a charge problem the app did not flag).

Common Charge Fault Patterns

High Superheat on Piston = Undercharge

This is the most common charge fault pattern. When a piston system is low on refrigerant, less liquid reaches the evaporator. The refrigerant fully evaporates before it reaches the end of the evaporator coil, and the remaining coil surface superheats the vapor beyond the target.

Measured superheat of 20-30F on a system targeting 10-12F strongly suggests undercharge or a refrigerant leak. Check the system for leaks before adding charge.

Low Superheat on Piston = Overcharge

Too much refrigerant in a piston system floods the evaporator. Liquid refrigerant does not fully evaporate before reaching the suction line, resulting in very low superheat (below 5F) or even liquid flooding.

Low superheat on a piston system is less common than high superheat but more immediately damaging - liquid refrigerant returning to the compressor causes slugging.

High Subcooling on TXV = Overcharge or Liquid Line Restriction

On a TXV system, high subcooling (above the target by more than 3-5F) indicates excess liquid refrigerant stacking up in the condenser. Two possible causes:

• Overcharge. Too much refrigerant in the system. The TXV limits how much enters the evaporator, so the excess backs up in the condenser.
• Liquid line restriction. A partially blocked filter-drier, kinked liquid line, or other restriction between the condenser and TXV creates a pressure drop that looks like high subcooling at the condenser outlet.

To differentiate: check the temperature drop across the filter-drier. More than 2-3F temperature drop across the drier indicates a restriction, not overcharge.

Low Subcooling on TXV = Undercharge

Low subcooling (below the target by more than 3F) on a TXV system means insufficient liquid refrigerant in the condenser. The system is low on charge. Check for leaks.

How DTD Supports Charge Diagnosis

DTD, or Condenser Temperature Over Ambient (CTOA), measures the temperature difference between the liquid line leaving the condenser and the outdoor ambient air temperature. It provides corroborating evidence for charge assessment.

• High DTD (condenser running hot relative to ambient): Suggests the condenser is working harder than expected. Could indicate overcharge, dirty condenser, or restricted airflow over the condenser coil.
• Low DTD (condenser running cool relative to ambient): Suggests less refrigerant is being condensed. Could indicate undercharge.

DTD does not replace superheat or subcooling as the primary charge indicator. It serves as a second opinion. When superheat says "undercharge" and DTD confirms with a lower-than-expected value, you have more confidence in the diagnosis. When the two disagree, investigate further - there may be a secondary issue like a dirty condenser or non-condensables in the system.

measureQuick displays DTD on the diagnostic screen alongside the other refrigerant measurements.

Why Charge Evaluation Requires Pressure and Temperature Probes

measureQuick can calculate superheat and subcooling from temperature-only measurements if the refrigerant type is known - the app derives saturation temperatures from the pressure-temperature relationship for the selected refrigerant. However, this approach relies on an assumption: that the system contains only the specified refrigerant with no contaminants.

For a reliable charge evaluation, measureQuick needs:

• Suction and liquid line temperature clamps - for direct temperature measurement
• Suction and discharge pressure transducers - for actual saturation temperature calculation

With live pressure data, measureQuick calculates saturation temperatures from measured pressures rather than assuming them. This catches conditions that temperature-only measurement misses:

• Non-condensable gases in the system (raise measured pressures above what the refrigerant alone would produce)
• Mixed refrigerant charges (actual P-T relationship does not match the assumed refrigerant)
• Compressor valve issues (abnormal pressure ratios)

When measureQuick has only temperature data and no pressure data, it notes this limitation. The source_pressure_condenser IS NULL flag in the database indicates tests where no physical pressure instrument was connected. These tests can still provide superheat and subcooling values, but they are calculated rather than directly measured, and they carry more uncertainty.

Outdoor Measurements form showing pressure and temperature fields

When "Fail" May Not Require Immediate Action

The charge subsystem can flag "Fail" in situations where the system is actually acceptable or where the charge reading is misleading:

Borderline Readings

Superheat at 18F on a system targeting 12F (+/- 5F tolerance) is technically a fail at 1F outside the range. After a few more minutes of stabilization, the reading may drift into the pass zone. If the system has not run for at least 7-8 minutes, readings may not reflect steady-state operation.

Extreme Ambient Conditions

Very high outdoor temperatures (above 100F) or very low temperatures (below 60F for cooling mode) push the system outside its normal operating envelope. Target superheat calculations assume a reasonable operating range. At extremes, the calculated target may not reflect realistic charge behavior.

Low Airflow Masking as Charge Fault

A dirty evaporator coil or restrictive filter reduces airflow across the evaporator. Reduced airflow lowers the evaporator's ability to absorb heat, which reduces suction pressure and raises superheat. This looks like undercharge on a piston system but is actually an airflow problem.

Run a static pressure screening (H1) before concluding that a charge failure is truly a charge problem. If TESP is high, fix the airflow issue first and re-evaluate charge.

Recently Charged System Not Yet Stabilized

If you just added or removed refrigerant, allow the system 10-15 minutes to redistribute and stabilize before evaluating the pass/fail result. Charge migration through the system takes time, and readings during redistribution are not reliable.

Video Walkthrough

• YouTube (measureQuick): (4,695 views, 9:22). How measureQuick's diagnostic engine works internally, including the refrigerant charge evaluation logic and fault classification

• YouTube (third-party): (13,085 views, 80 min). Full cooling commissioning walkthrough including charge evaluation, flag interpretation, and discussion of superheat vs subcooling methods

Tips & Common Issues

The app says "Fail" but the system seems to be cooling fine

A system can cool adequately with a moderate charge fault. Undercharged by 10-15% on a mild day, the system still produces cold air, but it works harder, runs longer, and wears the compressor faster. The charge evaluation catches the problem before it becomes a comfort complaint or compressor failure.

I only have temperature probes - no pressure gauges connected

measureQuick will still calculate superheat and subcooling using the selected refrigerant's pressure-temperature relationship, but the results carry more uncertainty. For a definitive charge evaluation, connect pressure transducers. Temperature-only evaluation is better than no evaluation, but it cannot detect non-condensables, mixed charges, or compressor valve problems.

Should I override the charge result if I just recharged the system?

No. Run the test-in first with whatever the system shows. Perform your repair. Then run a test-out. The before-and-after comparison is more valuable than a single overridden result. The test-in documents the fault; the test-out documents the fix.

My superheat is high but I suspect a TXV problem, not a charge issue

A failed or stuck-open TXV will produce high superheat because it is not metering refrigerant properly. If you suspect a TXV issue, check the TXV bulb location and contact, verify the sensing bulb is properly insulated and clamped to the suction line, and check for a plugged equalizer line. measureQuick evaluates charge based on the metering device you selected - it does not detect TXV mechanical failure directly. Your professional judgment matters here.

The override rate is 12.8% - does that mean the app is wrong 12.8% of the time?

Not exactly. Overrides include technicians who override a "Fail" on a borderline reading they believe is acceptable, technicians who override a "Pass" because other evidence suggests a problem, and cases where environmental conditions make the reading unreliable. The override mechanism is a designed feature that combines the app's analytical engine with the technician's field experience.

Reference Material

Download: Field Checklists Combined (PDF)

Related Articles

Prerequisites (complete these first):

• Superheat & Subcooling - What these measurements are and how they relate to charge
• Overcharge/Undercharge Recognition - Identifying charge faults from measurement patterns
• Metering Device Selection - How metering device type determines the charge evaluation method

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

• Fault Types: Minor vs Major - How charge faults fit into the broader fault classification system
• Understanding the Vitals Score - How charge evaluation affects the overall system score
• mQ Assist (AI Diagnostics) - Context-aware guidance that considers charge alongside other measurements

Related in the same domain:

• Component Pressure Drops - Diagnosing airflow issues that can mimic charge faults
• TESP Budget & 140% Rule - Airflow evaluation to rule out airflow-masking-as-charge issues

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
• Schedule a training session with the measureQuick training team