What You'll Learn

• The specific data patterns that indicate overcharge and undercharge on piston (fixed orifice) systems
• The specific data patterns that indicate overcharge and undercharge on TXV (thermostatic expansion valve) systems
• Why the primary charge indicator differs between piston and TXV metering devices
• What measureQuick's V12 data reveals about charge failure rates in the field (56.0% piston failure)
• How measureQuick flags charge issues and what pf_charge pass/fail means
• How to distinguish a real charge problem from an airflow-induced false reading

What You'll Need

• Device: iPhone (iOS 15+) or Android phone (Android 10+) with measureQuick installed
• Account: measureQuick account with active subscription
• Probes: Wireless pressure probes (manifold gauges), temperature clamps (suction line, liquid line, return air, supply air)
• Understanding of: Superheat and subcooling (E3), charge-airflow interaction (E6)
• Airflow verified: TESP within the equipment's rated maximum and CFM/ton in the 350-450 range before evaluating charge
• Time: 10-15 minutes to read and understand

Metering Device Determines Your Primary Indicator

The type of metering device installed in the system determines which measurement is your primary charge indicator. This is not a preference; it is how the physics works.

• Piston (fixed orifice): Superheat is the primary charge indicator. Subcooling provides supporting data.
• TXV (thermostatic expansion valve): Subcooling is the primary charge indicator. The TXV actively regulates superheat, so superheat is not a reliable charge indicator on TXV systems.

measureQuick handles this automatically. When you profile the system and select the metering device type, the app adjusts its target ranges and diagnostic logic accordingly. As Jim Bergmann explains when walking through the profile setup: "I set the type of metering device - thermostatic expansion valve, piston, capillary tube, the electronic expansion valve, or automatic expansion valve." The metering device selection directly controls which charge indicator measureQuick prioritizes.

Piston Systems: Superheat Is Primary

On a fixed orifice system, the metering device is a simple restriction (a brass plug with a calibrated hole, or a short capillary tube). It does not adjust to changing conditions. The amount of refrigerant that flows through it depends on the pressure difference across it.

This means superheat responds directly to charge level:

Piston Overcharge Pattern

What it looks like in measureQuick: Superheat reads below the target zone (green range). The superheat indicator turns yellow or red on the low side. Subcooling reads above the target zone. The diagnostics may flag "possible overcharge" or "low superheat."

measureQuick diagnostics screen on a piston system showing low superheat, high subcooling, with charge indicators flagged

Piston Undercharge Pattern

What it looks like in measureQuick: Superheat reads above the target zone. The superheat indicator turns yellow or red on the high side. Subcooling reads below the target zone. The diagnostics may flag "possible undercharge" or "high superheat."

Piston Target Superheat

measureQuick calculates the target superheat based on the outdoor ambient temperature, return air wet bulb, and the equipment profile. The allowable range is typically plus or minus 5 degrees F from the calculated target. As Jim Bergmann notes: "it's a plus or minus 5 degrees of superheat, so we look at 5 degrees of saturation, that's an allowable range."

TXV Systems: Subcooling Is Primary

A thermostatic expansion valve actively modulates the refrigerant flow to maintain a set superheat at the evaporator outlet. The sensing bulb on the suction line controls the valve opening. When superheat rises, the valve opens more. When superheat drops, the valve closes.

Because the TXV regulates superheat, superheat stays relatively stable regardless of charge level (within the TXV's operating range). The charge condition shows up in subcooling instead:

TXV Overcharge Pattern

What it looks like in measureQuick: Subcooling reads above the target zone. The subcooling indicator turns yellow or red on the high side. Superheat may remain in or near the acceptable range because the TXV is compensating. This is why subcooling is the indicator to watch.

TXV Undercharge Pattern

What it looks like in measureQuick: Subcooling reads below the target zone. The subcooling indicator turns yellow or red on the low side. On a severely undercharged TXV system, superheat will also rise because the valve is fully open and still cannot supply enough refrigerant.

TXV Target Subcooling

Typical subcooling targets range from 8 to 15 degrees F, depending on the manufacturer's specification. measureQuick uses the equipment profile to determine the target. When you profile the system as having a TXV, the app shifts its primary charge evaluation to subcooling.

What the V12 Data Shows: 56.0% Piston Charge Failure

measureQuick's V12 database of 115,706 quality-filtered cooling tests reveals that 56.0% of piston systems fail the charge evaluation. This is the charge failure rate after data quality filtering, on systems where airflow and other conditions were assessed.

This number is high, and it reflects the real condition of the installed base. More than half of the piston systems tested by measureQuick users had a detectable charge problem. The reasons include:

• Incorrect initial charge at installation. Systems charged by weight without verifying operating conditions.
• Refrigerant leaks over time. Slow leaks that reduce charge gradually.
• Prior technician adjustments without airflow verification. The misdiagnosis cycle described in Charge & Airflow Balance.
• System modifications without charge adjustment. Line set changes, coil replacements, or equipment swaps without recalculating the charge.

These are not edge cases. This is the state of the residential HVAC installed base.

How measureQuick Evaluates Charge: pf_charge

measureQuick records a pass/fail result for charge in the pf_charge field. This evaluation uses the following logic:

• Piston systems: Compares measured superheat to the calculated target superheat, considering the allowable tolerance (typically +/- 5 degrees F).
• TXV systems: Compares measured subcooling to the target subcooling range for the equipment.

A Pass means the primary charge indicator is within the acceptable range for the metering device type and operating conditions.

A Fail means the primary charge indicator is outside the acceptable range. The measurement record captures the actual values so you can see how far outside the range the system is.

The pf_charge_override field indicates whether the technician manually changed the pass/fail result. An override of 1 means the technician disagreed with the automated evaluation and changed it. This might happen when the technician knows about a condition (such as a restricted TXV) that explains the reading.

Distinguishing Real Charge Problems From False Readings

Before concluding that a system has a charge problem, confirm:

1. Airflow is verified. TESP is within the rated maximum and CFM/ton is in the 350-450 range. See Charge & Airflow Balance.
2. The system is stabilized. Readings taken before the system reaches steady state are unreliable. Wait 7-10 minutes of run time after startup. As Jim Bergmann notes, "it takes about seven to eight minutes to actually get stabilized."
3. Outdoor temperature is within the valid test range. Charge evaluation is unreliable at very low outdoor temperatures (below about 55 degrees F for cooling) because the condenser cannot reject enough heat.
4. The correct metering device is selected in the profile. If you profile a TXV system as a piston system, the target calculations will be wrong and the evaluation will be incorrect.
5. The TXV is functioning. On a TXV system with a stuck or failed valve, superheat and subcooling patterns will not follow the expected TXV behavior. A stuck-open TXV floods the evaporator (low superheat, low subcooling). A stuck-closed TXV starves it (high superheat). measureQuick may flag a TXV sensing bulb issue if the pattern matches.

Video Walkthrough

• YouTube: - Jim Bergmann demonstrates how measure quick evaluates charge on both piston and TXV systems, explaining: "we record the superheat of the sub cooling, primarily on a TXV system we'll record a sub cooling because there's a range."

• YouTube: (21:52) - Complete charging procedure with measurement interpretation for both metering device types

• YouTube: (9:22, 4,695 views) - Explains the diagnostic logic measureQuick uses to evaluate system performance, including how charge indicators are assessed

• YouTube: - Jim Bergmann walks through the full app, including how profiles and metering device selection affect charge target calculations

Tips & Common Issues

Superheat is in range but subcooling is high on a piston system

On a piston system, superheat is primary, but subcooling still provides useful information. If superheat is within the target range but subcooling is elevated, the system may be slightly overcharged but not enough to push superheat out of range. It can also indicate a partially restricted condenser or a dirty condenser coil. Check condenser airflow and coil condition.

Subcooling is nearly zero on a TXV system

Very low or zero subcooling on a TXV system is a strong indicator of significant undercharge, a major refrigerant leak, or a restriction in the liquid line before the TXV. This condition starves the TXV. The system needs a leak check before adding refrigerant.

The system is a heat pump - which readings do I use?

In cooling mode, a heat pump uses the same charge evaluation as a standard A/C: the indoor metering device type determines the primary indicator. In heating mode, the outdoor metering device type applies. measureQuick handles this automatically when you select the correct mode and profile.

measureQuick says "charge pass" but the system is not cooling well

A passing charge evaluation means the refrigerant side is within the acceptable range. Poor cooling with a passing charge usually points to an airflow problem, an oversized or undersized system, duct leakage, or a building envelope issue. Check TESP, CFM per ton, and the temperature split (return minus supply dry bulb).

I see the term "CTOA" in training materials

CTOA stands for Condenser Temperature Over Ambient. It is the difference between the condensing (saturation) temperature and the outdoor ambient temperature. Typical design CTOA is about 30 degrees F. An elevated CTOA with normal subcooling may indicate a dirty condenser or non-condensable gases in the system. measureQuick calculates and displays CTOA as part of the diagnostic screen.

Related Articles

Prerequisites (complete these first):

• Superheat & Subcooling - How measureQuick calculates these values and what they represent
• Charge & Airflow Balance - Why airflow must be verified before evaluating charge

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

• AC Cooling Commissioning Workflow - Complete commissioning process including charge adjustment
• Quick Charge Mode - Using measureQuick's guided charging feature
• Understanding Diagnostic Flags - What measureQuick's flags mean and how to interpret them

Related in other domains:

• Selecting the Metering Device Type - How to identify and profile the metering device
• Airflow: CFM per Ton - Verifying airflow before charge evaluation
• Probe Placement: Cooling - Where to place temperature clamps for accurate charge readings
• Static Pressure Screening - Quick airflow assessment before charge diagnostics

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