What You'll Learn

• How each stage of the refrigeration cycle maps to specific measureQuick measurement fields
• Which mQ columns store pressures, temperatures, and calculated values for each cycle stage
• How the app derives condensing temperature, evaporating temperature, superheat, and subcooling from raw probe data
• Why the metering device type (TXV vs piston) changes which diagnostic target matters
• How source columns track which probe brand captured each measurement
• What the pf_refrigerant pass/fail result tells you about charge state
• How real-world mQ data shows a 45.4% charge failure rate across 115,706 quality-filtered cooling tests

What You'll Need

• Device: iPhone, iPad, or Android with measureQuick installed
• Account: Any measureQuick account (free or Premier)
• Smart tools: Pressure probes and temperature clamps connected via Bluetooth (see Smart Tool Overview)
• Knowledge: Working familiarity with the vapor-compression cycle. This article does not teach the cycle from scratch; it maps it to mQ's measurement framework.
• Time: 10-15 minutes to read

New in mQ 3.6: System View provides an interactive, real-time visualization of the refrigeration cycle using your live measurement data. As you read about each stage below, System View shows that exact component with tappable detail. See System View: Interactive Thermal Cycle Visualization. System View is available to all users (free and paid).

The Refrigeration Cycle in measureQuick

You already know the four stages: compression, condensation, expansion, evaporation. measureQuick captures measurements at specific points in this cycle and uses them to calculate system health. Every field on the diagnostic screen corresponds to a physical location in the refrigerant circuit.

This article walks through each stage, identifies which mQ fields map to it, explains where those readings come from, and shows what they tell you about the system.

mQ diagnostic screen with labeled measurement points corresponding to cycle stages

Stage 1: Compression

What happens: The compressor draws in low-pressure, low-temperature refrigerant gas from the suction line and compresses it into high-pressure, high-temperature gas. This is the energy input that drives the entire cycle.

mQ Measurement Fields

Where the Readings Come From

Suction and discharge pressures come from your manifold gauges or wireless pressure probes. The source_pressure_condenser and source_pressure_evaporator columns record which probe brand captured each reading (e.g., "Fieldpiece," "Testo," "NAVAC").

If source_pressure_condenser is NULL, no physical pressure instrument was connected for that measurement. The app can still calculate some diagnostics using temperature-only methods, but pressure data produces more accurate results.

What It Tells You

Compression ratio indicates how hard the compressor is working. A normal compression ratio for residential A/C is typically 2.5:1 to 3.5:1. Higher ratios mean the compressor is working harder, which can indicate a dirty condenser, restricted airflow, or refrigerant overcharge. Lower ratios may indicate undercharge or a failing compressor. As Jim Bergmann explains in the "Enthalpy Method" discussion: with high-efficiency systems, you are looking at compression ratio back at the compressor to evaluate whether the system is operating within its design envelope.

mQ diagnostic screen showing suction pressure, discharge pressure, and compression ratio

System View (3.6): In System View, tap the compressor to see gas property information including inlet/outlet speed indicators. The visualization shows refrigerant flow density and speed changing as gas enters and exits the compressor, reflecting the compression process in real time.

Stage 2: Condensation

What happens: Hot, high-pressure gas from the compressor enters the condenser coil (outdoor unit on a split system). The refrigerant rejects heat to the outdoor air, changes phase from gas to liquid, and leaves the condenser as a high-pressure liquid.

mQ Measurement Fields

How mQ Calculates Condensing Temperature

measureQuick does not measure condensing temperature directly. It calculates it from two inputs:

1. Condenser pressure (pressure_condenser) from your high-side probe
2. Refrigerant type (refrigerant) from the system profile

Every refrigerant has a known pressure-temperature relationship. At a given pressure, the saturation temperature (the temperature at which the refrigerant changes phase) is fixed. The app looks up the saturation temperature for your measured pressure and refrigerant type. That is the condensing temperature.

This is why the system profile matters. If the wrong refrigerant is selected, every calculated value downstream is wrong.

Subcooling: The Key Condenser Diagnostic

Subcooling = condensing temperature - liquid line temperature.

Subcooling tells you how much the liquid refrigerant has cooled below its saturation point after fully condensing. For TXV systems, subcooling is the primary charge indicator. Target subcooling varies by manufacturer, but a common residential target is 10-15F.

• Low subcooling typically indicates undercharge. Not enough refrigerant in the condenser to fully subcool the liquid.
• High subcooling typically indicates overcharge, a restricted condenser, or reduced airflow across the outdoor coil.

The source_temp_liquid_line column records which probe brand captured the liquid line temperature reading.

mQ condenser section showing condensing temp, liquid line temp, and subcooling calculation

System View (3.6): In System View, tap the condenser coil to see its current state, including the liquid seal level. The visualization shows refrigerant transitioning from gas to liquid through the coil. Tap the subcooling display to see the live subcooling value and how it is calculated. Tap the liquid line for liquid line temperature details.

Stage 3: Expansion

What happens: High-pressure liquid from the condenser passes through the metering device, which drops the pressure and temperature rapidly. The refrigerant enters the evaporator as a low-pressure, low-temperature mixture of liquid and gas.

mQ Measurement Fields

No Direct Measurement at This Point

measureQuick has no probe at the metering device itself. The expansion happens inside the system between the liquid line and the evaporator inlet. You cannot clamp a temperature probe on the metering device orifice.

What mQ does capture is the type of metering device, and this is critical. The metering device type determines which diagnostic target matters for charge verification:

• TXV (thermostatic expansion valve): Use subcooling as the primary charge indicator. The TXV regulates superheat, so superheat stays relatively constant regardless of charge level. Subcooling changes with charge.
• Piston (fixed orifice): Use superheat as the primary charge indicator. A piston has no regulation mechanism, so superheat responds directly to charge level.

In the mQ database, 56.0% of piston-metered systems fail the refrigerant charge test. This is the single most common diagnostic finding across all 115,706 quality-filtered cooling tests.

System View (3.6): In System View, the metering device updates dynamically based on your system profile. If you select a piston, the visualization shows a fixed orifice. If you select a TXV, it shows the TXV with its sensing bulb and external equalizer line. Changing the metering device type in the profile immediately updates the visualization, so you can see how TXV and piston systems differ in their refrigerant flow behavior.

Stage 4: Evaporation

What happens: Low-pressure refrigerant enters the evaporator coil (indoor unit). It absorbs heat from the indoor air, changes phase from liquid to gas, and leaves the evaporator as a low-pressure gas headed back to the compressor.

mQ Measurement Fields

How mQ Calculates Evaporating Temperature

Same method as condensing temperature, but on the low side:

1. Evaporator pressure (pressure_evaporator) from your low-side probe
2. Refrigerant type (refrigerant) from the system profile

The app looks up the saturation temperature for the measured suction pressure and refrigerant type. That is the evaporating temperature.

Superheat: The Key Evaporator Diagnostic

Superheat = suction line temperature - evaporating temperature.

Superheat tells you how much the refrigerant gas has heated above its saturation point after fully evaporating. For piston systems, superheat is the primary charge indicator.

• Low superheat typically indicates overcharge or excessive indoor airflow. Too much liquid is reaching the compressor (risk of liquid slugging).
• High superheat typically indicates undercharge, low airflow, or a restricted metering device. Not enough refrigerant is reaching the evaporator.

The source_temp_suction_line column records which probe captured the suction line reading.

Temperature Split: Quick Evaporator Check

Temperature split (temp_split) = return air temp - supply air temp.

A typical residential cooling temperature split is 14-22F. This is a fast field check. If the split is outside this range, something is wrong with the evaporator side of the cycle: low airflow, low charge, dirty coil, or other issues.

mQ evaporator section showing suction pressure, evaporating temp, suction line temp, superheat, and temperature split

System View (3.6): In System View, tap the evaporator coil to see refrigerant level and bubble density, which reflect evaporation activity in real time. Tap the superheat display for the live superheat value and calculation. The visualization also shows airflow arrows across the evaporator, the air filter with face velocity ranges (low, ideal, acceptable, warning, excessive), and the condensate line. Tap the condensate line to see the dehumidification rate in gallons per hour.

Additional Cycle-Related Measurements

These mQ fields relate to overall cycle performance rather than a single stage.

Pass/Fail Results

measureQuick evaluates each subsystem and assigns a pass or fail result.

• pf_refrigerant - Pass or Fail for refrigerant charge (based on superheat and/or subcooling targets)
• pf_capacity - Pass or Fail for delivered capacity vs nominal
• pf_efficiency - Pass or Fail for measured efficiency

The _override columns (e.g., pf_refrigerant_override) flag cases where the technician manually changed the app's automatic result. This happens when field conditions justify a different conclusion than the raw numbers suggest.

The Complete Picture: Probes to Cycle to Diagnosis

Here is how data flows from your probes through the refrigeration cycle model to a diagnostic result:

1. Probes stream raw data - pressure, temperature, humidity, airflow, electrical
2. mQ identifies the refrigerant - from your system profile (refrigerant column)
3. mQ calculates saturation temperatures - condensing temp from high-side pressure, evaporating temp from low-side pressure
4. mQ calculates superheat and subcooling - from saturation temps and line temps
5. mQ evaluates charge state - compares superheat/subcooling to targets based on metering device type
6. mQ assigns pass/fail - pf_refrigerant result stored in the test record

More connected probes means more complete calculations. measureQuick requires a minimum of 9 physical probe channels for a cooling/heating Vitals score. With fewer probes, the app can still calculate individual measurements, but the Vitals score will not generate.

Video Walkthrough

These videos cover the refrigeration cycle in the context of measureQuick diagnostics:

• YouTube (HVAC School): (18,030 views, 1:55). Covers compression ratio, capacity, efficiency, and sensible heat ratio across the refrigerant circuit. Topics include static pressure, refrigerant behavior, airflow, and system profiling

• YouTube (HVAC School): (15,014 views, 1:41). Common charging errors and considerations, covering refrigerant, airflow, static pressure, system profiling, and probe placement

• YouTube: (15,726 views, 45 min). Full commissioning workflow showing how cycle measurements flow through the app

• YouTube: (4,695 views, 9 min). How mQ calculates diagnostics from cycle measurements, including the pressure-temperature lookup

• YouTube: (18,992 views, 53 min). Deep dive into interpreting superheat, subcooling, and charge diagnostics

Tips & Common Issues

Wrong refrigerant in the system profile breaks everything

Condensing temperature, evaporating temperature, superheat, and subcooling are all derived from the refrigerant's pressure-temperature relationship. If R-410A is selected but the system runs R-22, every calculated value is wrong. Verify the refrigerant type before trusting calculated results.

Subcooling vs superheat: know which one to use

For TXV systems, subcooling is your primary charge indicator. For piston systems, superheat is your primary charge indicator. The metering_device field in the system profile controls which target measureQuick uses. If this field is wrong, the charge diagnosis will reference the wrong target.

Temperature-only diagnostics have limits

If source_pressure_condenser is NULL, mQ did not receive pressure data from a physical probe. The app can derive some diagnostics from temperature alone using known refrigerant properties, but pressure-based measurements are more accurate. Connect pressure probes when possible.

Suction line vs vapor line on heat pumps

On a standard cooling system, the suction line runs between the evaporator and the compressor. On a split system heat pump, the connecting lines are the vapor line and the liquid line. The vapor line can carry cool suction vapor or hot discharge gas depending on the mode of operation (heating or cooling). In measureQuick, you map an additional temperature clamp to the vapor line on a heat pump: one clamp on the true suction, one on the liquid line, and one on the vapor line. This distinction matters when interpreting cycle measurements on heat pump systems.

Probe placement affects calculated values

Liquid line and suction line temperatures must be measured at the correct locations. A liquid line clamp placed after a long line set in direct sun will read higher than the actual condenser outlet temperature, inflating your subcooling reading. See Outdoor Probe Placement and Indoor Probe Placement for correct positions.

Charge failure is the most common finding

Across the mQ database, 45.4% of quality-filtered cooling tests fail the refrigerant charge evaluation. For piston-metered systems specifically, the failure rate is 56.0%. If a system fails charge, the cycle measurements in mQ tell you exactly where the problem is: superheat and subcooling pinpoint whether the system is over- or undercharged.

Related Articles

Prerequisites:

• None

Follow-up articles:

• Pressure-Temperature Relationship
• Superheat & Subcooling
• Design Temperature Difference (DTD)
• Charge & Airflow Balance
• System View: Interactive Thermal Cycle Visualization - Real-time visualization of the refrigeration cycle using live measurement data (new in 3.6)
• Outdoor Probe Placement
• Indoor Probe Placement
• A/C Service Workflow

Need Help?

Contact measureQuick support: support@measurequick.com