Five Parameters Instead of One Gauge: How Digital Refrigerant Gauges Are Changing Field Diagnostics

Low evaporator-side pressure does not necessarily mean a refrigerant shortage. It may be caused by a clogged metering device, a disruption in liquid-phase supply, or a heat-exchange anomaly in the evaporator. To correctly isolate the fault, a service engineer needs a set of five parameters, not a single gauge reading.

Five Parameters of Field Diagnostics

Analysis of the refrigeration circuit relies on the following values:

• Pressure on the evaporator side (low) and on the condenser side (high) — combined with the refrigerant type, these allow determination of saturation temperatures.
• Evaporator saturation temperature — calculated from the low-side pressure; characterises the state of the evaporator. A low value requires consideration of superheat, unit load, liquid-phase supply, and heat-exchange conditions before concluding that the evaporator is faulty.
• Condenser saturation temperature — calculated from the high-side pressure; assesses the efficiency of heat rejection in the condenser. A high value requires checking cooling conditions, fan operation, and cooling-water flow rate and temperature.
• Superheat (SH) — suction-line temperature minus evaporator saturation temperature; characterises the state of the refrigerant at the evaporator outlet.
• Subcooling (SC) — condenser saturation temperature minus liquid-line temperature; characterises the state of the liquid refrigerant at the condenser outlet.

What a Digital Gauge Adds Over a Mechanical One

A traditional mechanical gauge records only evaporator- and condenser-side pressure. For in-depth analysis, the engineer must separately measure pipe temperatures, consult pressure–temperature tables, and manually calculate superheat and subcooling.

A digital (electronic) refrigerant gauge combines these operations:

• automatic calculation of saturation temperatures from measured pressure;
• real-time SH and SC calculation when temperature sensors are connected;
• data storage and transfer via Bluetooth, and report generation.

In essence, a mechanical gauge measures pressure, whereas a digital gauge is a complete system: "pressure measurement + temperature measurement + automatic calculation + data archiving."

Testo 550s: Specifications and Features

The Testo 550s is positioned as a professional tool for inspection, commissioning, and acceptance of air-conditioning and heat-pump systems. Key specifications:

• dual-channel simultaneous high- and low-pressure measurement; range −1 to 60 bar; accuracy ±0.5 % of full scale; real-time display of pressure fluctuations;
• built-in refrigerant database (dozens of types); automatic saturation-temperature calculation upon refrigerant selection;
• temperature-sensor connection for real-time SH/SC calculation; pressure, saturation temperatures, pipe temperatures, and calculated values displayed on a single screen;
• Bluetooth connectivity with the testo Smart App: remote viewing, data storage, and generation of a standardised maintenance report;
• support for nitrogen leak-tightness testing with pressure and temperature curve logging;
• compatibility with the wireless vacuum sensor testo 552i for monitoring and saving the vacuum-pump-down curve.

The kit covers the full cycle: commissioning, vacuum pump-down, leak-tightness testing, and report generation. It is adapted for multi-zone systems, air-source heat pumps, cold storage, and chillers.

Practical Diagnostic Scenarios

When evaporator-side pressure is low, the combination of the remaining parameters narrows the range of possible causes:

• low pressure + elevated superheat + low subcooling — probable refrigerant shortage;
• low pressure + reduced superheat — check evaporator load and liquid-phase supply; rule out the risk of liquid return to the compressor;
• low pressure + elevated superheat + normal or elevated subcooling — check the metering device and filter-drier for blockage.

The digital gauge consolidates all parameters into a single report, eliminating the need for field reference tables and manual calculations.

Source: 制冷百科, November 5, 2026.