Triple Point In Water and CO2 w/ Andrew G. and Matthew T.

In this episode, Bryan is joined by Matthew Taylor, a supermarket refrigeration specialist whose expertise centers on CO₂ rack systems, and Andrew Greaves, Director of User Experience at NAVAC Tools, for the third installment of their series on triple-point evacuation. The conversation expands beyond water to bring CO₂ into the picture, exploring where the two refrigerants behave similarly around the triple point and “more importantly” where the differences can create serious field problems. Whether you’re pulling vacuums on residential equipment or servicing transcritical CO₂ racks at a grocery store, understanding what actually happens at these phase boundaries will change how you approach the work.

The triple point of water sits at 4,579 microns, corresponding to just above 32°F (273 Kelvin). Andrew shares a fascinating piece of metrology history: for decades, the Kelvin was formally defined as one two-hundred-and-seventy-third of the triple point of water, making it the most reproducible temperature constant ever established. In practical HVAC terms, this means that any time a technician hits industry-standard evacuation targets, they pass right through the triple point of water “every single time.” The danger isn’t the crossing itself but what happens when moisture is present in a cold ambient: the vacuum pump can create ice, and because sublimation requires enormous energy input from the surrounding environment, that ice can persist and deliver a falsely passing vacuum reading. Andrew explains why a decay test is the real proof of a dry system, since sublimating ice will continue to raise the micron reading after the pump is isolated. The conversation also covers how micron gauges actually work: they measure thermal conductivity, not pressure, which is why refrigerant vapor entrained in compressor oil can cause wild, erratic gauge behavior that mimics both moisture and a leak simultaneously.

For CO₂, the triple point arrives at roughly 70°F and 75 PSI “conditions that are easy to stumble into in the field.” Matthew explains that in an operating transcritical CO₂ system, the triple point itself isn’t the daily concern; the danger comes when a technician relieves pressure too quickly and liquid CO₂ flashes right through the triple point, instantly forming dry ice inside the lines and creating a hard plug. On the opposite end of the scale, CO₂ faces a critical point at just 87°F and 1,055 PSI, something that is essentially unreachable for most conventional refrigerants but is a routine operating reality in warm climates. Matthew walks through exactly what happens in a transcritical system: above the critical point, the CO₂ behaves as neither liquid nor vapor, the PT chart no longer applies, and a conventional condenser becomes a “gas cooler” that rejects heat but cannot condense the refrigerant. The refrigerant must be forced through a high-pressure valve to drop it back below the critical point, where it flashes instantly into liquid inside a flash tank. The episode also covers the “burping” behavior of CO₂ pop-off valves during high-ambient conditions, the dangers of isolating liquid CO₂, and how grocery stores have evolved from keeping spare CO₂ cylinders on hand to battery-powered fractional compressor systems that keep the flash tank subcritical through power outages.

The episode wraps up with takeaways that apply across both refrigerant worlds. Big hoses and fast pumps don’t eliminate moisture problems; they can actually create a false-confidence scenario where ice forms, the system still pulls deep, and the technician believes the job is done. The decay test remains the only reliable proof of dryness, and nitrogen sweeps serve multiple purposes: adding turbulence to help release refrigerant from oil, displacing refrigerant from the micron gauge sensor, and temporarily raising the system back above the triple point so ice converts to liquid before being removed as vapor. The group also briefly touches on the emerging CO₂ carbon capture industry, where the triple point is used intentionally to separate and harvest CO₂ from industrial flue gases. The world of CO₂ service is growing fast, from niche grocery racks to mini-split-sized transcritical units, and the tooling needed to work on the high-pressure side hasn’t fully caught up yet.

Topics Covered

• Triple point of water: 4,579 microns / ~32°F / 273 Kelvin, and its historic role defining the Kelvin scale
• Why technicians pass through the triple point of water during every standard evacuation
• How ice forms during deep vacuum pulls and why sublimation is slow without adequate heat input
• How micron gauges measure thermal conductivity, not pressure, and what causes erratic readings
• Distinguishing moisture, refrigerant-in-oil, and actual leaks during a decay test
• Triple point of CO₂: ~70°F / 75 PSI, and how dry ice forms when liquid CO₂ is vented through it
• Why liquid CO₂ must never be isolated and the dangerous pressure implications if it is
• The critical point of CO₂ at 87°F / 1,055 PSI and why transcritical operation is unavoidable in warm climates
• How transcritical CO₂ systems work: gas coolers, high-pressure valves, and flash tanks
• CO₂ system operating pressure ranges across the suction, liquid, and discharge sides
• Pop-off valves and the “burping” phenomenon in transcritical CO₂ systems during high-ambient conditions
• Grocery store power outage scenarios and battery-powered CO₂ subcooling solutions
• The role of crankcase heaters when pulling a vacuum on systems with oil-logged refrigerant
• Nitrogen sweeps: turbulence, sensor displacement, and temporary triple-point recovery
• Heated nitrogen as an emerging technique for improving vacuum quality
• CO₂ carbon capture: using the triple point industrially to harvest CO₂ from flue gases
• The rapid growth of smaller transcritical CO₂ systems and the current gap in high-side service tooling

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