Why does my pressure reading keep changing when I am testing superheat / subcooling?
If your readings are changing when measuring line pressure, in most cases it means the system has not reached a steady state. Suction line pressure continues to drop as a system is coming to equilibrium, while the liquid line pressure will continue to rise. A good rule of thumb is to let the system run for ~10-15 minutes before taking readings to determine actual superheat or subcooling (See “Q: When should I take my actual and target superheat/subcooling measurements?” earlier in the document). If pressure readings continue to fluctuate, check the system for leaks. If the system is stable and there are no leaks, your testing equipment itself might cause fluctuations in your pressure readings. Leaks in hoses or at a fitting are possible sources for fluctuations as well. A simple way to perform a hose test is to hook the unit up to a positive pressure source such as a refrigerant recovery unit and isolating the system (the pressure reading unit/gauge and the hose) from the recovery unit
