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Fridges and air conditioners work in similar ways. They use refrigerant to keep food fresh and air temperatures indoors bearable. Fridges rely on evaporation, or the constant compressing and depressurising of refrigerant to expel hot air outside. The same goes for air conditioners. Refrigerant collects heat inside the house, gets hot, transfers that heat outside, and cools.  Of course, there’s more science to this, but both appliances are effective at what they do. That is until there’s a drop in refrigerant or coolant pressure. 
In cases like these, you’ll see HVAC and refrigeration technicians use a manifold gauge. This is an instrument that measures the pressure of refrigerant in the system, assesses if there are leaks, and helps maintain the right pressure when the refrigerant or other coolant is topped up or replaced. 
Manifold gauges might not be everyday household tools, but they’re the spanners and hammers of HVAC and fridge diagnosis and maintenance. As such, there are more than a few types. Gauges differ by the number of ports they have and the accessories that can be attached to these ports. There are also analogue and digital types. Both are accurate but there are pros and cons to each. Digital manifold gauges from respected brands like German instrument giant Testo offer higher accuracy, unrivalled ease of use, and speedy operation. They also work with different types of refrigerants, so are also more versatile.