 At heat loads which are very low compared to the valve's ton of refrigeration rating, the orifice can become oversized for the heat load, and the valve can begin to repeatedly open and close, in an attempt to control the superheat to the set value, making the superheat oscillate.Cross charges, that is, sensing bulb charges composed of a mixture of different refrigerants or also non-refrigerant gases such as nitrogen (as opposed to a charge composed exclusively of the same refrigerant inside the system, known as a parallel charge), set so that the vapour pressure vs temperature curve of the bulb charge "crosses" the vapour pressure vs temperature curve of the system's refrigerant at a certain temperature value (that is, a bulb charge set so that, below a certain refrigerant temperature, the vapour pressure of the bulb charge suddenly becomes higher than that of the system's refrigerant, forcing the metering pin to stay into an open position), or even different kinds of bleed passages that generate a minimum refrigerant flow at all times, help to reduce the superheat hunt phenomenon by preventing the valve orifice from completely closing, at the cost, however, of determining a certain flow of refrigerant that won't reach the suction line in a fully evaporated state while the heat load is particularly low, and that the compressor must be designed to handle.A thermal expansion valve is a key element to a heat pump; this is the cycle that makes air conditioning, or air cooling, possible.