How do you tell if a TXV lost part of its bulb charge? If you are measuring a correct/high subcooling along with a high superheat and a low saturated temperature on the vapor line, there may be a liquid line restriction!
Usually when a TXV bulb looses its bulb charge, it looses it slowly. One of the most obvious visual signs that a TXV has lost its bulb charge is if the top of the txv is very rusted. That would be the leak point of the refrigerant.
If you are measuring a high superheat and are just not sure if the bulb is the culprit, you could try taking the bulb off of the vapor line and putting it in hot water. If suddenly the vapor pressure rises and the superheat lowers then this tells you that bulb is partially low on refrigerant but there is still enough refrigerant to push down on the head of the TXV. When the bulb pressure pushes down on the head of the TXV, it opens the orifice inside the TXV a bit and allows more refrigerant into the evaporator.
Remember that if the vapor pressure does not rise and the superheat does not lower, it may be a problem other than the TXV or it may be that the TXV bulb has leaked out all its pressure. The problem could also be the strainer screen or the filter drier. Usually, the first thing to check for when assuming there is a liquid line restriction is the filter drier. This can be checked by measuring if there is a temp drop across the drier. There should not really be any temp drop. If there is a significant drop, it is due to a restriction and pressure drop. Have fun HVACing!
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Best Wishes, Craig Migliaccio