In the standard liquid-line sequence, which component provides a visual indication of liquid refrigerant presence?

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In the standard liquid-line sequence, you place a sight glass in the liquid line after the receiver so you can visually confirm that liquid refrigerant is present before it reaches the expansion device. This gives a quick, on-system check of the refrigerant state: clear liquid seen through the glass means liquid is present; bubbles or a milky appearance can indicate gas in the line or charging issues. Some sight glasses also include a moisture indicator that changes color if moisture is present, signaling the dryer’s condition.

The other components don’t provide this visual check: a king valve is just a manual shutoff to isolate parts of the system; a filter dryer removes moisture and contaminants (and may have a dryness indicator in some designs, but its main role is filtration); a solenoid valve is an electrically actuated control valve that governs flow rather than indicating the refrigerant’s state.