TXV here requires a column of liquid from the condenser. When the refrigerant is low there are bubbles in the Liquid line, the high-pressure side. The two clamps on the electronic sightglass sense when the line is full of liquid, no bubbles. The device beeps once a second once the high side is full of refrigerant. The bar of LEDs is like the bubbles, a visual clue that the rrefrigerantis low. Thei device was bought about 21 years ago new from Grainger.