[practicality ]This is a replacement cylinder for automatic window opener [excellent performance ]High sensitive, automatic respond to weather changes, so you don't need to stay near the greenhouse [how it works ]Non electric, uses solar energy, open or close the greenhouse vents as temperature rises or drops [applicablity ]Great for most greenhouse to open or close window automatically [high quality]Time and labor saving, also can supply a better temperature condition of plant's growth size: 33.5cm Material: galvanized sheet aluminum colour: black Package Contents: 2 x window opener Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.