Specifications: These air checkers are very easy to install. Replace the valve caps and your done. How often do you check the pressure in your tyres? Tyre pressures should be checked every week as an under inflated tyre can seriously effect on the handling of your cars, motorcycles, vans. When the tyre is correctly inflated the air checker allows a green indicator to be clearly visible in the viewing window, showing that the tyre pressure is correct. If the air checker reduces by 15% the green indicator is replaced by a yellow one, giving a warning that the pressure is reducing. If there is a 30% reduction in tyre pressure then a red indicator becomes clearly visible, giving a clear warning that the tyre pressure is now dangerously low. Minimum order:500pcs $9.00 a set(set 4 pieces)