The handle of the the Astro Pneumatic Tool 3018 3.5″ Digital Tire Gauge serves two purposes. It will both inflate and deflate tires to the desired pressure. To inlfate the tire as desired, simply squeeze the handle completely and air will begin rushing out through the tire chuck and into the device being inflated. If you accidentally allow overshoot your pressure target, or have something that is over inflated, squeeze the handle halfway and the 3018 will allow the pressure to escape. Release the handle when you are back down to the desired pressure and you are finished.
Owning this tool for over a year now, I have am completely satisfied with the design and operation. I am still using the batteries that came installed on the tool. When they finally run out, I may replace with a set of rechargeable AAA batteries. I have also used this in all types of temperature environments the tool seems stable and reliable whether it is in the middle of summer, or winter. I will say that I have found it works best to always turn on the tool and allow it to zero itself out before use. Hook up to the air hose, turn the unit on, and then connect the tire chuck to the valve stem. It just seems more accurate that way.
Below are pictures of the display, both outside with the back light off, and inside with the back light on. Although I will admit the back light was not necessarily needed with the lighting in the picture, you can see how nice and bright this tool will be if you are using it in the dark.