The achievement "Entry-Exit Relay Repeater System" wants you to pass back and forth through the same portal pair 30 times without hitting anything - neither the ground, nor bridge elements, nor test objects. The easiest chamber to achieve this is chamber 20. Just hit the "drive" button and wait for your vehicle to pass the first portal 30 times. Be aware of the achievement won't pop up, so you have to count carefully (just spend five minutes waiting) and then leave the level to get the achievement.