Portal/Chamber 15

First room


The first room introduces you to the double fling.

Place an orange portal on the extended panel above the entrance, and create a blue portal on the floor. When you fall through, notice you tend to land on the checkered part of the floor. This time, create a blue portal on the floor on the checkered part, and when you fall through, aim to fall through the blue portal a second time, which will then give enough momentum to pass through the force field.

Second room


The second room has a pellet that needs to cross two forcefields. You can place a portal on the burn mark, however you cannot place a portal above the receiver without the one on the burn mark disappearing. Since to place the portal above the receiver, the other portal would have to be in that small area, you must get the pellet into that area first. Thus, place portals on the burn mark and another part of the wall, as shown in the picture, so that it flies into the other side of the room. Then, run to the receiver, and shoot a portal in front of both the pellet and the red light above the receiver. You must be sure to do this before the pellet disappears, or else you will have to start the process over.

Third room


As with the first room, you have to perform a double fling; however, you can't start the fling within the same room because of the flooring (technically you can (see ways to reduce portals below 14), but it's extremely difficult). Instead, after placing one portal on the high extended platform, you need to head to a side room to the left which has a staircase where you can create a portal and jump. When you go through, quickly fire the second portal (same color as the one on the other floor) on the checkerboard and you will be thrown through the force field.

Next, you have to perform a double fling to cross the second force field. Perform similarly to the first room.

You are now in a room where a pellet is bouncing against an angled wall, and scorching the side wall. If you try to place portals on the angled platforms, you will realize that not only is momentum conserved, but the angle of entry as well. Instead, shoot a portal at the scorch mark on the flat wall, as well as the same location on the opposite wall so that the pellet hits the other angled platform and continues to the receiver. Then double-fling yourself back to the other side.

You now need to cross a conveyor belt that's moving backwards, and a misstep will drop you into the water. The easiest way is to put one portal that you will constantly use just outside this corridor, and shoot the other color against the walls. When you can advance onto the platform, do so, and shoot another portal further down the hall. When the platform brings you all the way back, simply go through the portal again and advance a little further each time until you can get all the way into the final room.

Fourth room
A pellet launcher is firing a pellet downward, and the receiver is blocked by a double door.

The doors are opened by the buttons in the side rooms. You need to get a portal in both rooms to press the two buttons fast enough. Then, shoot the wall opposite of the receiver (the extended panel) followed by the scorch mark on the ground to get the pellet into the receiver. This has to happen very quickly, so it may take a few tries. The lift for the chamberlock will lower, and you can proceed to Chamber 16.

Alternatively, you can try a high jump to the exit. With your back to the platform that takes you to the exit, place an orange portal at your feet. Go to either of the other two moving platforms in the room. Stand on one until it reaches the top and place a blue portal at its base while looking down. While jumping through the blue portal hold the forward button to land on the exit platform.

Advanced
In the advanced chamber, most of the puzzles have been made more challenging, requiring alternate methods to solve them

Room 1 has a lot fewer portalable surfaces. Place a portal on the ceiling and the floor. You need to shoot a portal (the same color as your ceiling portal) as you fall so that you can fling over to the other side. You can do this on the first try, but if you miss, you can continue to fall through infinitely until you do manage to place a portal correctly.

Room 2 has an additional forcefield, requiring a 6-portal combination. The pellet moves a little slower, which requires you to place the next portals by the time the pellet hits the wall the first time, or else it will disappear and you will have to start this room again.

Room 3 has no portalable walls in the main room, and no staircase in the side room. Place an orange portal sideways as close to the emancipation field as possible. Place a blue portal on or below the camera, and come through to land on the side room’s ledge. Now place a blue portal on the floor below you. Take note of which direction the field is. If you jump off the ledge and into the portal, pressing the necessary direction, you can manage to come up with just enough momentum to walk through the field into the next section. Another method which is more guaranteed is to, after placing the orange portal similarly, place a blue portal on the ceiling of the side room above the low floor. As you fall through, put another blue portal under you to fly back through to the main room and maneuver over to the next section.

Now that you are with the pellet launcher and receiver, fling yourself over to the other side as normal. Notice the additional angled platforms, preventing the normal method. Trace a path backwards from the receiver, bouncing off the platforms as a pellet would, and you will discover that the pellet needs to be coming straight out of the wall next to the three angled platforms (on that same wall). So place a portal there, and another where the pellet hits the adjacent wall (not the angled platform), and watch it bounce its way to the receiver. Portal back over as normal.

The transition hallway’s platforms move significantly faster, but the same method can be used, just with more accurate timing.

The final room has only the ceilings in the side rooms usable. Also, the potential shortcut of flinging to the exit (see portals challenge, silver method), as in chamber 14, is prevented. The plan in this case will be to press a button, quickly jump out of the side room and into a floor portal, bringing yourself through a ceiling portal in the other side room, and then proceed as normal. Set up the ceiling and floor portals, and then let the platform raise you up into the appropriate side room (the one without a ceiling portal). Press the buttons, redirect the pellet, and exit.

Portals

 * Bronze at 23 portals
 * 1) Blue portal on the extended panel.
 * 2) Orange portal in the middle of the checkered floor. Fling over.
 * 3) Blue portal on scorch mark.
 * 4) Orange portal on wall above lift. Pass through field after pellet passes through portal.
 * 5) Blue portal where pellet will now hit the wall.
 * 6) Orange portal above receiver. Continue into next area’s side room.
 * 7) Orange portal on extended panel.
 * 8) Blue portal on floor of side room. Jump through.
 * 9) Blue portal where you would land to create double fling. You should now be in the section with the pellet launcher and receiver.
 * 10) Blue portal on the extended panel.
 * 11) Orange portal in the middle of the checkered floor. Fling over.
 * 12) Blue portal on left wall on scorch mark.
 * 13) Orange portal on right wall in corresponding spot.
 * 14) Blue portal high on back wall.
 * 15) Orange portal somewhere on the ground to fling back over.
 * 16) Blue portal on wall next to hallway entrance.
 * 17) Orange portal at end of first hall section. Come through when a platform is under it.
 * 18) Orange portal at end of second hall section. Ride back and come through when a platform is under it again.
 * 19) Orange portal anywhere in the final room. Ride back and go through.
 * 20) Blue portal in one upper side room.
 * 21) Orange portal in other upper side room. Press both buttons and create the final two quickly.
 * 22) Blue portal on extended panel.
 * 23) Orange portal on scorch mark.

Portals 1-19 are the same as for bronze. Blue portal in front of exit lift, as close as possible. Ride either side room lift to top. Orange portal on the ground. Jump through and maneuver onto exit platform. 
 * Silver at 21 portals

Portals 1-9 are the same as for bronze. You can skip the next pellet entirely with some clever use of portal movement. Blue portal on wall next to hallway entrance. Orange portal at end of first hall section. Now, you can partially enter the portal without falling. When you are halfway through, you will be able to shoot a portal somewhere on the other side. Look left so that it will be farther through the hallway (or forward if portal 11 was on the left wall). Inch up to the boundary between the two sides of the portal. Orange portal at end of second hall section. As you shoot this portal, press left (or back) so that you come back into the main section. If you do so correctly, you will be on solid ground with the portal placed farther down the hallway. This is called peek-a-portal. Orange portal on the floor of the final room, towards the close end of the left side or the far end of the right side. Place this portal in the same manner as 12, by peeking through, shooting, and coming back to safety before it lands. Then just go through. Blue portal in front of the exit lift, as close as possible. Ride the appropriate side room lift to the top, and jump through your existing orange portal, maneuvering to the exit platform. 
 * Gold at 14 portals

Additional reductions can also be made to this room to achieve sub-14 counts.
 * In the first pellet room, if you place a portal on the scorch mark and another one just left of the camera, but close enough that the camera still falls, when the pellet flies through, it may start to bounce in odd directions, including slightly downwards. If it does, you can pick up the camera and help redirect the pellet into the receiver, which can reduce the portal count by two.
 * In the second fling room, you can place a portal as close to the field as possible, and another on the floor of the side room. If you jump, it is possible to maneuver yourself through the field. This reduces the count by one.
 * It is also possible, in that same room, to fling yourself over the field without using the side room. Place a portal on the high panel, and another on the floor as close as possible. If you run and jump through this at the right location and angle, you can just reach it on the first fling to create the second fling.
 * Some ridiculously difficult jumping can get you from one raised platform in the final room, to the front of the pellet receiver box, to the side of it, to the exit platform. This can reduce the best count by two.

Steps
When going for gold, be sure to skip the second pellet as described in the least portals method, and also use the ending shortcut as in chamber 14.
 * Bronze at 75 steps
 * Silver at 65 steps
 * Gold at 55 steps

Time
This award can be achieved by doing every step in the basic method as quickly as possible. For these awards, you must use a number of shortcuts. Use peek-a-portal, as described in the gold least portals method, to travel through the narrow hallway without activating the moving platform. Once in the final room, portal yourself into one of the upper side rooms, and then use your portals to maneuver yourself onto the exit lift without activating it.
 * Bronze at 1 minute, 45 seconds (105 seconds)
 * Silver at 1 minute, 25 seconds (85 seconds)
 * Gold at 54 seconds

No Cheating SpeedRun in One Segment for Mere Mortals
Here's another chamber in which preparing for a speedrun is very similar to perfecting the time challenge. Remember that if you mess up in the middle of a single segment speedrun the implications are much more serious than messing up the challenge, so develop a path through the chamber that pretty-much always works for you. For example, if, when you try to peek-a-portal through the hallways, you have a habit of falling out of the portal and into the toxic goo, you need to either practice until this doesn't happen or use the regular, slow path (which will really mess up your time). Review all the hints and tips for the time challenge. In particular, don't walk through the chamber when you can portal yourself ahead.

In the first room, there isn't much you can do except to portal yourself forward (instead of walking) and executing promptly and perfectly. Get the orange portal on the high extended panel as soon as possible. Don't worry about exactly where you're standing, put the blue portal under your feet immediately. As soon as you pop out of the orange portal, move the blue portal to where you're going to land. Don't try to get the blue portal in the right place so that you can pass through it twice without moving it; most people will find it easier and quicker to move the portal in mid-fling.

The second room can be frustrating. Use a portal on the wall and one under your feet (in the first room) to get as far into the room as possible with the minimum of walking. Immediately place the portals to send the pellet into the room with the receiver. Stand close to the emancipation grid, but be careful not to touch it. Watch the pellet go into the first portal and as soon as it does move through the grid. Place the portals first over the red glow and then on the back wall. There are two ways to make sure that the pellet hits the receiver. Either make sure that the portal from which the pellet emerges is exactly over the middle of the opening to the next room (or at some other well-known position) and get the portal on the back wall to match this position. Or, look up so that you can see the pellet passing over your head, judge where on the back wall it will hit and place the portal there. This is all a lot to do in the short time it takes the first pellet to complete its journey. If you miss, you have to wait for the first pellet to dissipate and the second to be launched. Argh! When the pellet reaches the receiver and the platform begins to lower you should already be back out in the main room and clear of the emancipation grid. As soon as you can shoot a portal through the gap above the platform, do so. Put the other portal in the wall nearby and go through without waiting to ride the platform.

Again, portal yourself forward into the third room instead of walking. Portal yourself up into the side room, near the opening. Keep track of your orientation as you do this because you need to quickly turn and shoot a portal through the opening back into the main room to land on the high extended panel. Now you have to walk to the small pit, place the other portal at the bottom, fall in, move the portal as you fall and double-fling over the barrier. Practice until you can get this right just about every time. Having got past the first barrier, do not play the pellet game; this and riding the moving platforms takes too long. Instead, use peek-a-portal to get through the corridor without starting the platform. Put the blue portal to the left of the opening to the corridor and the first orange portal on the wall at the end of the first section of the corridor as far to the left and as high as possible. Putting it to the left gives you the maximum distance to the next position of the portal which gives you more time to pull back after shooting. Turn to the left and move sideways into the blue portal just until you see the glow of the edge change from blue to orange. This means that you are standing in the orange portal and looking down the second section of the corridor. Shoot the second orange portal at the wall at the far end (towards the right is better for the reason just explained) and immediately strafe left, through the existing orange portal before it closes, so that you are back in the third room. This all needs to be repeated to pass through the third section of the corridor. This time, turn to your right and slide left-ward through the blue portal until the glow changes to orange. Fire your third orange portal at the floor of the fourth room and, again, immediately strafe, this time to the right, to return to the third room before the orange portal lands and closes the one you were standing in. Now simply go completely through the blue portal and pop out of the orange one in the fourth room. Where you put the portal in the fourth room turns out to be important. If you get it in the wrong place, you pop out in the path of the energy pellet. You also need to reorient yourself quickly to execute the final fling to the exit promptly (see below). So, experiment a bit and find out what works best for you.

In the fourth room, none of the equipment is necessary; you can just fling up onto the exit platform. Place the blue portal anywhere on the ceiling in a clear area (clear of the pellet and any obstruction underneath it on the floor). Place the orange portal sideways right next to the grating under the platform by the exit area and fall through, looking straight down. As you fall, move the blue portal to the floor. As you fly up again through the orange portal, strafe and turn to land on the platform and walk out.

YouTube video Qb4rqGlNXps shows these techniques being used in the third and fourth rooms.