Plumbing

Well the plumber has come and done the first-fix plumbing, including gas pipework, but that's not what I want to talk about.

I'm talking about the polypipe conduits I want to use for network cabling. I installed one 13 mm pipe as a test to see how easy it would be to push CAT6 cable up it once it is installed. Answer: very bloody hard. I can get CAT6 up about 2 m, and CAT5 about 3 m (due to being slightly thinner). Once you get this much cable in the cumulative friction means the cable just bends in your hands as you try to jam more in.

Okay, so I decided to do some more tests at home. I have had lots of suggestions so I now needed to know if any of them would work. Lots of people have said I need to put a string in there to pull the cable through -- but how do you get a string in, especially if your pipe is already mounted in the wall and ceiling. John suggested fishing line and I wondered if I could use a pump at one end to suck the line through, if you attached it to a suitable ball of fluff. Here I am with my test rig. I used a tiny ball of scrunched up paper towel (no strings attached first) and it worked a treat. Pump, pump, Fump!

Next I tried attaching the fishing line. This worked at first, but the nylon tends to get tangled easily and eventually the drag within the pipe seemed to overcome the suction. But by putting the polypipe directly over the pump inlet (avoiding the use of a restricting nozzle) I managed to get enough suction to pull the nylon line through. Hooray!

Then the simple job of pulling a stronger piece of string through... turned out to be not so simple. Just the friction of the nylon line inside 15 m of pipe was too much to overcome -- it just stretched. But once I straightened out the pipe a bit, so that the line wasn't in contact with the entire length of the pipe, I did managed to pull the string through. Nevertheless, at this point I was thinking that this is a hell of a lot of effort and I will probably just fix the CAT6 cable in directly.


Anyway, a greater concern is how am I going to insert the cables with thermometers into the identical pipe conduits I have already put in the slab -- I can't even get to the other end of these. Instead I need a better way to push the cable in up to 8 m (I had planned to use CAT5 cable for this since it happens to be cheap, and I thought it would be easy to push down the pipe). I wondered about using fencing wire, since I have some, and I thought it might be rigid yet flexible enough to push down a bendy pipe, pulling 8 m of cable behind it. But John happened to have handy something called a 'drain snake'. This is a very long, flexible strip of flat steel about 3 mm wide, but less than 1 mm thick, so it easily bends in one direction, but is strong enough to push through blockages in kitchen drains. It has a funny-looking 'snout' on the end you push into the drain so it can clear a blockage, and also to help it navigate corners.

Anyway, I tried this in the installed conduit and it works -- you can push this thing all the way into the 13 mm conduit. However it is only 8 m long; not long enough to reach to the other end of the conduit I have already installed in the walls + ceiling, and this is by no means the longest run I have to do. 8 m is long enough to reach the end of the thermometer conduits under the slab though. So I returned home for some more testing.

The plan is to use the drain snake to pull the CAT5 cable into the conduit, by attaching the end of the CAT5 cable to the end of the drain snake and pushing them into the pipe together. But I need to be able to remove the drain snake afterwards without pulling the cable out with it, so I can't use the end of the drain snake with the snout on it. Instead I needed some way to attach the other end to the CAT5 cable which would be strong enough to pull 8 m of cable through the pipe, but be easy to disengage by pulling on the drain snake. I made a new snout for the drain snake out of a plastic pen lid, but I attached the lid firmly with duct tape to the CAT5 cable. The drain snake itself was not attached to the lid, it is simply inserted into the lid, next to the cable. This way it can push very hard, but be easily pulled out.

Incredibly this worked. I had to file down the sides of the lid a bit to make it fit better, and I also used another of John's suggestions: talcum powder in the pipe to reduce the friction. With all these steps implemented I managed to successfully insert 8 m of CAT5 cable and remove the drain snake from a 13 mm polypipe that meandered around a few gentle corners.

The main way this test differs from what I have to do in practice is that the cable will have two or three transistor-sized digital thermometers attached at precise locations along it's length. I hope if I wrap each of these 'knuckles' with Teflon tape it won't make too much difference. But John has another suggestion: use telephone cable instead of network cable. That should reduce the friction significantly, and these digital thermometers only need two wires for up to 64 thermometers so it should be fine.