The Waterless Membrane Trap

The whole device is one moulded part, and you can make a working equivalent from flat sheet. THE MATERIAL. Silicone rubber sheet, 1 to 2 millimetres thick, shore hardness around 40A. Thinner is more compliant and opens at a lower head; the notebook in step 3 shows stiffness going as the CUBE of thickness, so 2 mm stock needs noticeably shorter seams than 1 mm does. Silicone is right because it stays flexible over a huge temperature range, resists the alkalis and detergents that go down a drain, and does not take a permanent set the way natural rubber does. Nitrile is cheaper and dies faster in hot detergent water. Avoid anything that stiffens with age, because stiffness is exactly what kills this device. MAKE THE SLEEVE. Cut a rectangle 120 by 90 millimetres. Roll it into a tube 90 millimetres long around a 32 millimetre former and bond the overlap with silicone adhesive — a 10 millimetre lap, clamped, left overnight. Silicone bonds only to silicone adhesive; ordinary contact adhesives will peel off in a week. FLATTEN THE OUTLET. This is the part that matters. The top of the sleeve stays round, clamped to the incoming pipe. The bottom 25 millimetres must be flattened to a slit. Do it by bonding the two walls together along two short seams at the sides, so the cross-section transitions from a circle at the top to a flat lens at the bottom. The two lips should touch along their whole length with no gap and no tension. Getting this right is the build. Too much bonded length and the slit is stiff, so it needs a big head of water to open and the basin drains slowly. Too little and the lips do not close reliably. FIT AND TEST. Clamp the sleeve into a length of 40 millimetre clear waste pipe with a jubilee clip at the top and let the flattened end hang free inside the pipe. Clear pipe matters — you want to watch the lips. MEASURE THE CRACKING PRESSURE. Fill the pipe above the membrane slowly and note the water depth at which the slit first opens and passes flow. That head, in millimetres, is your cracking pressure, and it is the single number that decides whether the device is usable. Below about 25 millimetres it is a good trap. Above 50 it will hold water in a shower tray and annoy everyone who uses it. Then measure it in the reverse direction. Blow gently into the pipe from below with a length of tube and a manometer — a U-tube of clear pipe with water in it is enough. Find the back pressure at which gas starts to pass. It should hold well over 100 millimetres of water gauge, which is far more than any sewer will ever present. Those two numbers together define the device: it must open easily one way and hold hard the other, and the ratio between them is the entire engineering.

Build an S-bend as well, or take the one from the sibling blueprint, and set the two up next to each other on the same bench fed from the same tap. Everything from here is a paired comparison, and paired is the only way to get numbers you can trust — same water, same room, same day. TEST ONE: DOES IT SEAL AT ALL? Put a few drops of peppermint oil or a cotton wool ball soaked in ammonia in the pipe below each trap, seal the bottom, and smell above. Neither should pass anything. Establish that both work before you try to distinguish them. TEST TWO: DRAINAGE RATE. Pour two litres into each and time how long it takes to clear. The S-bend will be faster, because it presents no resistance at all once the water is moving, while the membrane has to be held open against its own elasticity. Record the difference. If your membrane is much slower, the bonded seams are too long — go back and shorten them. TEST THREE: THE ONE THAT MATTERS. Leave both traps standing, unused, in a warm dry room. Check them every day: pour nothing in, but test for gas passage with the ammonia at the bottom. The S-bend will fail. In a warm room a 50 millimetre water seal evaporates in roughly two to four weeks; near a radiator, faster. One day the smell comes through, and from then on that drain is an open pipe to the sewer with nothing between. Record the day it happens and the room temperature. The membrane trap will not fail this way, because there is nothing in it to evaporate. Leave it a month and it still seals. That is the whole case for the device, and having watched it happen is worth more than reading it. TEST FOUR: THE OTHER FAILURE MODE, WHICH GOES THE OTHER WAY. Now put a small amount of hair, tissue and cooking fat down each. Run them for a fortnight, flushing daily. Inspect. The S-bend collects debris in its belly, which is ugly and eventually blocks, and is cleared with a plunger or by unscrewing the trap. The membrane is worse in a specific way: fibres catch across the slit and hold the lips apart, and once the lips cannot close the seal is gone entirely and silently. A partially blocked S-bend still seals. A partially fouled membrane does not. That asymmetry is the honest reason S-bends remain standard. The membrane's failure is less likely but more complete.