Recently, I had to have my home swimming pool recoated in new fibreglass and I took the opportunity to make a few changes.

One thing I did was to put in a new weir. My old weir had been hand-made out of cement, with pipes placed in the cement. It had worked perfectly for decades.
One thing I always wanted was to be able to fill the pool automatically. I had actually started to build an automatic filler when the pool fibreglass fellow told me that one could buy a weir with an automatic filler attached. So, I told him to buy one and to install it.

So he did. It has a separate chamber with a float arm, which, essentially, operates the same way as a toilet cistern. The one that I had built myself had a conventional toilet cistern valve and float in it.

When the pool fibreglass coating was done, I connected up the plumbing myself because the pool guy said that he was not a plumber, so could not do it.

As I connected the weir, and over the following few days, I examined the weir. I was appalled at the really bad engineering design in the weir. In fact, I think that there is none. It seems the bookkeeper did the design during lunch break. There seems to be a total lack of any engineering design in the device. This is shocking. Physics and engineering principles apply not only to giant cranes and locomotives, but also to small things like a kettle, a fridge, and a pool weir. All these need professional scientific attention.

The main weir chamber is circular. That is okay. It is attached by two horizontal pipes to a second, smaller circular chamber in which the float arm is housed. There is no reason for the second chamber to be circular. In fact, it limits the length of the float arm. A longer rectangular tank would have been better. The float arm is rather short. I would have made it longer to provide more sensitivity. It has a float which you can screw up and down to adjust the water height. But, when you move it up about 50%, the screw adjustor hits the lid of the container, which then stops it from operating. So, I had to remove it and cut some of the thread off. The lid is almost impossible to get out using one’s fingers, so I had to make a special rod and insert it so that one could get a grip without having to fetch a screwdriver or knife to open it.

Then, after a few days, I noticed that the float chamber was always clogged with leaves and floating debris to such an extent that it jammed the mechanism. So I carried out a simple experi- ment with coloured liquid and discovered that there is a natural flow of surface water from the main chamber into the float chamber. So, virtually all debris collected by the weir ends up in the float chamber. This is really dumb. Clearly, nobody in the ‘design team’ ever carried out the experiment. So I had to design and insert a filter to stop this.

Returning to the main chamber – when you want to remove the leaf trap basket, there is a plate above it that has to be rotated to unblock it. This is a battle. The flanges which you have to grip are about 5 mm high, so they are almost impossible to grip. There is no reason why they cannot be 20 mm or 30 mm high to provide a good grip.

So then I get the plate out. The leaf trap basket is under it. But the basket is less dense than water, so it floats. It rises up and reaches the larger diameter, whereupon it tips sideways, dumping half the debris back into the weir chamber, which defeats the purpose of the basket. I had to design a weight to attach to the basket. The holes in the basket are not small, but quite large square slots. This means that many bits of leaves get through – they just slide through the slots. So I had to insert a net liner. When the basket handle folds down, the bit below the hinge point sticks up. This has to be removed, or the cover plate jams.

The main chamber has no water overflow. As a result, after heavy rain, when the pool is overfull, water flows over the top of the chamber, spilling everywhere. Because of this, I had to drill a hole and insert an overflow pipe to lead the water away.

The flap which should come up when the pump switches off – to stop leaves going back into the pool – does not. There is not enough buoyancy. So I had to add extra buoyancy.

There are more bad design features, which I have not mentioned. This whole weir would be wonderful to present to an undergraduate engineering class as an exercise to find all the design errors.

Even something apparently simple, such as a pool weir, needs professional engineering in the design. Many items that appear simple have subtle science factors hidden in them that only a professional eye will spot.