There’s a Hole in My Bucket… Is There?
By: David Eliston
Senior Consultant, Product Design
8th April 2020
3 minute read
We all know that when we buy something, we want the best we can get for the price. The same is true for engineers specifying products. The problem is that there isn’t really a ‘best’. There’s only ever something that will meet, or preferably, exceed your requirements under a specific set of criteria.
For example, when there’s a requirement for a product or piece of equipment to have to operate outside in all weathers it’s sensible to want to make it waterproof. The decision then is “How waterproof does it have to be”? The natural environment isn’t quite as well controlled as a test lab, so it’s understandable to want to add an element of safety to the waterproofness rating and over-specify it; say, by increasing an IPX5 requirement to IPX6 or an IPX6 requirement to IPX7. Unfortunately, this isn’t always a valid approach. The test conditions for IPX6 and IPX7 are quite different and IP X7 isn’t necessarily better (or more “waterproof”) than IPX6.
The water ingress part of the test for IPX6 demands that powerful water jets from all directions for 3 minutes cause no harm to the product. The test for IPX7 is that immersion under 1m of water for 30 minutes causes no harm. These are two completely different tests. The water jet test blasts 100 litres of water per minute on to the device under test and could force water through seal interfaces. A typical home shower flows more like 10 litres a minute by way of comparison. The IPX7 immersion test under 1m of water only applies the pressure from the head of water. But that doesn’t mean it’s a lower spec test either. It’s just different.
When designing a product to meet an IPX5 or IPX6 test, the engineer might put flanges or shrouds around vulnerable areas to prevent the direct impact of the jet forcing water through the sealing interface. But these mitigations wouldn’t help with the IPX7 test, as the water pressure will be more or less equal on all surfaces and seals regardless of this. So does this mean that IPX7 is harder to achieve than IPX6?
Imagine we were trying to seal the lid onto a plastic sandwich container with a rubber seal. The pressure from the 1 metre head of water over the surface area of the lid would be giving additional compression to the seal and actually help to keep the container watertight. So does this mean IPX7 is easier to achieve than IPX6 then? If the test doesn’t represent the intended environment, then the test method is distorting the results you are getting and could be causing the designer to make inappropriate choices.
The message is, understand what the environment is and specify product requirements and tests accordingly. This gives the designer the freedom to use their knowledge and experience to best effect rather than designing to an artificial set of product needs.
And if your bucket still leaks…Then don’t try to fix it with straw.