How measurements are made

Last week, when I got all my assorted measuring tools out, I was reminded of something that has always bothered me. My tape measure is not an expensive one (I'm sure I got it for £8), but nonetheless you expect a certain standard. If it's not the quality of a Christmas cracker gift, you can demand a certain level of precision at least. But this tape measure has its flaws. 

The hook on the end of the tape, which you place on the end of whatever you mean to measure, is slightly loose. There's enough play between the rivets and the tape that there's an inaccuracy level that I estimate to be at least 2-3mm. Now I haven't got a machine shop, nor do I get to make things that are big enough to require a tape measure and accurate enough to require accuracy of less than 2-3mm. But as a perfectionist, it bothers me. Never mind the fact that I am now a journalist in the field of industrial metrology, which I believe should mean I keep my own standards of measurement at a reasonable height.

The tape measure serves its purpose. For instance, I have used it very effectively to demonstrate to my partner that upgrading to a king size mattress only really gives us 15cm extra, and thus we should probably look at a super king. But most of the time when I need to measure something, I resort to other tools at my disposal which are much more precise. In any case, I always seem to find projects that are on such a scale that my big fingers struggle to handle the details. My callipers are undoubtedly the most accurate piece of equipment I have, but they're also specialist enough that I don't get to use them very often. Honestly, my set square is probably my most used measuring tool besides a regular ruler. The ease of use of a set square is quite simply brilliantly convenient. 

The irony is, it's made by the same brand as my tape measure. So can I really rely on it as being accurate? It has failed me in the past, I regret to say. Any carpenter will know the trick to tell if a set square is made properly: draw a line and flip the set square over. If it still shows as ninety degrees on both sides, you know it's straight. Unfortunately, midway through a project a long time ago, I figured out that when you adjust the long arm of this tool, it may end up one or two degrees off a right angle. Very frustrating when you find that parts don't fit, but I've since learned that if you tighten the adjustment screw very tightly, it seems to be fine. 

I thought I would write this week about how rulers are made, or specifically what tools are used for the quality assurance in making rulers. Surely not every plastic ruler that comes out of China is exact in its millimetres? Just from common sense I'm guessing that not every ruler which is made gets compared to the original master metre. But although I did find this interesting video showing the creation of a special CNC'd ruler for building PCs, there wasn't actually that much information out there on the way regular rulers are made. I suspect it's probably because a lot of them are just made in factories in China, and most don't need to be accurate down to micron levels. Maybe I'm wrong, but it's an interesting thing to think about: any ruler you're given, whether plastic, metal or wood, you assume that 1mm = 1mm, because that's what it's made to do. 

But in the case of my tape measure, 1mm = 1mm+/-3mm. 

The problem I now face is that I have a project this weekend that will require precision on tape measure-scale. In the last house move, my bookshelf got damaged and warped. I need to replace the back board in order to reinforce it, which requires me going to the local DIY store and asking an employee there to cut a board to size (I would do it myself, but it won't fit in my car before it's cut down). So what should be a very simple fix has quickly become a project with far too many variables and error margins for my liking. There's the human error of me taking measurements of my bookshelf, the human error of the employee cutting the board to size and then there's the unknown error margin of their saw mount. 

At least the error margin of my cheap tape measure is a known variable...